CHAPTER I.

TARDIGRADA, OR SLOTHS.

The South American Forests--Discovery of the Sloth--How it derived its Name--Peculiarities of Dentition--Food--Fore Limbs and Fingers--Hind Limbs and Heel--Other Modifications of Structure--Kinds of Sloth--Waterton’s Captive Sloth--Habits of the Animal--Burchell’s Tame Sloths--Manner of Climbing Trees--Disposition--Activity among Trees--Naturalists’ Debate about Anatomy--Probable Conclusion regarding it--Skeleton--Vertebræ--the Rudimentary Tail--Most Distinctive Skeletal Characters--Arm, Wrist, Hand, Fingers, Claws--Mode of Walking--Great Utility of the Claws--Face of Sloth--Skull--Teeth--Classification--_TARDIGRADA_--BRADIPODIDÆ--Genus BRADYPUS--Characteristics--Genus ARCTOPITHECUS--Characteristics--CHOLŒPODIDÆ--THE COLLARED SLOTH--Description--Skull Bones--Habits--Circulation of the Blood--_Rete Mirabile_--THE AI--THE UNAU--Appearance--Skull and Teeth--Skeleton--Interesting Anatomical Features--Stomach--HOFFMANN’S SLOTH--Description--Habits.

When the dense forests of the northern parts of South America were first explored by Europeans, it was observed that active Spider Monkeys, Howlers, and their Quadrumanous allies, were not the only climbing animals which frequented the trees. For every now and then, hunters came in sight of creatures about the size of a large Monkey, but whose sluggish movements, long hair, short heads, small ears and tail, and very long claws, enabled them to be distinguished at once from their very lively companions. It was noticed that these new creatures, instead of climbing quickly and swinging from branch to branch and running along the boughs, moved very slowly, by hanging head and body downwards and grasping the branches with their long claws. During the daytime, these quiet animals were constantly found asleep, huddled up in the fork of a branch, and looking like great balls of tow, or else hanging by two legs, the rest of the body being curled up. Now and then, one was seen at the foot of a tree, and it appeared to run along the ground with great difficulty; for the arms were so long that it walked on the elbows, and the hind feet were turned in, so that it supported itself on the sides of its great hind claws. Naturally, the animal took its time in moving, and as it was never seen to be lively, it received the name of Sloth. Interesting from being so different in its habits from other arboreal animals, it became much more so, to naturalists, when its remarkable construction was ascertained; but still the hairy creature with a short face, small head, long neck, hardly any tail, and very long front limbs, retained its popular name.

A very slight examination of one of the Sloths showed that it had no front teeth, that is to say, neither incisors nor true canine teeth, and that the hinder teeth--the false and true molars--were not like those of any other mammal. The back teeth, few in number, have since been ascertained to be exceptionally simple in their structure, and evidently the masticating process is very simple. But when it was noticed that the Sloth fed upon leaves and young twigs, the absence of the necessity for more elaborate teeth was acknowledged. Then it was observed that they had very long arms, or rather fore limbs, for the fore-arm bones and the humerus are all unusually long, and also that they had great power of movement. Moreover, it was seen that the fingers were reduced to three in number in some kinds, and to two in others, and that they were furnished with long and strong claws, which did not interfere with a great amount of mobility in the wrist. The length of limb, the mobility of the wrist, and the great claws, enable the Sloth to bring the leaves to its mouth, to hang on, and to walk, as it were, beneath the branches. An examination of the hind limbs showed that they were shorter than the others, and always furnished, in all kinds of Sloths, with three great claws. But the ankle seemed to be turned in, as if there was a state of “club-foot.” This condition would enable the toes to clasp a bough without effort, but it would prevent the sole from being placed flat on the ground. As the knowledge of the anatomy of these constant tree-livers progressed, other modifications of structure, equally important in relation to the peculiar arboreal life and food, were gradually discovered. For instance, a remarkable flexibility of the neck, produced by the peculiar arrangement of the vertebræ; a _rete mirabile_, to a certain extent, in the limbs, resembling somewhat that in the Lemurs (Vol. I., pages 213, 245), and a complicated stomach suited for the digestion of leaves, and foreshadowing that of the Ruminants.

Two different kinds of Sloths were described in the first instance, and subsequently, several others. The first kinds known were the Ai, a Sloth with three claws on the fore limb, and the Two-toed Sloth, with two claws on the fore limb. The Ai was called _Bradypus tridactylus_, and the other the Unau, or _Bradypus didactylus_, names which have been changed somewhat, as will be seen further on.

Sloths are caught without much difficulty, and their habits, in captivity, have been observed in South America, and also after their removal to Europe. Waterton writes[56] on the subject:--

“Some years ago I kept a Sloth for several months. I often took him out of the house and placed him on the ground, in order to have an opportunity of observing his motions. If the ground were rough he would pull himself forward by means of his fore-legs, at a pretty good pace, and he invariably shaped his course towards the nearest tree; but if I put him upon a smooth and well-trodden part of the road, he appeared to be in trouble and distress. His favourite abode was the back of a chair, and often getting all his legs in a line upon the topmost part of it, he would hang there for hours together. The Sloth, in its wild state, spends its whole life upon trees, not upon the branches, but under them; he moves suspended from the branch, he rests suspended from it, and he sleeps suspended from it; hence his seemingly bungled conformation is at once accounted for. One day, crossing the Essequibo, I saw a large Two-toed Sloth on the ground upon the bank, and although the trees were not twenty yards from him, he could not make his way through the sand in time enough to make his escape before we landed. He threw himself on his back and defended himself with his fore-legs. I took a long stick and held it for him to hook on, and then conveyed him to a high and stately Mora. He ascended with wonderful rapidity, and in about a minute he was almost at the top of the tree. He now went off in a side direction, and caught hold of the branch of a neighbouring tree, and then proceeded towards the heart of the forest.”

At Santos, in Brazil, in 1826, Mr. Burchell kept a tame Sloth, a _Bradypus tridactylus_, which at the end of two months pined and died. It fed exclusively on the buds and leaves of a species of _Cecropia_, a tree having a slender stem of thirty or forty feet long, with horizontal branches, hollow internally, and naked, except at the extremities. It ate only the young shoots and terminal buds of the unexpanded leaves, rejecting the old leaves on the boughs, which were brought to it daily. It was always perfectly silent, and its countenance and manners were most expressive of melancholy. It fed by day, and slept much; being kept in a room, it sat upright upon its short tail, embracing the legs of a chair with its arms and legs.

When resident at Para, near the mouth of the Amazons, Mr. Burchell also kept two full-grown Sloths, and a young one of a three-toed species (not _Bradypus tridactylus_, but of nearly similar form and habits), in a garden enclosed with strong stockades. They were kept tied up to the pillars of a verandah, to prevent their escape. Against these pillars they always placed themselves in an erect position, embracing the pillar with all four legs; when not tied to the verandah, they got up into trees in the garden. They slept both by day and night, always fixing their arms round something or other. Their food, consisting of branches, was brought to them in the verandah. They appeared extremely stupid, and would never come to the food. They would eat no leaves but those of the _Cecropia_.

They did not mount very large trees, and they ascended with their breasts pressing the trunk of the tree, advancing the hind-leg beyond the fore-leg. On the ground, they could neither stand nor walk, but lay sprawling on their belly, and dragged, or rather warped, themselves along, laying hold of a bunch of grass or stone with their three claws, which operated like grappling-irons, or, rather, pincers. All these died in a month or two. In their wild state they are seldom seen, from their colour mingling with the grey foliage of the trees, and from their being so extremely quiet and slow. The tame Sloths never willingly remained on the ground, except to pass from one tree to another. All the movements of the animal are slow. It moves its claws slowly. In eating it chews slowly; it also climbs slowly. The moisture of the leaves it eats suffices it for drink, without descending to obtain water. None of those kept by Mr. Burchell were ever seen to drink. The full-grown animals were never heard to utter any sound, but the young one occasionally, though rarely, gave a short cry or whistling squeak, of a single note.

They showed no indication of fear, and seemed to give attention only with their eyes. They took no notice of the boy who often carried them across the garden to their place in the verandah, with their long arms sprawling; the only objects of their regard were trees. They fight on their backs, and grapple their enemy to strangulation. The use of the long wool that covers the body, and even the face, seems to be to guard them from the annoyance of insects. Possibly it may preserve them from the attacks of Snakes, which are, doubtless, their greatest enemies.

The Sloth spends nearly the whole of its life in the trees, and travels along the branches body downwards. It rarely comes to the ground, on which it walks with difficulty, and it occasionally takes to the water and swims. It looks slothful enough when asleep, for it then resembles a bunch of rough hair, and a jumble of limbs close together, hanging to a branch; but when awake, it is industrious in its search for nice twigs and leaves, and moves along the under side of the branches of the trees with some activity. It seizes the ends of adjoining branches, clinging to the leafy mass, and moves from tree to tree quickly enough, when it is requisite, and it has a very singular power of moving the head and neck backwards in seeking food. When the atmosphere is still, the Sloth keeps to its tree, feeding on the leaves and twigs, but when there is wind, and the branches of neighbouring trees come in contact, the opportunity is seized, and the animal moves along the forest, under the shady cover of the boughs. The Indians have a saying that “when the wind blows the Sloths begin to crawl,” and the reason is thus evident enough--the animal cannot jump, but it can hang, swing, and crawl suspended. Mr. Waterton states, however, that “the Sloth travels at a good round pace, and were you to see him passing from tree to tree you would never think of calling him a Sloth. Being born up in a tree, living amongst the branches, feeding on leaves, and finally dying amidst the foliage, and enjoying life as much as any other animal, its structure and conformation are, of course, admirably suited for this arboreal existence. Its power of grasp is great, and is assisted by the great bent claws as it hangs by its feet when asleep, and also often when it is dead. One which was much frightened by being taken from the forest had a pole placed near it at a little distance from the ground, on two supports. It clung directly to the pole and hung on. A Dog was then made to attack the Sloth, which seized it in its long claws, and did not let go until the enemy died.”

Leading thus a very unusual kind of life, up amongst the dense foliage, and having some very unusual peculiarities of construction, much debate occurred many years since regarding the general conformation as well as the special anatomy of the Sloths. One school of anatomists considered the Sloths incomplete and abnormal animals, moving with “pain” on the ground, and another regarded their unusual and peculiar anatomy as singularly beneficent.

But whilst it is perfectly evident that the long limbs and their joints, and the peculiar turning in of the ankles, and the structure of the clawed hands and feet, are all admirably adapted for the peculiar life which the animal leads, it appears to be consistent with anatomical reasoning to believe that the Sloth is an instance of retrograde development; that, in fact, the peculiar formation of the skull, neck, wrists, and ankles, is the result of the laws of disuse and adaptation operating on ancestral animals, which once had their anatomy more consistent with a perfect mammalian type.

When the Sloths were first carefully watched and studied, their length of neck and their ability to turn the head, so as to look at a person standing directly behind or beneath them, without swerving the body, struck Mr. Burchell especially. This curious peculiarity led to a careful examination of the skeleton of the different kinds, and much discussion followed, for it was found that in the Sloth examined (the Three-clawed Ai) there were more neck bones (vertebræ of the cervical region) than in other Mammalia. Instead of the common number of seven neck bones, there were nine. This elongation of the neck of course permitted a greater amount of twisting than could occur in an animal with fewer neck bones. But there are other reasons why the head can be so much twisted round, for the spines on the neck bones are small, and the joint between the skull and the first vertebra is so fashioned that this remarkable motion is possible. There was a great deal of discussion about the extra neck bones, and as the last two had rib-like projections from their sides, some anatomists considered them to belong to the true rib-bearing vertebræ, or those of the back (the dorsal). But when the other Sloths were examined it was found that the number of the bones of the neck in all the two-fingered kinds was not as great as in other animals. There are only six neck vertebra in one well-known species (_Cholœpus Hoffmanni_, for instance), whilst there are seven in another two-toed Sloth.

Eating largely and of bulky substances, the Sloths require a large digestive cavity, and the ribs are numerous, and the body is long and broad. There is much variation, however, in the number of the back bones in the dorsal and lumbar regions. Thus in the Ai there are sixteen dorsal and three lumbar vertebræ, whilst in the Two-fingered Sloth there may be twenty-three or twenty-four dorsal bones, and two, three, or four lumbar vertebræ. The ribs are close together and are broad. As the hind limbs require strong muscles, for the animal hangs on by them whilst it is feeding itself with the fore hands, the pelvis is large and is strengthened by having the hip and haunch bones (ilia and ischia) united to the conjoined sacral vertebræ, which may be six, seven, or eight in number. Moreover, all the strength of the pelvis is behind, the fore part or pubic bones being slender and united in front.

Some small tail bones exist, for that organ is rudimentary in all the Sloths, there being a stump in the Ai, and eleven very small bones; but in the Two-fingered Sloths the tail is not visible, and there are four little ossicles. There are no long and very prominent spines to any of the back bones, and the whole bony column of the spine is readily curved and bent. The animals so constantly bring the hands and feet close together, when hanging, that a ready bending of the spine is absolutely necessary. Moreover, in sleeping they often rest in the fork of a tree, or on a branch, and place the head between the hind legs, rolling the body up as it were in a ball, and this is facilitated by the peculiar construction of the long chain of back bones with small spines.

The most distinctive character of the skeleton of the Sloth is the excess of length of the fore limb over the hind one. An examination of the slender bones of the arm shows that they are more solid than those of most Mammals. The arm bone (humerus) has a hole through it in the inner expanded part, just above the elbow (inner condyle), in the Ai; but this is not found in the two-toed kinds. The wrist and hand are long and narrow, and this is produced by the union of some bones which are separate in other Mammalia, and the slight development of others. Thus there are six bones in the wrist instead of eight (the scaphoid and trapezium, and the os magnum and trapezoid have coalesced). In the Ai there are three clawed fingers, and the bones of the thumb and of the little finger are absent, and their corresponding hand bones (metacarpals) are very small, and are joined on to the next, that is, to the metacarpal of the index and third finger.

The three fingers are, moreover, strengthened for their peculiar uses, the first two joints being united, and the tip or last joint being very long, and supporting the claw. Moreover, as the long claws are constantly half closed in the hand, and they are never required to be widely open, the tip of the finger is so made that flexion is possible, but not unclasping widely. The skin comes up to the base of the claws, and encloses the fingers, and the base of each claw is protected by a bony sheath. They form capital hooks; they grasp, and although there is no opposable thumb, they hold the food; and a tame Sloth may be seen holding a carrot very firmly between them and the wrist. In the case of the Unau Sloth, the outer claw is the longer.

The Sloths walk on the outside of the extremities of the fore and hind limbs, and their claws are always curved in, and, as it were, retracted. Consequently, the animal cannot place the soles flat on the ground, and it cannot open its foot-claws to a great extent. This fixing of the claws assists in the clasping and hanging, which are the usual and commonest attitudes. The claws surpass the foot in length, and are so sharp and crooked that they readily seize upon the smallest inequalities in the bark of the trees and branches upon which the animals habitually reside. They and those of the fore limb are no mean weapons of offence and defence, for, situated at the end of long and muscular arms, they can drag, cling, and hold with great tenacity. The thigh bone (femur) of the Sloth is straight, and is thicker and shorter than the arm bone (humerus); it has no ligament to unite it to the joint (no ligamentum teres). On examining the lower bones of the leg (the tibia and fibula), they will be found to be bent, so as to leave a space between them, and they are shorter than the bones of the fore arm. The bones of the ankle joint, are united together immovably--that is to say, the usual bones seen in other Mammalia are there, but are united by bone. Moreover, this union includes the complete and ill-developed feet bones (metatarsals), and the first bones of the second, third, and fourth fingers. One bone is not included in this strange union. It is the astragalus, or the bone immediately jointed with the ends of the bones of the legs. The outer or small bone of the leg (fibula) fits into a pit in the outer part of the upper surface of this bone, and thus prevents any movement of the foot like a twisting outwards, and favours, but does not produce, the usual position of twisting inwards. Moreover, there are two powerful muscles in the front of the leg which are not opposed by others as strong, and they, by their contraction, keep the foot twisted inwards, as in club-foot (the anterior tibial and the long extensor of the great toe).

In the Unau, or Two-fingered Sloth, there is the same general arrangement of the bones and muscles, with some important differences, which result in there being a greater amount of bending and extending of the foot, although the foot rests on its outer edge.

A Sloth’s face is short, and there is a broad snub nose, with round nostrils, which are widely open. The cheeks become wide suddenly, and the forehead slopes rapidly backwards, the eyes being wide apart and small, but looking forwards. The head is small and round, and as it is covered with hair behind, it cannot be distinguished well from the upper part of the back of the neck. The expression of the face is always the same, and the method of masticating and eating is disagreeable to observe. The animal having no front teeth, and moving its jaws usually only upwards and downwards, and not from side to side, places the morsel, such as lettuce leaf or carrot, well into its mouth, and chews at it, dragging out the food every now and then, when it is covered with moisture. On examining the skull, the short cut off or truncated appearance of the face is very evident, and it will be observed that the teeth are wanting in the front bones of the face (the pre-maxillaries), and that only the palatal part of these bones exists. The lower jaw is strongly jointed to the upper, and the back part is large: there are teeth at the sides, but there are none in the front part of it. A very singular-looking cheek bone (zygoma) exists on either side. It is not attached behind to the ear bone, so as to cover the jaw muscles, but it has two processes behind--an upper and a lower--which differ in shape and size according to the species. The central bone of the nose does not reach to the nasal outlet, and there is a system of air-cavities which is continued from the nose into the forehead bone. In some kinds, the lower jaw ends abruptly in front, as in the Ai; but in the Unau Sloths it is slightly angular, and projects.

The back teeth of the Sloths are very simple, and consist of three structures, called vaso-dentine, hard dentine, and cement, there being no proper enamel. The vaso-dentine is a kind of bony substance in the centre of the tooth, in which there are the passages and tubes of blood-vessels. The dentine is outside this, and consists of more earthy particles than the vaso-dentine, and of fewer tubes; it is all the denser and more resistant. Wearing away more slowly than the vaso-dentine, it forms a ridge which grinds easily. The cement is a kind of bony structure on the outside of the tooth. The teeth of the Sloth continue to grow from below as they are worn above, and there is no entire milk set which are replaced by those of a permanent kind.

(From the _Proceedings of the Zoological Society_.)]

The term Sloth is commonly applied to all the kinds of animals whose general shape and habits have just been noticed. It is evident, however, that this union of several species under one term is not correct in zoology, and it is necessary to distinguish them by peculiarities which are permanent. A very ready method of distinction is to separate the Sloths into two families, one containing those which have three claws on the fore limbs and the same number on the hind limbs, and the second including those which have only two claws on the fore feet and three on the hinder.

The first family is called the BRADYPODIDÆ, from βραδύς (slow), and πούς (foot), and the second CHOLŒPODIDÆ, from χωλός (halting, lame), and πούς (foot), and both are included in the group TARDIGRADA, or slow-moving _Edentata_.

The BRADYPODIDÆ include two genera, but many naturalists only acknowledge one. The first is _Bradypus_. This includes the Sloths with three-clawed fingers on the fore limbs, whose males and females are alike in their fur, and which have the cheek bone (malar bone) with two processes. The upper one is long and dilated at the end, and the lower is long and triangular, and neither of the processes reaches the ear bone. There are in these Sloths, when full grown, five molar teeth on each side in both jaws, and the first is very short. There are two mammæ on the chest.

The second genus is _Arctopithecus_ (Gray), and it contains species which have the males and females dissimilar in their colour and ornamentation, and the malar bone has a thin and narrow upper process.

The second family of the Sloths (the CHOLŒPODIDÆ) contains but one genus, _Cholœpus_ (the Unau), whose species have two claws on the fore limbs and three on the hind ones. The front of the lower jaw is stuck out, and not cut short, and the first molar teeth are long.

The genus _Bradypus_ probably contains several species, but it is only necessary to mention one, which is called

THE COLLARED SLOTH, OR THE HAIRY SLOTH.[58]

This Sloth lives in the densest forests of Brazil, Peru, and Para, and is found not far from Rio Janeiro.

It is a kind of the Three-clawed Sloths, in which there is little or no difference between the fur of the males and females. The neck is surrounded by a large collar of long black hair, and underneath this is a fur of a dark-brown colour. The face is naked, and is of a black colour, and the hair of the body is not very flattened, but is withered-looking to a certain extent. The forehead, temples, chin, throat, and breast are covered with reddish or rust-coloured hair, slightly grizzled. On the crown of the head it is long and yellow, and pale orange on the rest of the body. This Sloth produces one at a birth.

The lower jaw has a kind of blunted lobe in front, and the angle of this jaw is broad, triangular, with a rounded lower edge, and it projects backwards beyond the joint which connects the bone with the skull. The cheek bone has those peculiarities which have already been mentioned. The teeth are peculiar, for the first or foremost grinders are smaller than the others, and the second upper grinder is the largest of all. The first grinder on the lower jaw is broader than the rest, and the hinder are the largest, being also cylindrical.

It has the general method of living of the Sloths, being perhaps not quite so lively or active as the Unau, and feeds mainly on _Cecropia_ leaves, finishing those of one tree as far as it can before commencing those of another. Like all the Sloths, it has the power of long and sustained muscular action, and can cling on, or grasp, for a very long time without perceptible fatigue, and this gift is associated with a structure of the blood-vessels which supply the muscles, resembling, as we have said, that noticed in some of the Lemurs. The main artery which supplies each of the fore limbs is the axillary, so called from its being found in the armpit or axilla. In quickly-moving animals this vessel reaches into the upper arm, and divides into a few rather large ones lower down, and these give off others, so that a certain quantity of blood is supplied and removed quickly. But in the Sloths the axillary artery divides at once into a number of cylindrical vessels nearly as large as it is, and they are united here and there. These unusual arteries are found in contact with the surface of the muscles, and their branches go in and amongst the muscular bundles. As many as forty-two of these large vessels were counted by Sir A. Carlisle, on the surface of the muscles on the front of the arm, and probably about twenty were inside. These arteries thus carry an immense supply of blood to the muscles, but blood which, although it is finally removed by the veins, does not move very rapidly. In fact, the muscles are turgid with blood. The same arterial structure is seen in the hinder limbs, and the arteries of the thigh form as numerous a set as those of the arm.

It seems to be in accordance with careful investigation, to state that the species of Sloth called _Bradypus tridactylus_ (the Three-toed Sloth, or Ai) has too large a meaning, and that it really refers to the Collared Sloth, as well as to others which have been placed in the next genus. It is as well to remark here, that although there are three clawed fingers to the fore limb, there are vestiges of two other ones by their side in the form of two rudimentary metacarpal bones.

GENUS ARCTOPITHECUS.--THE AI.[59]

Several kinds of three-clawed Sloths have been called Ai; for instance, the Yellow-throated Ai, and De Blainville’s Ai, and all have been named _Bradypus tridactylus_. Dr. Gray, however, satisfied himself that the kind which was first described by Cuvier as the Ai, and which is figured in Prince Maximilian of Neuwied’s “Animals of Brazil,” is the same as one which has since been called _Arctopithecus Ai_, or _Arctopithecus flaccidus_. The word _Ai_ is taken from the noise made by the animal, and the term _flaccidus_ relates to its long hair. The true Ai inhabits Venezuela and Peru, and has very long flaccid grey hair mottled with white. There is an abundant under-fur of a blackish-brown colour, which has white and black in spots and blotches.

There is a small spot between the shoulders on the back, where the fur is soft and woolly, and a broad, short, blackish streak there, with a white or orange ring around it. The claws are coloured brown. The head has a curiously-cut short and turned-up nose appearance, and is furnished with coarse shaggy hair, disposed on the crown in a diverging manner. The short hair of the face contrasts with the long, shaggy, shrivelled, dry, hay-looking hair of the body. This hair is coarse and flattened at the ends, but it is exceptionally fine at the roots, and it greatly resembles in colour and texture some of the vegetation of the trees on which it lives. The eyes are bright, and are surrounded by a dark ring. Several species of the genus Arctopithecus have been described which live in Guiana, Bolivia, Brazil, Peru, and Venezuela.

The next genus of the Sloths is represented by

THE TWO-FINGERED SLOTH (THE UNAU).[60]

There are several kinds of Sloths with two “toes,” or rather with two fingers ending in claws on the fore limb, but the differences between the species are not very readily appreciated. They are differences which can be recognised, but it is doubtful whether the possession of dark brown or pale brown hair is sufficient to decide that there are more than one species.

The common Unau Sloth is usually of a darker tint than the others, but there is no doubt that the specimens in museums of all these Sloths vary much in the colour and length of the hair. Thus the hair may be generally dark, and the hairs of the crest on the back of the head may be white, and more or less tinted with bright green; or the hair may be short, of a dark brown colour, paler on the rump, much paler on the head, cheeks, and chin; a band may be across the nose, and the orbits dark brown. Others of the same species have very long hair, of an uniform dark tint, paler on the head and redder beneath, whilst one from Juan de Fuca has short hair, without any indication of a crest. From Brazil there are specimens with long paler hair and a crest. All these specimens, however, have pale whitish claws.

A Unau from Columbia is of a pale and whitey-brown paper colour, darker at the root of the hairs, and it has pale fawn-coloured claws.

In all these animals with different kinds of furs, the two-clawed condition is peculiar to the fore limbs only, for on the hinder there are three claws, and it is to be remarked that the hair and skin unite the fingers and toes close up to the base of the claws. The skull of the Unau is rather projecting in front, and not, as it were, quite cut off close; and there is a great gap in the upper and lower gums in front, the incisor teeth being absent, of course. But at the side of the mouth there is a longish tooth above and below, looking like a canine, but really it is the front molar, which in both jaws and on both sides is longer and larger than the others. The under teeth belonging to the lower molar set are placed behind the corresponding upper ones when the mouth is shut.

The cheek or malar bone is seen, on looking at the skull, to be separated from the ear bone, and to have a forked end posteriorly, the lower part of the fork extending downwards and backwards.

The lower jaw is very straight: it projects a little, in front and behind, where it is jointed with the upper jaw, there is no upright portion or branch, or ascending ramus. The last back tooth is just in front of a curved piece of bone called the coronoid process, the base of which is on a level with the line of the teeth.

This Sloth has seven neck bones (cervical vertebræ), and the last one has a very small and rudimentary rib attached to it on either side. There are no less than twenty-three dorsal vertebræ found to be with ribs. The Unau has a clavicle which is much smaller in the other group. It has no tail. The structure of the ankle joint enables it to turn in, even more than that of the Three-clawed Sloth. As the habits of the Unau Sloth are the same as those with three claws, and all live in the same great district, these anatomical distinctions are very interesting, and relate to their remote ancestors, being hereditary legacies, which are of little or no importance in assisting the creature merely to live. One of the differences between the Sloths is singular. The Unaus have a very remarkably formed stomach, which may be said to be double. The first stomach is large and rounded, but it is contracted behind, and then formed into a kind of conical appendix. This appendix is doubled from left to right, and its cavity has a fold at its opening into the stomach. It forms a special part of the first stomach. Then it is to be noticed, that where the food enters the stomach, or at the opening, which is called the cardia, there is a pouch, looking like a bag at the end of the tube which runs down from the gullet to the stomach. This is the second part of the first stomach: and the third is a tube-like space which connects the cardia with the stomach far away to the left. These three cavities form the first stomach. The second stomach is of a slender form, and is very much smaller than the other. Its walls are thin for the first half of its length, but towards the spot where the gut commences (the pylorus) they are thick and muscular. A small fold occurs midway. There is a fold in the body of the smaller or second stomach, and there is a little hollow there with glands in it, and it is called the appendix to the second stomach.

The stomach is thus rather complicated, and its internal mucous membrane is so thrown into folds, and made into hollow spaces, that it occupies much more space than if it were a simple bag. This plan is also well seen in those ruminating animals which, like the Ox, live entirely upon vegetable substances; and it is evident that the diet of the Sloth bears some relation to the complicated stomach.

In the Ai, the appendix to the second stomach is larger than that of the Unau Sloth, and is more complicated.

HOFFMANN’S SLOTH.[61]

This is a Sloth with two clawed fingers on the fore, and with three claws on the hinder extremities. Living specimens are occasionally brought to Europe, especially from Porto Rico, so that its general appearance may now and then be studied at the Zoological Gardens, in the Regent’s Park. If it be looked at there in the day-time, it certainly merits the name of Sloth, for it resembles a bundle of long, light, brown hair, fixed on the top of a bar of wood close to an upright branch, or huddled up in a corner on the ground; but in the morning, and also late in the evening, the creature begins to move slowly, and to look out for the food put for its use on the floor of the den. All the Hoffmann’s Sloths have pale brown hair, whiter at the tips, and a white face, showing a brown band across the nose, extending to a ring round each eye. They have also a long and full crest of hair on the neck, and the hair on the limbs is darker than that of the rest of the animal. Dr. Peters, who discovered this Sloth, examined the skeleton, and found only six vertebræ in the neck, and in this it differs from the _Cholœpus_ just noticed.

When its food, consisting of carrots and lettuce, and bread-and-milk, is put down in the morning it is soon in movement, and enjoys its meal hanging down from a bar with its hind legs, and resting its back on the floor of the cage. It seizes the food between the claws and the long straight palm of the fore-foot, and passes it into its mouth, chewing actively with the molar teeth, especially with the first, which are sharp. It cares little for the spectators, and when it has finished, slowly mounts up into a corner of its little den and settles down to sleep. In the evening it becomes lively, for it is, and, indeed, all Sloths are, nocturnal in habit. The hairless snout, of a light red tint, the absence of “smellers,” the little eyes with a few hairs around them, and the broad forehead, give the animal a curious appearance. The hair is brushed back on the forehead, and comes around the very small ears on to the cheeks, and is whitey-brown, and this same tint is seen over the whole of the back in long slender hairs. But the under hair is light red or red-brown. The long and slender hand, with its two claws, contrasts with the rather bulky upper part of the limbs, and the flesh-coloured palms are very remarkable.

The whole of the Sloths lead very monotonous lives; their food is ever within their reach, and it is abundant, and they do not appear to have to compete much or at all in the struggle for existence with other animals. Their enemies are Snakes and the Carnivora, but it is evident that they are much more readily preserved by their habits from the latter than from the former. Leading such an uneventful existence, there is no great call upon their nervous energies or intelligence, and these are at a low pitch. The brain consequently is very simple in regard to convolutions, which are few in number and shallow.