Part 17
The last class of the Articulata, the Arachnida, includes the Spiders, Scorpions, Centipedes, and Millipedes. The Arachnida differ from insects, in having eight or more legs, more than two eyes, no wings, and no head; the Spiders have four pairs of legs, and six, eight, or more, simple eyes, which appear as minute spots on the upper part of the thorax. Spiders nearly all live by preying upon small insects, which they catch in several ways, but by far the most general mode is by constructing a web or net to entangle them; the threads of this web are made of a fluid glutinous substance, which is secreted within the abdomen, and which they project from extremely minute orifices, several of which exist on the point of little projections called "spinnarets," this liquid dries into a solid thread immediately it comes into contact with the air; it is extremely strong, and so fine that several thousand of the single fibres will only make the size of a human hair. The Garden Spider (Epeira diadema) is often called the Geometrical Spider, from the regularity of its web. These Spiders will often place a thread from one spot to another, where it would seem impossible that they could attach it, such as from the branch of a tree to the corner of a house twenty feet or more from the ground; it has never been accurately determined how the Spider does this, and it is the more difficult to observe, as it is always done in the night, but it has been supposed that a fine thread is spun, and that the wind carries it across, when it is strengthened and afterwards made tight. The Spider (Araneæ domestica) that makes "cobwebs" in the corners of rooms, is called the Weaving Spider. Some Spiders are called Hunting Spiders, they do not construct webs, but pounce upon their prey with a leap; the Mygale avicularis of South America is one of this kind; it is said that some grow so large that their feet spread out a foot wide. The Long-legged Spider, often found in our houses, and of which fig. 32 is an exact copy, measures 3 1/2 inches, and is the largest species we have. There are also aquatic Spiders, which feed upon insects in the water and dive down for them; their surface being hairy and unctuous, retains sufficient air to bring them up and prevent them being wetted. The eyes of Spiders are different from those of other insects; they appear as simple, shining spots, or "ocelli," and not compound eyes, having a great many "ocelli" compounded into one. These "ocelli" are arranged in a group of eight or more, and placed on the upper part of the thorax, for Spiders have no head, although generally represented with one. The eggs of Spiders are generally deposited in some sheltered place, fastened together and partly covered with a net-work of a sort of yellow silk, very much resembling the cocoon of the Silkworm. The Scorpion, so terrible in appearance, and in its sting so deadly, and which is sometimes a foot in length, belongs to this order. It has eight legs, and a pair of claws like a lobster, the body is lengthened into a sort of tail, at the end of which is a claw or sting, like the poison fang of a serpent, with a poison bag at its root; its effects are often fatal to man, and always so to the insects and other small animals upon which the Scorpion feeds, its usual mode of killing which, is to catch them with its claws and then to sting them to death. The Centipede is a scarcely less frightful creature; in South America and the West Indies it is sometimes found to grow to eight or ten inches in length, it is of a long jointed form, with generally twenty-one pairs of legs, although some have double this number; its bite is much to be dreaded, as it is poisonous, especially the "Scolopendræ," which have a poison gland, like the snake; in this country they are very small indeed, and are generally found in moist places.
The Centipedes and Millipedes are, by some, placed in the class Arachnida, but by others in a separate class called "Myriopoda," divided into two orders, the Chilognatha, which have rounded cylindrical bodies, and generally more than one pair of legs to each segment of the body, and the Chilopoda, which have a distinct flattened head and but one pair of legs to each segment.
The last of the three great divisions of the Invertebrata is formed by the Mollusca; it is divided into six classes, (see page 218). The Mollusca are characterised by having no internal skeleton, nor external horny case, as in the Articulata; they are rather soft, and either void of solid covering, or possessed of "shells" composed chiefly of earthy matter (chalk), and in one or two pieces, called valves, hence the names, uni-valve and bi-valve; all shells of this description belong to the Mollusca, some of which have no means of locomotion, as Oysters; others have a "foot" covered with a muscular expansion, called the "mantle," by which they glide onwards, as the Snail.
1. The Tunicata have no shell or hard covering, but are of a pretty firm consistence; they are either fixed to rocks and sea-weeds, or float about freely, and are either solitary, social, or compound. The "Ascidians" are united in groups, and are all connected by a common stalk or "Stolon," from which they grow by buds.
2. The Brachiopoda. These Molluscs are furnished with a pair of shells, within which the animal lives; one of these shells (the ventral one) has a small hole in it, close to the hinge, and through this a long tendinous cord passes, which fixes the creature to some stone or rock, hence the name Brachiopodous, which means arm-footed. The young of this Mollusc are not fixed, but float about. The Lingula has horny shells, and the foot passes out between them; these creatures were some of the first created, if not the very first; the shells of Terebratula are found in vast quantities in the oldest fossiliferous strata.
3. The Lamellibranchiata include a great many of our most ordinary Mollusca, commonly known as "shell fish." They have a pair of shells, and are thence called "bivalve." The Oyster (Ostrea edulis), Mussel (Mytilus edulis), Cockle (Cardium edule), Scallop (Pecten), belong to this class. Oysters form a considerable article of commerce, thirty or forty thousand bushels are brought each season to London; they are dredged up from "beds," where they are found in great quantities. They spawn in May and June, and are not then good. There is an old and a well-known saying, that "Oysters are not fit to eat, unless there is an R in the month," all the names of the months containing an R but May, June, July, and August. The spawn is collected and placed in artificial beds, consisting of shallow places or hollows in the sea, where the tide will not wash them away, and whence they can be easily removed when sufficiently grown, which is in five or six years; these creatures have no powers of locomotion, but remain where the tide washes them; but Scallops, Cockles (fig. 34), and Mussels (fig. 33), have the power of fixing themselves to any substance they wish, by means of the "byssus" or beard, which is a tuft of fibres passing out from between the shells; and it is said, the Scallops have the power of progression, by suddenly opening and shutting the shells.
4. The Pteropoda are Mollusca which have no shell, or a very thin one; the Clio borealis (fig. 33), which forms the chief food of the Whale, is found in great multitudes in the Arctic seas, it swims about by means of two extensions, similar to wings. The Hyaloea has a small round transparent shell.
5. The Gasteropoda are extremely numerous; most of them have univalve shells, but many of them have none, as the Garden Slug (Limax, fig. 35). The Whelk (Buccinum undatum), Perriwinkle (Littorina littoria), Garden Snail (Helix aspersa, fig. 35), and the Wentle-trap (Scalaria, fig. 36), are the most familiar examples of this class. These Mollusca walk by means of the mantle, which is muscular, and capable of alternate contraction and expansion; they breathe by means of lungs on their back beneath the shell, and to which there is an opening in their side. What are usually called the horns are four in number, two short and two long; they are tentaculæ, but what is peculiar in them is the circumstance of having the eyes placed at their ends. These Mollusca have a sort of valve, which, when they retire into the shell, closes it like a lid; it is called the "Operculum;" in some cases it is horny, as in the Perriwinkle, and in others, resembles shell; the shells of these mollusca are coiled into a spiral, this is caused by the shell always growing by additions to the edges of the mouth, and in nearly every case this spiral turns in the same direction that a screw does, but in a very few it turns in the opposite direction, as in the "Fusus contrarius."
6. The Cephalopoda include the "Cuttle-Fish," and Nautilus (fig. 33). They are furnished with eight or ten tentacula or arms, which spring from the head, and which are covered with suckers; the mouth is in the centre, and these tentacula are used to lay hold of their prey and convey it to the mouth; they have two perfect eyes, and they breathe by gills. The Nautilus has a univalve shell, which is of a very graceful and elegant form; its interior is divided into chambers, with a syphon running through them, by which the air is exhausted or compressed so as to cause them to sink or swim; the animal occupies only the outer chamber; the extinct Ammonite belonged to this class. It is from the Cuttle-Fish (Sepiæ) that the beautiful dark brown pigment, known as "sepia," is obtained, and also the material of which Indian-ink is made.
THE VERTEBRATA.
The Vertebrate animals are those which have a brain and spinal cord inclosed within a bony case, an internal skeleton, to which are affixed the muscles, and which is moved by them; they all possess four extremities, more or less modified in form, with the exception of the Serpents. They are divided into four classes, Fishes, Reptiles, Birds, and Mammals.
FISHES.
The Fishes live in water, and are unable to live in the air; they receive oxygen from the water, but this is received much more slowly than by air-breathing animals, and as the combination of oxygen with the carbon of the system gives the elevated temperature of animals, so in Fishes, the oxygen being received slowly, their temperature rises but little above the temperature of the water they inhabit; they have thence received the name of "cold-blooded." Fishes live upon the marine animals which they capture, the various sea-weeds which grow in the water, and insects which fall into, or breed there. They have fins instead of legs, but the two pairs of fins known as "Pectoral" and "Ventral" are analogues of them, and in a pectoral fin may be found bones analogous to all the bones of the arm and hand of Man, or the fore-feet of the other Vertebrata. Fishes urge themselves through the water chiefly by the action of the tail from side to side, and in those members which have more extended forms (as Eels), by a wavey motion of the body itself; and the rapidity with which they swim is such, that Sharks have been known to go round the ship they are following many times in a very short space, although it may be sailing at a great speed before the wind. Fishes are covered with scales, and the head is united to the body without any constriction which can be considered as a neck; they are mostly of a form such as can easily pass through the water, without offering much resistance, and their specific gravity is so nearly that of water, that, by the compression of a small vessel filled with air, called the swimming bladder or "sound," they can rise and sink with little effort. Many tribes of the human family, such as the Esquimaux and Polynesians, almost live upon Fish, and even in Great Britain, in fishing districts, it forms a great part of the food of the inhabitants, and their capture gives employment to thousands.
Fish inhabit all kinds of water, the sea, rivers, and ponds, and many which live habitually in the sea come into rivers at certain seasons; and with respect to those which dwell always in the sea, some inhabit certain tracts only, and confine themselves to them, just as quadrupeds do to certain localities on the land. It has been a matter of wonder how the Fish get into ponds formed by the drainage of lands; but most ponds are supplied by springs which run underground from adjacent lakes or rivers, or by rills running on the surface, these bring the ova or eggs, which in favourable situations become hatched. Fish are exceedingly prolific, the eggs in the roe of a Codfish were estimated by Lewenhoec, to be upwards of 9,000,000, but not one in a thousand comes to maturity; there are enemies on all sides to devour them before they are hatched, and others to attack them afterwards. Some of the rivers of France have lately been artificially stocked with Fish by causing the fecundated spawn to be preserved till they are of a bulk and strength to protect themselves. This was effected by confining them in tanks floating in the water and perforated with holes, and afterwards placing them in a part of the river, inclosed both above and below by means of nets, so as to keep away their enemies--the larger Fishes.
Fishes were the first of the Vertebrated animals created, and as a proof of the incorrectness of the doctrine of development, these first-created Fish were of a class corresponding to the most highly organised Fishes of the present day.
Fishes are divided into two families: 1, Osseous Fishes (with a bony skeleton); 2, Cartilaginous Fishes (with a cartilaginious skeleton).
The Osseous Fishes are divided into six Orders:--
1. ACANTHOPTERYGII (Thorny-fin).
The Acanthopterygii include a great number of our ordinary useful Fishes, as Mackerel, Perch, Tunny, Gurnard, &c. (fig. 37). The Perch is a good example of this order, the sharp spines on its dorsal fin being well developed, and dangerous for anglers to handle.
2. MALACOPTERYGII ABDOMINALES (Abdominal soft-fin).
The abdominal Malacopterygii include the Pilchard, Herring, Pike (fig. 38), Salmon (fig. 39), Anchovy, Trout, Roach, Chub, Tench, and many more of the Fish which inhabit our streams and the sea.
The following account of the Salmon, is extracted from Dr. Baird's Cyclopædia of the Natural Sciences:--"The Salmon is exceedingly abundant in all the northern oceans. It is found in France, England, Germany, Bohemia, Switzerland, Scandinavia, Iceland, and Greenland. It exists also in Labrador and Canada, and down the American coast to the Connecticut river, in latitude 41-1/2 deg., and either it, or a species so closely allied as not to be distinguished except by close comparison, abounds in the Arctic seas washing the northern shores of the American continent. It does not frequent the rivers falling into the Mediterranean, and, though it has been stated to be the case, it does not occur in the Persian Gulf, or the Caspian Sea. In entering the mouths of rivers in order to spawn, the females are always observed to precede the males, depositing their ova in little pools or sort of nests, which they form in the sand at the bottom, for the males to fecundate. The young grow very rapidly, and at the present time, the subject of the changes which they undergo in their progress to maturity is one exciting much interest. When first hatched they are about an inch in length, and during the first year are called "Parr." When they remove to the sea, they receive a more brilliant dress, and then become the "Smolt," varying from four to six inches in length. After a residence in the sea of from two months to ten weeks, they revisit the fresh waters, and are then from two-and-a-half to four pounds' weight, and are called 'grilses' or 'gilses.' During the ensuing winter they spawn, and are then known as Salmon. Salmon grow to a large size, full-grown specimens weighing about forty pounds, though individuals are mentioned which have weighed seventy or even eighty pounds. Such are now excessively rare, owing to the perfection of the means of capturing them at the mouths of our rivers, by which the chances are very greatly against any fish escaping the various dangers by which it is environed for such a succession of years as is likely to admit of its attaining to its full dimensions. The power these fish have in overcoming obstacles to their progress up large rivers at spawning time is very great; they shoot up rapids with the velocity of arrows, and clear considerable waterfalls, leaping a height of ten, twelve, and even as much as twenty feet. The usual time at which the Salmon leaves the sea is the autumn; it remains in the rivers during the winter, and returns to the sea, after having deposited its spawn, in the spring. The fishing of this excellent and universally admired fish gives employment to many thousands of people, throughout the whole of the north of Europe. Salmon fisheries, it has been said, rank next to agriculture. In this country the Scotch rivers supply by far the greater proportion of the salmon brought to market. The total value of the Scotch fisheries has been calculated at £150,000 a year."
Many salmon are caught by stake nets, and also by the practice of "spearing;" this is a very picturesque sight, as it is conducted by torchlight.
The Herring (Clupea Harengus) makes its visit to this country about September, in enormous shoals, extending many miles each way; they are caught in great quantities and salted in the boats immediately; when brought on shore they are suspended in the smoke of wood fires, and become "red herrings." The Herring fisheries are very important, and afford--like the Salmon fisheries--employment to thousands; the greatest quantities are caught off the coast of Norway, where it is calculated about 400,000,000 are taken yearly. A great many are caught off Lowestoffe, and Wick, in Caithness. The Pilchard (Clupea pilchardus) is a kind of Herring, and is caught in enormous quantities off the coasts of Cornwall. The Sprat and Whitebait are species of "Clupea."
3. MALACOPTERYGII SUB-BRACHIALES (Sub-brachial soft fin).
The brachial Malacopterygii include most of the Flat Fish; as the Turbot, Plaice, Sole, &c. (fig. 40). The Turbot, and Halibut are caught out in the deep waters, but the Plaice and Flounders, chiefly in shallows; these fish have one side dark and the other light, and both eyes are placed on the dark side; they always swim with this side upwards; in the Turbot this is the _left_, while in Plaice it is the _right_ side. The Cod (fig. 40) is also included in this order; it is a large fish, often weighing forty or fifty pounds, and three or four feet long; it is chiefly caught in northern regions, the banks of Newfoundland being the most celebrated for its Cod fisheries, and supply the London market. These fish are caught by a long line and hook let down to a great depth.
4. MALACOPTERYGII APODES (Footless soft fin).
To this order belong the Eels (fig. 41). They have no abdominal fins, and are therefore called apodial, or footless; they very much resemble the serpent in their elongated form; they live chiefly in rivers and ponds, in the mud of which they obtain worms, &c., upon which they feed, they have been known to leave the water at night and travel for some distance in search of worms, frogs, &c. These fish live a long time after they are removed from the water, and are extremely tenacious of life; their skins are covered with such fine small scales that they appear quite smooth, and are extremely slimy. The Conger Eel is much thicker in proportion than the common Eel, and has a dorsal fin like a fringe all down the back; it grows to the weight of ninety or a hundred pounds sometimes. Eels are often caught by thrusting many-pronged spears (having the spears barbed) into the mud, and the Eels are brought up sticking between the prongs. The Electrical Eel (Gymnotus electricus) belongs to this section; it somewhat resembles a large ordinary Eel four or five feet long, but has a broad ventral fin running all along its lower part; it is famous for its power of giving electrical shocks when touched, and seems to have the power of using this influence at pleasure, but after giving a certain number of shocks its power becomes exhausted, and it has to remain some time before it can again give out shocks of any strength; these shocks are sufficiently strong to prove fatal to small animals, and even to mules and horses crossing the streams which they frequent, when a number will attack them at once. This creature is a native of the rivers and pools of South America, Demerara, &c.; the natives kill them by driving in a great number of horses, and when the Eels have exhausted their electrical powers, they are then caught and killed.
5. LOPHOBRANCHI (Tufted gill).
To the Lophobranchi belongs the Pipe-fish (Hippocampus, or Sea-horse, fig. 42). The following is Dr. Baird's description of these curious little creatures:--
"Most of these species are straight, but others of the family contract after death, so as to form a grotesque resemblance to a horse in miniature. These form the genus Hippocampus, and from their shape are generally called Sea-horses. Hippocampus brevirostris (the short-nosed Sea-horse) is occasionally met with on the British coast. It is from six to ten inches long, the body much compressed, short and deep; the whole length of the body and tail divided by longitudinal and transverse ridges, with tubercular points at the angles of the intersection. The snout is slender, and the tail long, quadrangular, and terminated in a naked or finless tip. When swimming about, the Hippocampus maintains a vertical position, but the tail is ready to grasp whatever meets it in the water; it quickly entwines in any direction round weeds or other objects, and darts at its prey with great dexterity."
6. PLECTOGNATHI (Plaited jaw).
The Sea Porcupine (Diodon hystrix) belongs to this order. These Fish are of a peculiar thick rounded form, and occasionally inflate their stomachs with air, which makes them almost spherical, in which state they float with the belly upwards, and being covered with spines, present rather a formidable object to attack; the flesh is unwholesome, and, it is said, the liver is violently poisonous; some of the species of this fish are phosphorescent, and from this appearance they derive the name of Sunfish, as they shine very brightly in the dark. The short Sunfish (fig. 43) turns round as it swims; it is very large, sometimes weighing from three to four hundred pounds. They yield a considerable quantity of oil, for which they are caught. Their jaws are not divided into teeth, but consist each of a single piece; they live upon seaweeds and various crustaceans.
The Cartilaginous Fishes are divided into three orders.
1. STURIONES (Sturgeons).