CHAPTER XX.

SOME CRUSTACEANS, MOLLUSCS, AND WORMS.

65. SOME COMMON CRUSTACEANS.

1. =The crayfish and lobster.=—(_a_) _Habits._—Readers in limestone districts will probably be able to find crayfishes in the streams, and the habits of the animals in their natural surroundings should be observed and described. Other readers will be able to obtain live specimens from dealers.[35]

Place the animal on the table or desk. Notice that the body consists of an anterior, unsegmented portion, the _cephalothorax_ (corresponding to the head _plus_ the thorax of an insect), and a posterior, jointed _abdomen_. Watch the movements of the stalked eyes, the feelers and legs; allow the claws of the largest pair of legs (the “pincers”) to grasp a pencil. Put the crayfish in a white dish, with an inch or two of water, and by means of a pipette discharge a few drops of water containing some suspended colouring matter, such as carmine or indigo, near the point of attachment of the last walking leg. Describe the movements of the coloured water. When the animal is startled, notice the sudden vertical flexure of the abdomen, by means of which the body is pulled backward in the water. Feed the crayfish with meat or worms, and try to see the action of the jaws.

(_b_) _External characters._—Kill the crayfish instantaneously by dropping it into boiling water. Compare the animal with a cockroach (p. 349). Examine more closely the stalked eyes, the two pairs of feelers, the _four_ pairs of walking legs and the single pair of large pincers, and the fusion of the head and thorax to form the cephalothorax. Count the segments of the abdomen.

(_c_) _The appendages._—Examine the ventral surface of the abdomen, and notice that each segment, except the last, bears a pair of small jointed organs; these are called _swimmerets_. Remove one of the pair carried by the last segment but two, examine and draw it; make out that it consists of a stalk and two branches. Notice that the branches of the swimmerets of the last segment but one are expanded, and form, with the last segment, the _tail fin_.

Study the appendages of the cephalothorax from behind forwards. They consist of (i) four pairs of legs used for walking, and a larger pair of pincers; (ii) three pairs of foot-jaws or _maxillipedes_; (iii) two pairs of _maxillae_; (iv) one pair of _mandibles_; (v) two pairs of _feelers_.

(_d_) _The gills._—With strong scissors cut off the side part of the exoskeleton of the cephalothorax, and notice the plume-like _gills_ in the _gill-chamber_ thus laid open. Move the adjoining legs, and see that some of the gills also move.

2. =The crab.=—Obtain a crab and compare it, point by point, with the crayfish, or lobster. Notice the great width of the shell of the cephalothorax; much of this width is due to the gill-covers, which stand out from the sides of the true body. Look for the opening of the gill-chamber at the base of the pincers. Make out the stalked eyes (in sockets), the two pairs of feelers, and the five pairs of legs. Notice how the animal runs; in what respects does the method differ from the gait of the crayfish? Examine the ventral surface of a dead specimen, and notice that the abdomen is tucked under the cephalothorax. Stretch out the abdomen; compare it with that of the crayfish, and count the segments. Notice the absence of the tail fin; it is not required, as the crab does not swim. Extend the abdomen, and make drawings of the animal (i) from above, and (ii) from below, and label the parts. Carefully remove the foot-jaws and the true jaws, and compare them with the corresponding appendages of the crayfish. With strong scissors cut off one of the gill-covers, and examine the gills. Observe that in the crab the gills are not in any case attached to appendages; they all spring from the sides of the body-wall.

Tabulate the respects in which you have found the crayfish and crab (i) to agree with, (ii) to differ from, insects.

The student who has worked through Chapter XIX. will at once recognise in a crayfish, or a lobster, an animal possessing many features in common with the insects, and will find it interesting to try to discover for himself why it is not called an insect, but is placed by naturalists in a different class.

=The crayfish.=—The crayfish (Fig. 229), or the lobster (which agrees closely in structure with the crayfish), is obviously an =arthropod= (p. 351), for it is covered by an exoskeleton of chitin, and has hollow jointed limbs and a segmented body. In these respects it agrees with the insects. On the other hand, it plainly possesses at least five pairs of legs, and has two pairs of feelers (_ant._ 1 and _ant._ 2, Fig. 229), whereas no insect has more than three pairs of legs (when adult), or more than one pair of feelers.

These differences are apparent at the first glance, and closer examination reveals even greater contrasts. The three primary divisions of the body into head, thorax, and abdomen, which are so characteristic of insects, are not obvious in the crayfish; for the head and thorax are here fused into one mass, the =cephalothorax=[36] (Fig. 229, _c.th._), which is covered by a shield called the =carapace=. Moreover, in the crayfish _every_ segment except the last bears a pair of =appendages=, which vary in form in different regions of the body according to their duties, but which can all be shown, by careful comparison, to be modifications of one primitive form, which is =Y=-shaped and consists of a basal stalk and two branches. This form of appendage is well seen in the _swimmerets_ (17, Fig. 229) of the abdominal region; further forwards the appendages become _walking legs_ (9-13, Fig. 229); next come three pairs which combine the characters of legs and jaws, and are called _maxillipedes_ (8, Fig. 229); then are the true jaws—two pairs of _maxillae_, and one pair of biting and crushing _mandibles_; and lastly, in front of the jaws, two pairs of _feelers_ (_ant._ 2 and _ant._ 1). It is believed that the first pair of feelers corresponds to the single pair of feelers of the cockroach; that the jaws correspond to the jaws; that the maxillipedes of the crayfish are the equivalents of the walking legs of the cockroach; while the remaining appendages of the crayfish have no representatives in the insect.

Lastly, the crayfish differs essentially from the insect in its method of =respiration= (p. 355), for it is an aquatic animal and breathes _dissolved_ oxygen. It therefore possesses neither lungs nor spiracles but =gills= (Fig. 230). These are situated at the sides of the true body-wall, in gill-chambers formed by the downgrowth of the sides of the carapace. The gills are delicate plumes, containing fine blood-vessels, so that an exchange of gases readily takes place between the blood and the surrounding water. On each side, a scoop (Fig. 230, _ep._ 5) on the second maxilla is continually baling water out of the front of the gill-chamber, fresh water flowing in from behind to take its place. Certain of the gills are attached to the legs, so that the motion of the legs also is some assistance to respiration.

=The crab= (Fig. 231) is markedly different in shape from the crayfish or lobster, but is nevertheless easily seen to be built on essentially the same lines of structure. It also consists of twenty segments, of which the first thirteen are fused to form a cephalothorax; and the appendages of this region are quite comparable with those of the crayfish. The great width of the shell is largely due to the gill-covers, which stand out from the sides of the body much further than do those of the crayfish. As a result the crab finds it easy to run side-first. The crab is essentially a walking, not a swimming, animal, and the abdomen—upon which the crayfish and lobster so much depend in swimming—is in the crab reduced in size and kept tucked under the cephalothorax.

=Crustaceans.=—These facts are sufficient to show that the title “insect” cannot with any propriety be given to either the crayfish or the crab, unless, indeed, the term is to be applied to all arthropods indiscriminately. The crayfish, lobsters, shrimps, prawns, crabs, barnacles, and many less-familiar animals, are placed in the =crustacean= class of arthropods. Crustaceans have usually a distinct head, thorax, and abdomen; but some of the thoracic segments may be fused with the head to form a cephalothorax (p. 375). Like other arthropods, the animals are covered with an armour of chitin, and in many cases this is so hardened, except at the joints, by mineral matter that it becomes a rigid shell or crust. The head bears two pairs of feelers in addition to the jaws; and the segments of the thorax and abdomen are provided with appendages which are variously modified as jaw-feet, legs, swimmerets, etc. Typically, the animals breathe by gills and are aquatic, but forms are known which are able to live comfortably on land if the gills are kept moist. One of the most interesting examples of this is found in the common =wood-louse= (Fig. 232), which lurks under stones and logs in damp and dark situations, and breathes by plate-like gills on the abdominal segments.

=Other arthropods.=—Other familiar arthropods, which are neither insects nor crustaceans, are spiders and centipedes. The spiders belong to the =arachnid class=, and the centipedes to the =myriapod class= of arthropods.

66. SOME COMMON MOLLUSCS.[37]

1. =The fresh-water mussel.=—(_a_) _Habits._—Study the habits of the living animal (Fig. 233) in the aquarium. Notice the muscular _foot_ which is protruded between the halves (_valves_) of the shell, and by means of which the mussel slowly makes its way along the sandy bottom. With a pipette, discharge some water, coloured with carmine or indigo, close to the more pointed end of the shell. Where is the coloured water taken in, and where is it expelled? Notice that the shell closes when the animal is handled.

(_b_) _The regions of the shell._—The rounded end is anterior, the more pointed end posterior, the straight hinge-line dorsal, and the gape ventral. Notice the knob-like _umbo_ on each side near the anterior end of the hinge; it is the oldest part of the valve. Make out the concentric lines of growth, showing the successive positions of the margin. Draw the shell from the right side and from above.

(_c_) _General structure._—Kill the mussel by putting it for a few minutes into hot, but not boiling, water. Notice that the valves of the shell now gape apart somewhat. Hold up the animal and see, lining the shell, a thin soft membrane, the _mantle_. Carefully separate the mantle from one valve and notice, near each end, a thick white pillar which passes from one valve to the other. These pillars are the _closing muscles_. Pass the blade of a knife between the mantle and valve, and cut through each closing muscle quite near to the valve. Turn the valve back and remove it from the other valve by cutting through the elastic _ligament_ (at the hinge) with strong scissors. Clean the inside of the separated valve and examine it; notice (i) the line of attachment of the mantle; (ii) the impressions of the closing muscles; (iii) the lines of shifting of the closing muscles—triangular depressions stretching from the umbo to the muscles.

Examine the animal as it lies in the other valve. Make out: (i) The right and left lobes of the _mantle_ which line the valves and (in the natural position of the mussel) hang down from the sides of the body; (ii) the two plate-like _gills_ on each side, lying between the mantle and the foot; (iii) the median _foot_; (iv) the reddish, triangular _palps_ surrounding the _mouth_—an aperture at the anterior end of the foot. Push a stout pin into the mouth and upwards into the gullet. Make a drawing showing the relative positions of the parts.

2. =The garden snail.=—(_a_) _Habits._—Where have you seen snails? At what time of the year are they most active? Upon what do they feed? Place a snail upon a lettuce leaf and carefully watch its method of eating. How does it move about? Put a live snail upon a sheet of glass, and look through the glass to see the wave-like action of the flat sole—the _foot_—by means of which it moves.

Have you ever found snails in winter? How much of the animal was visible? How is the mouth of the shell closed in winter?

(_b_) _General appearance._—Make a drawing of the animal from the left side and another from the right side, and notice the differences between the two. Is the shell placed over the middle of the animal, or does it lie to one side? How many turns has the spiral of the shell? In a fully expanded snail observe the fleshy “collar” round the margin of the shell; it is the edge of the _mantle_. Notice the rounded _head_, the two pairs of _tentacles_, the _eye-spot_ at the tip of each of the larger and upper tentacles, the _mouth_, and, near the base of the shell on the right side, the _respiratory pore_, which opens into the _lung_. Touch various parts of the body in turn to see if they are irritable; how are the tentacles retracted? Can the animal close and open its respiratory pore at will?

3. =An air-breathing pond-snail.=—Obtain several fresh-water snails and study their habits in glass aquaria. The commonest fresh-water snails are species of _Limnaea_; identify some of these by comparison with Fig. 234, _A_, and try to make out the parts already seen in the garden snail. Watch the action of the mouth as the animal feeds on the green scum which often collects on the sides of aquaria. Notice how it creeps over the glass or along the surface-film of the water. When the snail comes to the surface it replaces the air of its lung by fresh air, and a bubble may often be seen escaping from the respiratory opening under the lip of the shell. Look for the spawn (_egg-masses_) of this snail on leaves or on the sides of the aquarium, and examine the eggs frequently with a hand lens.

=Molluscs.=—Molluscs are soft-bodied animals, in most cases protected by a hard, external shell, but they differ essentially from crustaceans and all other arthropods in not being segmented, and in not possessing jointed limbs. Familiar and instructive examples of two classes of the group are found in the fresh-water mussel and the garden snail.

The =fresh-water mussel= (Fig. 233) is to be found in streams, along the bed of which it ploughs its way by means of a muscular =foot= (_ft._). The body is enclosed in a brown shell, which consists of halves called =valves=, hinged together along the straight, dorsal edge by an elastic =ligament= (_lg._). The action of the ligament is to separate the valves slightly unless they are forcibly held together by the contraction of closing muscles which run from one valve to the other. Hence the shell of a dead mussel always gapes open. The rounded end of the shell is anterior, the more pointed end posterior. The oldest part of each valve is the =umbo= (_um._, Fig. 233), a knob just in front of the ligament; and concentric lines surrounding the umbo mark successive positions of the margin of the valve as the animal increased in size. The valves are formed by the activity of the =mantle lobes= (_m._)—a pair of delicate membranes which hang down from the sides of the body. The foot is a median prolongation of the body itself, and on each side a pair of plate-like =gills= lies between the foot and the mantle lobe of that side. “Thus the whole animal has been compared to a book, the back being represented by the hinge, the covers by the valves, the fly-leaves by the mantle lobes, the first two and the last two pages by the gills, and the remainder of the leaves by the foot.”[38]

When the living mussel is undisturbed, the mantle folds project slightly at the hinder end of the shell, their edges being so placed in contact that they form two short tubes. A current of water flows in at the lower of these (_in. sph._, Fig. 233), carrying to the mouth a supply of food-particles, and to the gills and mantle a store of dissolved oxygen; while an outgoing current leaves by the upper tube (_ex. sph._).

The =garden snail= (Fig. 234, _B_) seems at first sight to have but little resemblance to a mussel; but it also is a mollusc—consisting of a soft, unsegmented body, which is produced ventrally into a =foot=, and is protected by a =shell= formed by the activity of a =mantle fold= of the body. In this case, however, the shell is one piece, and is spirally coiled. The snail has also a distinct =head=, which bears two pairs of tentacles; at the tip of each of the longer and upper tentacles (_t_) is an eye (_e_). The animal crawls about by wave-like contractions of the muscular foot; it feeds upon vegetation, which it rasps into small particles by means of a toothed tongue, and then swallows. The snail is entirely adapted to a terrestrial life, and breathes air—the mantle fold under the shell enclosing a =lung chamber= with blood-vessels in its walls, which opens to the exterior by a respiratory pore (_p.o._, Fig. 234) on the right side. The snail spends the winter, in a state of torpor, under logs or stones, the body being entirely retracted into the shell, the mouth of which is closed by a plate of hardened slime.

=Slugs= (Fig. 235) are of very similar structure, but in them the shell has almost disappeared, even the trace which remains being concealed by the mantle fold.

Among the commonest of fresh-water snails are various species of =Limnaea= (Fig. 234, _A_). They may be found abundantly in ponds, and are often kept in aquaria, where they perform a useful service by devouring the minute plants which are apt to accumulate to an undesirable extent and form green scum on the sides. These snails, like the garden snail, breathe air, and often come to the surface to take a fresh supply into their lung chambers. Some other water snails, however, breathe _dissolved_ oxygen by means of gills beneath the shell.

67. EARTHWORMS.

1. =External characters.=—Dig up several earthworms from the soil and examine them. What is the _length_ of the largest and of the smallest specimens? What is the _thickness_ of the body? Watch the worms crawling about, and describe the method of locomotion. Has the animal any legs? Do the length and thickness of any one worm vary during its movements? Is the variation connected with locomotion? How? Can you distinguish between the fore (anterior) and hind (posterior) ends, and between the upper (dorsal) and lower (ventral) surfaces? How do they differ?

Kill a large worm by immersion in methylated spirit and examine it more closely. Notice the segmented character of the body, and estimate the number of segments. Which part of the body has the largest segments? Observe the swollen appearance of segments 32 to 37; this region is called the _clitellum_. Notice the _mouth_ (overhung by a short lobe) in the first segment, and the _vent_ in the last segment. On the ventral surface of segment 14 try to see a pair of small pores; these pores are the openings through which the _eggs_ are discharged from the body. Pull the worm gently between your fingers, and notice the bristly feel; in which direction of motion is this most apparent? Examine the ventral surface with a strong lens in a good light, and try to see four double rows of very small _bristles_.

2. =Habits.=—Examine the surface of the ground of a garden or lawn at night by help of a lantern or lamp, being careful to tread lightly, and observe the actions of any worms you see. Are more worms to be seen at night than during the day? Do the worms seem disturbed by the light of the lantern? Try to grasp one; is it easily caught? Why not? Where does the worm retreat? If you can grasp a worm before it has time to withdraw completely into its burrow, observe the difficulty of drawing it out without tearing it. Try if the worms are disturbed by a loud shout (be careful not to blow upon them when shouting), or by a heavy stamp of your foot.

Carefully lay open several _burrows_ and notice:

(_a_) Whether the mouths of the burrows are plugged in any way, and, if leaves are used for this purpose, whether the leaves have been dragged into the hole (i) by the broad ends, (ii) by the narrow ends, or (iii) by the sides; do you find any signs of intelligence in the method adopted?

(_b_) The length and width of the burrow and the character of its lining.

(_c_) The end of the burrow; is it enlarged?

Examine _worm castings_. Mark out a square yard of surface and collect, dry, and weigh all the castings found on this area in a certain time, say a month. Such observations should be made at different periods of the year, and over different kinds of soil, and comparisons made. Estimate the weight brought up per acre by worms during a year. Are the particles composing the castings very fine, fine, or coarse?

Place worms in glass-covered flower-pots with earth of different degrees of firmness, and observe the methods of burrowing in loose and in firmly compacted earth respectively. Place pieces of leaf of carrot, onion, and cabbage on the surface, and at night observe how the worms grasp the pieces and drag them into the burrows.

=Earthworms.=—Few people except naturalists have any idea of the vast number of earthworms living in the surface soil of this and most other countries, or of the importance of the work which they do.

The common earthworm (Fig. 236) is much simpler in structure than any of the animals previously considered in this book. It is roughly cylindrical in shape, though somewhat flattened on the ventral surface. It is divided into about 150 segments, which are marked on the exterior by grooves running round the body. The mouth is an opening in the first segment (Fig. 236, 1) and is overhung by a short fleshy lobe. The worm crawls along by alternate elongation and shortening of its body, being aided by short bristles which are directed backwards and act as pivots; limbs are entirely absent. The animal is very sensitive to touch, even to the vibrations of the ground; but it is stone-deaf, only just capable of distinguishing between light and darkness, and has very little sense of smell.

During the day, the earthworm generally remains in its burrow in the soil, with its head just inside the entrance. Its method of forming the burrow depends upon the texture of the ground. In loose soil the earth is simply pushed aside, but where the material is too compact for this, the animal actually eats its way through. The burrow is lined with soft earth or little stones, and is plugged at the mouth with leaves or other convenient objects. The animal was found by Darwin to display distinct intelligence in its manner of drawing leaves into the mouth of its burrow, seizing them in most cases by their narrow ends, so that they could be pulled in with as little difficulty as possible. Objects are generally grasped between the lobe, which overhangs the mouth, and the lower part of the first segment, the hold being maintained by a sucking action. The inner end of the burrow is enlarged to allow the worm room to turn round. At night, the fore part of the body is protruded in search of food, the tail being generally retained in the burrow, ready for instantaneous retreat in case of alarm.

The earthworm feeds upon leaves—which are first softened by a fluid discharged over them, and then sucked into pieces small enough to be swallowed, for the animal has no jaws—and upon the half-decayed organic matter which is always present in ordinary soil. The soil itself is swallowed in large quantities; the nutritious portion is extracted, and the undigested matter deposited upon the surface of the ground, near the mouth of the burrow, in the form of =castings=. As a result of numerous experiments, Darwin estimated the weight of castings thus thrown up by earthworms on an acre of land as 15 tons annually. The following passage is worthy of very careful attention: “When we behold a wide, turf-covered expanse, we should remember that its smoothness, on which so much of its beauty depends, is mainly due to all the inequalities having been slowly levelled by worms. It is a marvellous reflection that the whole of the superficial mould over any such expanse has passed, and will again pass, every few years through the bodies of worms. The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures.”[39]

The earthworm lays its eggs in a small cocoon formed by the hardening of a viscid material which is discharged by a swollen part of the body called the clitellum, extending from the 32nd to the 37th segments. After the formation of the cocoon the worm moves backwards, and the eggs leave the body by small pores on the ventral surface of segment 14, as this region passes the cocoon. A small amount of food-material is also enclosed in the cocoon, and forms a store of nutriment for the young worms during their early development.

EXERCISES ON CHAPTER XX.

1. Compare the legs of a cockroach with those of a crayfish and a vertebrate. (1900)

2. Describe the respiratory organs of the crayfish. How are they continually supplied with fresh water? (1898)

3. Examine a spider. How many legs has it? Of what divisions does its body consist? Why do you consider that a spider is not an insect?

4. Make observations, and write descriptions, of the habits of spiders, paying special attention to the methods of construction of the webs, the manner of catching prey in different cases, and the care of the young by the parents.

5. Compare a centipede with an insect, pointing out the features of resemblance and difference. (1897)

6. How does a pond-mussel open and close its shell? (1900)

7. Compare an oyster with a fresh-water mussel, and try to find points of resemblance and difference, making careful notes and sketches of these. How many closing muscles has the oyster? Are its gills plate-like? Why do you consider the oyster a mollusc?

8. Where is the lung of a snail situated? How do we know that it is really a lung? (1897)

9. How does an earthworm resemble and differ from a caterpillar? (1900)

10. Explain the action of an earthworm in the formation of vegetable mould. (1897)

11. Classify the animal described below:

A land animal with a long narrow soft body and no legs; two pairs of tentacles on the head; a breathing hole on one side? (1904)

12. Give the characters by which insects can be distinguished from crustaceans. (1905)

FOOTNOTES:

[35] Living specimens may be obtained from Mr. T. Bolton, 25 Balsall Heath Road, Birmingham.

[36] Greek, _kephalon_, a head.

[37] See footnote, p. 372.

[38] Parker and Haswell’s _Text-book of Zoology_ (Macmillan).

[39] Darwin’s _Vegetable Mould and Earthworms_ (Murray).