Elementary Zoology Part 9

TECHNICAL NOTE.--Lift carefully to right and left the reproductive organs, thus exposing the sophagus.

Note three pairs of bag-like structures projecting from the sophagus.

The front pair is the _sophageal pouches_; the next two pairs are the _sophageal_ or _calciferous glands_. They communicate with the alimentary ca.n.a.l, and their secretion is a milky calcareous fluid.

Make a drawing that will show all the parts so far studied.

TECHNICAL NOTE.--Cut transversely through the alimentary ca.n.a.l in the region of the c.l.i.tellum and carefully dissect the anterior portion of the ca.n.a.l away from the surrounding organs.

Note the dorsal fold of the intestine, _typhlosole_, extending into the lumen. This fold gives a greater surface for digestion, and in it are a great many _hepatic_ or special _digestive cells_. The entire alimentary ca.n.a.l is lined with _epithelium_. Observe just beneath the alimentary ca.n.a.l the _ventral blood-vessel_, and still beneath this blood-vessel the _ventral nerve-cord_. There is a slight swelling on the nerve-cord in each segment of the body. These swellings are the _ganglia_. How many pairs of nerves are given off from each ganglion?

Observe in each segment, posterior to the first three or four, the successive pairs of convoluted tubes, the _nephridia_, or organs of excretion. Each nephridium opens internally through a ciliated funnel, the _nephrostome_, within the body-cavity, while it opens externally by a small excretory pore between the setae on the ventral surface of the segment behind that in which the nephridium chiefly lies. The function of the nephridia is to carry off waste matter from the fluid which fills the body-cavity.

[Ill.u.s.tration: FIG. 26.--Dissection to show alimentary ca.n.a.l in section and nephridia of earthworm.]

Trace the ventral nerve-cord forward to its connection with the cerebral ganglion. Note the throat nerve-ring or _circ.u.msophageal collar_ connecting the ventral cord with the brain.

Make a drawing of the nervous system showing its relation to other organs.

[Ill.u.s.tration: FIG. 28.--Cross-section of earthworm.]

=Life-history and habits.=--The earthworm lives in soft moist soil which is rich in organic matter. Its food is taken into the mouth mixed with dirt and sand. As this mixture pa.s.ses through the long alimentary ca.n.a.l the organic particles are taken up and digested. As we have already seen, there are in each worm two sets of reproductive glands, namely, male and female organs. Each earthworm produces both egg-cells and sperm-cells, but the sperm-cells of one worm are not used to fertilize the eggs of the individual producing them. When the eggs are ready to be discharged from the body, the c.l.i.tellum becomes very much swollen and its glands begin an active secretion which hardens and forms a collar-like structure about the body of the worm.

As this collar moves forward toward the anterior end of the body it collects the eggs and also the sperm-cells previously received from another worm, and finally slips off the head end of the animal. The entire structure with the contained eggs and sperm-cells as it pa.s.ses off from the body becomes closed at both ends, thus forming a h.o.r.n.y capsule which lies in the earth until the young worms emerge. Only a part of the eggs develop in each capsule, the rest being used as food for the growing young. The young earthworms, though of very small size, are fully formed before they leave the egg-capsule. Earthworms are more or less gregarious, large numbers often being found together.

For an interesting account of the habits of earthworms see Darwin's "The Formation of Vegetable Mold."

OTHER WORMS.

The branch Vermes comprises so large a number of kinds of animals presenting such great differences in structure and habit that it is impossible to give a brief statement in general or summary terms of their external body-characters, of the structural and functional condition of their various organs and systems of organs, and of the course of their development and life-history as has been done for the preceding branches. Many zoologists, indeed, do not include all the worms or worm-like animals in one branch, but consider them to form several distinct branches.

[Ill.u.s.tration: FIG. 29.--A group of marine worms: at the left a gephyrean, _Dendrostomum cronjhelmi_, the upper right-hand one a nereid, _Nereis_ sp., the lower right-hand one, _Polynoe brevisetosa_. (From living specimens in a tide-pool on the Bay of Monterey, California.)]

In certain very general characters all of the animals which compose the branch Vermes do agree. All, or nearly all, have an elongate body which is bilaterally symmetrical, that is, which could be cut by a median longitudinal cutting in two similar halves. In most of them also the body is composed of a number of successive segments or somites which are more or less alike. This kind of segmented or articulated body is also possessed by the insects and crabs. Almost all of the worms have the power of locomotion; usually that of crawling. For this crawling they do not have legs composed of separate segments or joints as do the higher articulated animals, the crabs and insects, but either have fleshy unjointed legs, or various kinds of bristles or spines, or suckers, or even no external organs of locomotion at all. As regards their internal structure they have well-organized systems of organs, which show great variety in character and degree of complexity. The special sense-organs are usually of simple character and low degree of functional development. Reproduction occurs both s.e.xually and as.e.xually; in some species the s.e.xes are distinct, while in others both sperm-cells and egg-cells are produced by the same individual. As.e.xual reproduction is by budding or by a kind of simple division or fission. The worms live either in salt or fresh water, or in moist, muddy or slimy places or as parasites in the bodies of other animals or in plants. While most worms feed on animal substance either living or dead, some feed on living or decaying plant matter.

=Cla.s.sification.=--There is great lack of agreement among zoologists in the matter of the cla.s.sification of the worms. Not only are the various groups which by some are called cla.s.ses held by others to be distinct branches, co-ordinate in rank with the Echinodermata, Clenterata, etc., but the limits of these groups are also constantly called in question.

It will require a great deal better knowledge of the structure and life-history of these diverse animals before the matter of their cla.s.sification is satisfactorily settled. We shall consider briefly four of the various groups (which we may consider as cla.s.ses) which include worms either specially familiar to us or of special interest or importance. One or two examples of each group (the groups being selected primarily because of the examples) will be described in some detail. By this means we may get an idea of the extremely diverse character of the animals which are included in the heterogeneous branch Vermes.

=Earthworms and leeches (Oligochaetae).=--The various species of earthworms, an example of which has been studied are found in all parts of the world; they occur in Siberia and south to the Kerguelen Islands.

They are absent from desert or arid regions, and some can live indifferently either in soil or in water. Some near allies of the earthworms are aquatic, living in fresh or brackish water, some in salt water near the sh.o.r.e. In size earthworms vary from 1 mm. (1/25 in.) to 2 metres (2-1/6 yds.) in length. All show the distinct segmentation of the body noticeable in the common earthworm already studied.

The leeches, some of which are familiar animals, are closely related to the earthworms, although at first glance the similarity in structure is not very noticeable.

TECHNICAL NOTE.--Some common water-leeches, alive or preserved in alcohol, should be examined by the cla.s.s. The animals are not unfamiliar to boys who "go in swimming" in the small streams of the country. The body of a leech should be examined carefully, and drawings of it showing the external structural characters should be made.

The body of a leech is flattened dorso-ventrally, instead of being cylindrical as in the earthworm, and tapers at both ends. In the live animal the body can be greatly elongated and narrowed or much shortened and broadened. It is composed of many segments (not as many as there are cross-lines however; each segment is transversely annulated), and bears at each end on the ventral surface a sucker, the one at the posterior end being the larger. These suckers enable the leech to cling firmly to other animals. The mouth is at the front end of the body on the ventral surface and is provided with sharp jaws.

Leeches live mostly on the blood of other animals which they suck from the body. The common leech "fastens itself upon its victim by means of its suckers, then cuts the skin, fastens its oral sucker over the wound and pumps away until it has completely gorged itself with blood, distending enormously its elastic body, when it loosens its hold and drops off." Its biting and sucking cause very little pain, and in olden days physicians used the leeches when they wanted to "bleed" a person. A common European species of leech much used for this purpose is known as the "medicinal leech." All leeches are hermaphroditic, that is, the s.e.xes are not distinct, but each individual produces both sperm-cells and egg-cells. Most of the leeches lay their eggs in small packets or coc.o.o.ns. This coc.o.o.n is dropped in soil on the banks of a pond or stream so that the young may have a moist but not too wet environment. The young issue from the eggs in four or five weeks, but they grow very slowly and it is several years before they attain their full size. Leeches are long-lived animals, some being said to live for twenty years.

=Flatworms (Platyhelminthes).=--TECHNICAL NOTE.--Collect some live fresh-water planarians (see fig. 30), which are to be found on the muddy bottom of most fresh-water ponds, and examine them while alive in watch-gla.s.ses of water. Make drawings showing the external appearance, and as much of the internal anatomy as can be seen. The branching alimentary ca.n.a.l can be seen in more or less detail, and with higher power of the microscope parts of the nervous system can be seen also. Have also a tapeworm preserved in alcohol or formalin to show the very flat and many-segmented body.

The flatworms include a large number of forms which vary much in shape and habits. They are all, however, characteristically flat; in some this condition is very marked. Some are active free-living animals, as the planarians (figs. 30 and 31), while many live as parasites in the alimentary ca.n.a.l of other animals, as do the sheep-fluke and the tapeworms.

[Ill.u.s.tration: FIG. 30.--A fresh water planarian, _Planaria_ sp. (From a living specimen.)]

The fresh-water planarians (fig. 30), which live commonly in the mud of the bottom of ponds, are small, being less than half an inch long.

They are very thin and rather broad, tapering from in front backwards.

On the upper surface near the front they have a pair of eyes; the mouth is on the under surface a little behind the middle of the body.

The alimentary ca.n.a.l is composed of three main branches, each with numerous small side branches. One main branch runs forward from the mouth, and the other two run backwards, one on each side of the body.

There is no a.n.a.l opening, and the alimentary ca.n.a.l thus forms a system of fine branches closed at the tips, and extending all through the body. The nervous system is composed of a ganglion or brain in the front end of the body from which two main branches extend back throughout its whole length. From these main longitudinal branches arise many fine lateral branches.

[Ill.u.s.tration: FIG. 31.--A marine planarian, _Leptoplana californica_.

(From a living specimen.)]

Of the parasitic flatworms the tapeworms are the best known. There are numerous species of them, all of which live in the bodies of vertebrate animals. In the adult or fully developed stage the tapeworms live in the alimentary ca.n.a.l, holding on to its inner surface by hook-like clinging organs and being nourished by the already digested food by which they are bathed. In the young or larval stage tapeworms live in other parts of the body of the host, and usually, indeed, in other hosts not of the same species as the host of the adult worm.

The common tapeworm of man, _Taenia solium_ (there are several other species of _Taenia_ which infest man, but _solium_ is the common one), may serve as an example of the group. In the adult condition its body, which is found attached to the inner wall of the intestine, is like a long narrow ribbon: it may be two or three metres long. It is attached by one end, the head, which is very small and provided with a score of fine hooks. Behind the head the thin ribbon-like body grows wider. The body is composed of many (about 850) joints called proglottids. There is no mouth or alimentary ca.n.a.l, the liquid food being simply taken in through the skin. Each proglottid produces both sperm-cells and egg-cells; one by one these proglottids or joints with their supply of fertilized eggs break off and pa.s.s from the alimentary ca.n.a.l with the excreta. If now one of these escaped proglottids or the eggs from it are eaten by a pig, the embryos issue from the eggs in the alimentary ca.n.a.l of the pig, bore through the walls of the ca.n.a.l and lodge in the muscles. Here they increase greatly in size and develop into a sort of rounded sac filled with liquid. If the flesh of the pig be eaten by a man, without its being first cooked sufficiently to kill the larval sac-like tapeworms, these young tapeworms lodge in the alimentary ca.n.a.l of the man and develop and grow into the long ribbon-like many-jointed adult stage.

The life-history of the other tapeworms which infest the various vertebrate animals is of this general type. There is almost always an alternation of hosts, the larval tapeworm living in a so-called intermediate host, and the adult in a final host. Of the domestic animals the dog is the most frequently attacked. At least ten different species of tapeworms have been found in the dog. The intermediate hosts of these dog tapeworms include rabbits, sheep, mice, etc. Some of the domestic fowl, ducks, geese and chickens, for instance, are also infested by tapeworms, and the intermediate hosts in these cases are usually insects or small aquatic crustaceans like the familiar _Cyclops_.

=Roundworms (Nemathelminthes).=--TECHNICAL NOTE.--Vinegar-eels from mouldy vinegar, and hair-worms from fresh-water pools, can usually be readily obtained. They should be examined, and drawings should be made of them, showing their shape and simple external structural character. If a specimen of trichinosed pork be obtained, the encysted stage of the _Trichina_, described in the following account, can be shown.

The roundworms are slender, smooth, cylindrical worms pointed at both ends. They are all very long in proportion to their diameter, although their actual length may be short. Some species are of microscopic size; as the _Trichina_ worm, which is about 1/20 in. long; while the guinea-worm, one of the worst parasites of man, may reach a length of six feet. Many of the roundworms are parasites living in the various organs of other animals. Some, however, lead an independent free life in water or in damp earth.

[Ill.u.s.tration: FIG. 32.--A vinegar eel, _Anguillula_ sp. (From a living specimen.)]

Familiar examples of roundworms are the so-called vinegar-eels (_Anguillula_) (fig. 32) to be found in weak vinegar, and other species of this same genus which live in water or moist ground or in the tissues of plants, doing much injury. The hair-worms (_Gordius_) or horse-hair snakes, which are believed by some people, to be horse-hairs dropped into water and turned into these animals, are also familiar examples of roundworms. They are often found abundantly in little pools after a rain, and it is sometimes said that these worms come down with the rain. They have in reality come from the bodies of insects in which they pa.s.s their young or larval stages as parasites.

The hair-worms all live as parasites during their larval stage, and as free independent animals in their adult stage. Some of them require two distinct hosts for the completion of their larval life, living for a while in the body of one, and later in the body of another. The first host is usually a kind of insect which is eaten by the second host. The eggs are deposited by the free adult female in slender strings twisted around the stems of water-plants. The young hair-worm on hatching sinks to the bottom of the pond, where it moves about hunting for a host in which to take up its abode.

[Ill.u.s.tration: FIG. 33.--_Trichina spiralis_, encysted in muscle of a pig. (From specimen.)]

The terrible _Trichina spiralis_ (fig. 33), which produces the disease called trichinosis, is another roundworm of which much is heard. This is a very small worm which in its adult condition lives in the intestine of man as well as in the pig and other mammals. The young, which are borne alive, burrow through the walls of the intestine, and are either carried by the blood, or force their way, all over the body, lodging usually in muscles. Here they form for themselves little cells or cysts in which they lie. The forming of these thousands of tiny cysts injures the muscles and causes great pain, sometimes death, to the host. Such infested muscle or flesh is said to be "trichinosed," and the flesh of a trichinosed human subject has been estimated to contain 100,000,000 encysted worms. To complete the development of the encysted and s.e.xless _Trichinae_ the infested flesh of the host must be eaten by another animal in which the worm can live, e.g. the flesh of man by a pig or rat, and that of a pig by man.

In such a case the cysts are dissolved by the digestive juices, the worms escape, develop reproductive organs and produce young, which then migrate into the muscles and induce trichinosis as before. But however badly trichinosed a piece of pork may be, thorough cooking of it will kill the encysted _Trichinae_, so that it may then be eaten with impunity. Some people, however, are accustomed to eat ham, which is simply smoked pork, without cooking it, and in such cases there is always great danger of trichinosis.

=Wheel animalcules (Rotifera).=--TECHNICAL NOTE.--Live specimens of Rotifers can be found in almost any stagnant water. Examine a drop of such water with the compound microscope, and find in it a few small, active, transparent creatures, larger than the _Paramcium_ and other Protozoa in the water and which have the appearance shown in fig. 34. They may be known by the constant whirling, or rather vibrating, circlet or wheel of cilia at the larger or head end of the body. These wheel animalcules may be studied alive by the cla.s.s.

Although usually darting about, the animalcules occasionally cease to move, when, because of their transparency, almost the whole of their anatomy can be made out. Their feeding habits can also be readily observed, and the food itself watched as it moves through the body. Make drawings showing as much of the anatomy as can be worked out. Note especially the "mastax" or gizzard-like masticating apparatus in the alimentary ca.n.a.l.

The wheel animalcules (fig. 34) or Rotifers look little like the other worms we have studied. But they are nevertheless more nearly related to the worms than to any other branch of animals. They are all small, about 1/3 mm. long, and have a compact body. They are aquatic and feed on smaller animals and plants or on bits of organic matter which they capture by means of the currents produced by the vibrating cilia of the "wheel." Small as they are they have a complex body-structure, with well-organized systems of organs. For a long time, however, they were cla.s.sed by naturalists with the Protozoa on account of their size. They are found all over the world, mostly in fresh water; a few are marine. More than 700 species of them are known.

[Ill.u.s.tration: FIG. 34.--A wheel animalcule, _Rotifer_ sp. (From living specimen, Stanford University.)]

An interesting thing about the Rotifers is their remarkable power to withstand drying-up. When the water in a pond or ditch evaporates some of the Rotifers do not die, but simply dry up and lie in the dust, shrivelled and apparently lifeless, yet really in a state of suspended animation. On being put into water they will gradually fill out to their full size and shape, and finally resume all their normal activities. In this dried-up condition Rotifers may persist for a long time, several years even, although otherwise their natural life is short, being probably of not over two weeks' duration. Certain other of the lower animals have this same power of withstanding desiccation.

FOOTNOTE:

[8] The author recognizes the untenability of the group Vermes as a group co-ordinate with the other branches of the animal kingdom, and that "Vermes" has been discarded in modern text-books. But because of the very scant consideration which can be given the various kinds of worm-like animals the course of the older text-books will be followed, and all of the worm-like animals, as far as referred to in this book, be considered under the group name Vermes.

CHAPTER XX