Part 9 (1/2)
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.