110 STRUCTURE OF MYRIAPODA AND ANNELIDA. through a series of metamorphoses, in which they lose their eyes and legs, and become fixed for the remainder of their lives. 103. We now pass back to another class of the higher group of Articulata, adapted to breathe air and to inhabit the land, the MYRIAPODA or Centipede tribe (fig. 42). Both these names are derived from the great number of legs possessed by these animals, which often amount to 60 pairs or even more. In this class we see a more perfect equality of the segments or divisions of the body than in any others among the higher Articulata; and the similarity is scarcely less complete in the internal arrangement, than it is in the external form. In its lower tribes (fig. 51), the legs are so weak as scarcely to be able to sustain the body, which moves, therefore, partly in the manner of that of a worm. The animals of this class undergo no proper metamorphosis; but there is a considerable addition to the number of their segments and legs after they have come forth from the egg. 104. We now pass to the lower division of Articulata, in which the body possesses no jointed members; and the animals belonging to this group are for the most part included in the class of ANNELIDA, the Leech and Worm tribe. We here find the body enveloped,-not in a hard casing, formed of distinct pieces united by a flexible membrane, but in a skin which is altogether flexible, and which gives little indication of a division into segments. This class includes several distinct tribes, which all agree, however, in the long worm-like form of the body, and in the similarity of the different ganglia or their nervous system. The Earth-worm and its allies are adapted to live on land and to breathe air; but the greater number of Annelids are purely. aquatic; and these breathe by gills, which form tufts that are disposed on various parts of the body. In the Nereis, or Sea-centipede (fig. 52), these STRUCTURE OF ANNELIDA AND ENTOZOA. 111 tufts are arranged regularly on the several segments, and the animal can swim by the motion that it gives them; besides these, it has a kind of bristle-shaped appendage, that seems S Fig. 52.-NEREIS. like a rudimentary leg, which assists it in crawling. But there are others of these marine-worms, that form a tubular shell, in which they reside during the greatest part of their lives; and in these the gills, if disposed along the body, would have been removed from the access of water; they are therefore arranged round the head, often forming (as in the Serpulæ, fig. 145) tufts of great brilliancy and elegance. 105. Below the Annelida are other worm-like tribes of yet greater simplicity of conformation, but still presenting the same general plan of structure. Of one of these the common Leech may be taken as an example; of another, the Tape Fig. 53. TAPE-WORM. worm (fig. 53). This last belongs to a group termed ENTOZOA, from the circumstance that they inhabit the bodies of other animals. They are remarkable for the very low development of their digestive apparatus,-their nourishment being appa- 112 GENERAL STRUCTURE OF MOLLUSCA. rently imbibed through the whole surface of their bodies from the juices in the midst of which they live; whilst, on the other hand, their reproductive apparatus is enormously developed, the multiplied segments of the Tape-worm (for example) containing this alone, and the head (as it is commonly termed, though really the body) being able to reproduce these to an indefinite extent after they have been thrown off. The group of ROTIFERA, or Wheel-Animalcules, which is one of great interest to the Microscopist, also belongs to this lower section of the Articulated sub-kingdom. 106. The general character of the animals composing the group or division MOLLUSCA, is, in many respects, the very opposite of that which prevails in the Articulated animals. The body is soft (whence the name of the group is derived), neither possessing an internal skeleton, nor any proper external skeleton. In some of the most characteristic specimens of the group, such as the Slug, there is no hard frame-work or skeleton whatever, the body being alike destitute of support and protection. In most Mollusks, however, the body has the power of forming a shelly covering, which serves for its protection; but this does not give any assistance in its movements by affording fixed points for the attachment of the muscles; in fact, when the animal puts itself in motion, it is obliged to make its locomotive organs project beyond the shell. We must not regard the shell as an essential part of the Molluscous animal; because there are many tribes entirely destitute of it; and also because some of the Articulata have the power of forming a shell (§ 102), which bears a close resemblance to that produced by the animals of this group. Not un Fig. 54.-TESTACELLA. frequently we see that, of two animals whose general structure is almost exactly the same,—as that of the Snail and Slug,— STRUCTURE OF MOLLUSCA. 113 one possesses a shell into which it can withdraw its whole body for the sake of protection, whilst the other has none; and several intermediate forms exist, in which the shell bears a larger or smaller proportion to the body, sometimes being able to contain nearly the whole of it, and sometimes being a mere rudiment, as in the Testacella (fig. 54). 107. The external form of the body of Mollusks is subject to great variation; and generally has a good deal to do with the degree in which the organs of sense and the instruments of motion are developed in the particular animal. For these are almost always symmetrical, being arranged with equality on the two sides of a middle line; whilst the rest of the body, containing the organs of nutrition, is often unequal on the two sides. But in the lower Mollusca, which have little or no power of moving from place to place, even this degree of symmetry is altogether lost. Few of the Mollusca have any powers of active movement; in fact, the term sluggishness, derived from a characteristic member of the group, very well expresses their general habit. The Gasteropods, which may be regarded as the types of the whole series, crawl upon a fleshy disk, by the successive contractions and relaxations of which they advance slowly along the surface over which they move; this kind of action is easily studied, by causing a Snail or Slug to crawl upon a piece of glass, and by looking through this at the under side of its foot. Hence, there is a great contrast between the inertness of the Mollusca, and the high activity of the Articulata. This contrast shows itself in the structure of their bodies; for whilst the chief part of the interior of an Insect is made up of the muscles which move its legs and wings, the apparatus of nutrition being small,— the chief part of the bulk of a Slug or Snail is given by its very complex apparatus for nutrition-there being no other muscles (except some small ones connected with the mouth and head) than the fleshy disk already mentioned. The blood of the Mollusca is nearly colourless, as it is in the Articulata ; but the organ by which it is circulated through the body is much more powerful and complete, bearing more resemblance to the heart of Vertebrated animals. The skin is usually thick and spongy in its texture; having muscular fibres interwoven in its substance, so that it can contract or extend itself in any part; and having the power of exuding shelly matter Ι 114 STRUCTURE OF MOLLUSCA, from its surface, in those species which form such a protection. This envelope, which is called the mantle, is very loosely applied round the parts which it contains; and it frequently extends itself into folds or duplicatures, which wrap round the gills, and sometimes meet and adhere so as to inclose them within a cavity of their own. In the Cuttlefish, the water within this cavity is renewed from time to time by the muscular movements of its walls; but usually a current of fluid is kept up over the surface of the gills, by the action of the cilia (§ 45) with which they are covered. 108. The accompanying figure of the interior of a Turbo will show the very large size of the digestive apparatus, and of the other organs of nutrition. The muscular disk or foot is seen at p; and this carries the operculum o, which serves to close the mouth of the shell when the body of the animal is drawn within it. At t is shown the proboscis, on either side of which are the tentacula or feelers, ta, bearing the eyes at y. Just behind the tentacula is seen the large cephalic ganglion, sending nerves to the eyes; and behind this again are the salivary glands. The mantle, m, is opened and folded back to show the respiratory cavity, in which lie the gills |