DEVELOPMENT OF ANNELIDA. 301 greenish tint; and in many instances it is colourless, as in the Annelida in general. 836. Little is known of the history of the development of the Annelida; but there appears to be considerable variation in the amount of change they undergo after coming forth from the egg. The Nereidans, in their early stage, seem to bear a very close resemblance to Infusory Animalcules; for they are described as consisting of a simple disc, surrounded with two rows of vibratile cilia, which serve for locomotion, and for the acquirement of food. The mouth, which is situated near the edge of the disc, leads to a short, straight alimentary tube, which terminates in a sort of cone projecting from its lower side. In a short time this conical projection begins to increase in length, and to show a division into segments; and by a gradual increase in the number of these, the body is at last formed. At the same time, the formation of the head takes place by a gradually-increasing projection of the other surface of the disc. Whilst the head and body are being developed, this first-formed disc remains for a time as a sort of appendage on each side of the head; but it finally disappears. In the Leech and Earth-Worm, on the other hand, the development of the young seems to be nearly complete, by the time that they leave the egg. We find in this group the first appearance of that gemmiparous mode of reproduction, which is especially characteristic of Zoophytes (ANIM. PHYSIOL. §§ 723-730). The accompanying figure represents the mode of propagation of the FIG. 524.-NEREIS PROLIFERA. Nereis prolifera; in which a young one is formed from the hinder part of the body of the parent, its head being produced by a conversion of one of the segments of its body; and this is gra 302 PROPAGATION AND CLASSIFICATION OF ANNELIDA. dually detached by a narrowing of the preceding joint. Previously to its separation, however, the young one often forms another bud from its own hinder part, in a similar manner; and even three generations have been seen thus united.-From observations recently made on another Marine Worm, in which a similar occurrence takes place, it would appear that the buds thus detached are, like the flowers of Plants, destined to Reproduction only; for whilst the parent continues to grow, to obtain its food, and to develope new buds from its hinder portion, these buds do not seem to grow into new individuals like their parents, -but, although possessing a head, mouth, and alimentary canal, they do not take in nourishment, their functions being restricted to the laying of eggs, from which new Nereids spring. Something analogous to this is met with among the Entozoa; the posterior segments, containing the ova, being detached from the rest of the body, and thrown off altogether. 837. This Class is subdivided into Orders, according to the differences in general conformation and habits exhibited by the tribes which compose it; and especially by the character and distribution of the respiratory organs. The first Order, DORSIBRANCHIATA, includes those which have the branchial appendages or gill-tufts disposed regularly along the body, sometimes extending along its entire length, and sometimes restricted to the segments about the middle. This Order has been also denominated Errantia, from the active habits of the animals composing it.-II. In the next, TUBICOLA, we find worm-like animals inhabiting fixed and permanent residences, like the testaceous Mollusca. The disposition of the gill-tufts around the head, and the absence of them along the body, is the principal character which distinguishes the animals themselves from those of the first Order. III. In the third Order, TERRICOLA, the body is destitute of all external appendages, except some minute and almost imperceptible bristles; for the respiratory organs are here developed internally, the animal being usually formed to crawl upon the ground, instead of swimming through the water.-IV. And in the last, SUCTORIA, the body is destitute even of these bristles, but is furnished with a sucker at each extremity. ORDER DORSIBRANCHIATA ;-NEREIS, ETC. 303 ORDER I.-DORSIBRANCHIATA. 838. Of the foregoing Orders, the Dorsibranchiata appear, on the whole, to possess the most complete structure, as well as the most varied faculties; and they also exhibit the forms most characteristic of the class. The head is almost always quite distinct from the trunk, and is furnished with one or two pairs of minute eyes, which are seen as black or reddish specks upon its upper side; and it is also provided with several pairs of appendages (Fig. 525), as well as with complex instruments of mastication. These Marine Worms do not attain any considerable dimensions upon our coasts, rarely exceeding a few inches in length; but in tropical climates, species FIG. 525.-HEAD are to be met with of comparatively gigantic proportions, having their bodies composed of 400 or 500 segments, and occasionally measuring four feet from one end to the other. They generally crawl with facility, and swim rapidly; their chief habitations are among rocks and masses of shells; but some of them bury themselves in the sand, forming a sort of burrow, lined by mucus secreted from their bodies, which they quit in search of their prey. They are all carnivorous, and live on various small marine animals. AND ANTERIOR SEGMENTS OF 839. The NEREIDE, commonly known as Sea-Centipedes (Fig. 520), may be regarded as characteristic examples of this group. They are distinguished by having the gills in the form of leaf-like appendages or laminæ, which are traversed by a network of vessels; and every segment has, on either side, two tubercles, two cirrhi, and two tufts of bristles. They have none of that venomous power, which their name might be supposed to indicate; but they are extremely voracious, and thread the most intricate passages and crevices among rocks and stones, in pursuit of their prey. In their turn, they are devoured by Fishes. The Eunice is nearly allied to the Nereida; but has gills composed of filamentous tufts (Fig. 521, b), instead of leaf 304 DORSIBRANCHIATA ;-EUNICE, ARENICOLA, APHRODITA. like plates.-The Eunice gigantea of West Indian seas is the largest Annelide known; sometimes measuring four feet in length. There are smaller species upon our own coasts. In the Amphinome, the gills have the form of branching or arborescent tufts; and these are disposed along the whole of the body. They are frequently coloured very brilliantly.-The allied genus Euphrosine has these branchial tufts very much developed; and : gills, like those of the preceding genera, have an arborescent form; but they are disposed on the middle part only of the body. As its name imports, it lives in the sand, in which it burrows to the depth of twelve or eighteen inches its situation may be known by a little coil of sand, which is seen on the spot beneath which it is to be found. The Arenicola is one of the Annelidans most distinguished by the red colour of its circulating fluid; and this colour is peculiarly seen in the branchial tufts, which have a beautiful crimson hue during the life of the animal.-The Aphrodita is an animal well known on our coasts under the name of "Sea Mouse;" probably on account of the large quantity of silky hairs with which it is covered. These hairs are of a very brilliant metallic lustre; and their colours vary with the play of the light; so that this animal is scarcely surpassed by any, in beauty of colouring. The back is furnished with two rows of large membranous scales, which somewhat resemble the elytra of Insects; these inclose the gills; but they are themselves concealed by the hairy covering just mentioned. The form of the body much resembles that of the Euphrosine (Fig. 526). A large number of Aphroditæ are not unfrequently thrown up on our coasts after a gale of wind. In many species, APHRODITA, PERIPATUS.-ORDER TUBICOLE. 305 the lateral setæ or bristles exhibit a beautiful structure, which admirably fit them for weapons of defence, being barbed on each side at their tips; and each of these barbed setæ is inclosed in a smooth horny sheath, composed of two blades.-Lastly, we may mention a very curious genus, Peripatus, which is probably to be placed in this order, although it is a resident on land. In FIG. 527. PERIPATUS IULIFORMIS. some respects it bears a resemblance to the Iulidæ (824), having legs adapted for walking, which present some appearance of articulation or jointing; but in the softness of its body, and in the termination of the legs in tufts of bristles, it evidently corresponds with the Nereidans. ORDER II.-TUBICOLE. 840. The animals of this order never attain to the same dimensions with the preceding; and more is known of the casings which they form, than of the structure of the animals themselves. One of the commonest of these is the shelly tube exuded by the Serpula; which is formed of calcareous matter, resembling that of the shells of Mollusca, and apparently secreted from the surface of the body in a similar manner. In fact, it is often scarcely possible to distinguish between the shell of a Serpula and that of a Vermetus (Fig. 583). The tubes of the Serpulæ (Fig.528) are generally found clustering in masses, attached to the surface of stones, shells, or other bodies, which have been immersed for any length of time in the sea; they are usually more or less contorted in form, varying in this respect according to the position in which they grow; but they are always closed at one end, which tapers to a point, the wide end being open to give exit to the head and mouth of the inhabitant. The animal which forms this shell, and resides in it, has its branchial filaments or gilltufts all assembled round the head; where they form a pair of |