Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/79

Rh blood from behind forwards, and a ventral or sub-intestinal conveying the fluid in the opposite direction. The former is broken up into a plexus of vessels in the anterior seg ments, and generally gives off a large branch (perivis- ceral) on each side in the others. The periviscerals in certain segments behind the anterior plexus are enlarged, and so evidently contractile as to have received the name of " hearts," and propel the blood into the ventral vessel. In the other segments the periviscerals pass to the latter, either with or without division into branches in the form of cutaneous plexuses, which are greatly developed in this group. Branches pass from the ventral to the intestinal wall, inosculating with others from the dorsal, and forming a rich capillary network. In the Lumbrici there is also a third longitudinal vessel underneath the nerve-cord, and it is in communication with both dorsal and ventral, especi ally by the cutaneous system. The blood is generally reddish, devoid of corpuscles, and coagulable. Respiration seems to be effected by the cutaneous and special plexuses, aided, probably, in some by the currents of water in the ciliated digestive canal. The digestive system is in the form of a simple straight, more or less muscular tube, differentiated into oesophagus, crop, gizzard, and sacculated intestine in Lurtibricus, and coated internally with the usual glandular elements (certain glands anteriorly being supposed to be salivary), and lined internally with cilia. In the same group a peculiar closed fold of the chamber constitutes the typhlosolis of authors. The anus is situated at the posterior end of the body. In the Limicolous group the structure is less complex pharynx, oesophagus, and intestine only being present. The nervous system consists of a pair of cephalic ganglia, from each of which a trunk passes by the side of the oesophagus to join the ventral chain, which gives off branches in each segment to the surrounding parts. In Lumbricus another small gangliated chain lies over the commencement of the ali mentary tract. The nervous system agrees in intimate structure with that of the Polychseta. Besides touch, and in some sight, the special senses are not much developed. Several have epidermal papillae connected with cutaneous nerves, which probably aid tactile sensibility. The Oligochata are hermaphrodite, the sexual elements being developed in certain anterior segments in the form of testicles, deferent canals, receptacles, copulatory papilla) in some males, and ovaries and accessories in the female. In each segment, with a few exceptions (while there are two in each in Lumbricus), the segmental organs occur. Claparede thought that in the Limicola the latter are absent from the segments bearing the oviducts and seminal receptacles, but present in those having male organs ; while in Lumbricus they are present in all the segments except the four last. In some, special glands exist for secreting the egg-capsules. A cincture or clitellum occurs in certain segments anteriorly (in Lumbricus from the eighteenth to the twenty-ninth or thirtieth, and in the others from the tenth to the fifteenth segment). The ova are deposited in chitinous capsules (containing one or more), and the young issue therefrom in a tolerably complete con dition. Besides the ordinary development by ova, it has long been known that Nais and Chcztogaster exhibit fissi- parous reproduction. In Nais, after a certain degree of growth has been reached, budding takes place, so that several tolerably complete young forms may be found at tached to the adult. The process goes on until the JTais has been reduced to twelve or fourteen rings ; then a pause occurs, and the animal increases in length to forty or fifty rings, when a new cycle commences by division in the middle of the body Chcetogaster shows similar features, the budding taking place between the third and fourth segments in the androgynous form many zoids or buds being attached in line ; but while the second in position is one of the newest, the second in age is near the middle of the series. Reproduction of lost parts readily takes place in this group. Parasites, such as Gregarinee, Opalince, Nema- toids, and larval cestodes, are met with internally, and Vorticellce are common externally. Commensalism is seen in Chcetogaster, which lives on the pond-snails (Limnosus and Planorbis), and Stylaria is stated also to be ectoparasitic. They are distributed over the land and fresh waters (in sand and mud) of the whole world : very few are marine. Two of the most important classifications are those of D Udekem and Claparede. The former arranges the sub orders according to the gemmiparous, or non-gemmiparous condition of the constituents. The latter group live in earth or mud, are unable to swim or follow their prey, and have always their organs of generation. The former are elegant little worms, living in stagnant or running water, able to swim, and their generative organs are developed only at certain periods. Claparede divides them into two families, viz., the 0. Terricola (including the earthworm and its allies), and the 0. Limicola, an arrangement much resembling Grube s Lumbncina and Naidea.

III. The are represented by a single aberrant family, Peripatus. They are hermaphrodite forms (with claws instead of bristles) approaching the Arthropods, and inhabiting land and moist places in Chili, the West Indies, and other parts. The ventral nerve-cords remain separate.

IV. The (leeches) are ringed forms (fig. 17), without lateral appendages (except in Branchellion), but possessing an anterior and posterior sucker. There is no specialised respiratory system. They are generally hermaphrodite, and often ectoparasitic. FIG. 17. llintdo meilicinalis, L. Externally there is a delicate porous cuticle ; beneath lies a thick glandular cutis, which pours out an abundant secretion of mucus, as in the Nemerteans. The mus cular system consists of an external circular coat of considerable strength, from which various vertical or decussating bands pass at intervals, powerful longitudinal bundles being held in the intermediate spaces, especi ally laterally. There are also special muscles connected with the pharynx and other parts. Each end of the body is generally formed into a flattened muscular sucker, consisting apparently of several amalgamated segments. By aid of the latter organs they move from point to point, and they can also progress rapidly by swimming like the Xemerteans. The perivisceral space is quite absent. The circulatory system shows a main dorsal, a ventral, and two lateral trunks, all com municating by branches in each ring. In Branchiobdella the blood is corpusculated. The blood is probably aerated on the cutaneous, and perhaps, in some cases, on the digestive surface. The digestive system consists of a mouth opening into the anterior sucker, a muscular pharynx, oesophagus, a large stomach with various caeca! diverticula, and an anus which opens in front of the posterior sucker, though sometimes into it. In some (e.g. the common leech), the mouth is furnished with three horny serrated teeth. The nervous system is composed of two cephalic ganglia, which supply branches to the eyes when present, and the usual gangliated ventral cord giving off branches to the surrounding parts. The gullet passes through the connecting trunks as in the former groups. The last ganglion in the ventral cord of Clepsine is larger than the others; and, as in the Polychaeta. the ganglia do