Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/720

Rh 67 8 WORM areas. 1 As might be imagined from their soft perish able bodies, nothing is known respecting the distribution of the Oligocli&ta in past time. The most prominent characteristic of the Oligoch&ta, as of the Chsetopoda generally, is the segmentation of the body : the body is divided into a series of segments or metameres, which resemble each other most closely in the lowest forms. This metamerism is seen externally in the presence of transverse furrows, corresponding with the internal divisions of the body cavity, and in the dis position of the setae. The mouth opens into the first segment, which is usually unprovided with sete ; in front of the mouth is a preoral lobe ; this latter is aborted in some Oligochasta (Urocliseta, TJiamnodrilus). Body- Body- Wall.Ii all OUgochttta three layers can be distinguished wall. in the body-wall (1) an outer epidermis, which secretes a delicate cuticle, (2) a circular muscle layer, (3) a longitudinal muscle layer. Within the last-named is the peritoneal lining of the ccelom. Clitellum. During sexual maturity certain of the segments of the body in Lumbricus and other earthworms undergo a change in appearance which is caused by the development of several layers of unicellular glands beneath the epidermis. Among the Limicolse &quot; (e.g.,uLimnodrilus, Rhynchdmis, Enchytrseidie) the clitellum is, on the contrary, furnished with an epidermal layer only one cell thick ; some of these cells become large and glandular. In these points Criodrilus agrees with earthworms. The secretion of these glands may form the cocoon in which the eggs are deposited, but it appears to be also used to attach individuals togetherduring copu lation. The clitellum is universal among the Oligochazta. Monili- gaster and Criodrilus were for some time considered as an exception to this rule, but a clitellum has recently been demonstrated in these two genera by Bourne (i;) 2 and Benham (7). The clitellum in earthworms never occupies less than two segments, and in Trigaster it extends over twenty-seven. Sette. These are universally found in the Oligochseta, but differ in their shape, as well as in their number and arrangement, in different families. In the majority of forms the setae are disposed in four longitudinal rows ; the seta; in each of these rows may be comparatively numerous (Naidomorpha), or may be limited to two FIG. 2. Setse of OHgochxta. a, penial seta of Periclixla ccylonica ; 6, extremity of penial seta of Acanthodrilus (after Ilorst) ; c, seta of Urochxta (Perier) ; d, seta of Lumbricus e, seta of Criodrilus ; /, g, setos of Bohemilla comata ; h, i,j, setse of Psammoryctes barbatus (/to./ after Vejdovsky). Lunibriculidgs and many earthworms). In Acanthodrilus multi- poms and other earthworms the eight setae are no longer in pairs, but separated by nearly equivalent intervals. In Urochieta (fig. 1, D) and Diachxta each segment has also eight setae, but these are dis posed more or less alternately in successive segments. In the former of these two genera, as also in Eudrilus, peculiar structures of a chitinous nature exist between the individual setae of 1 The genus Acanthodrilus is almost entirely Antarctic in its range. It occurs in Patagonia, S. Georgia, Kerguelen s Land, New Zealand, Cape of Good Hope and other parts of Africa, Madagascar, and New Caledonia. Perichseta is characteristic of the Old and New World tropics, particularly of the former ; in the Old World it ranges from India to China and Japan, and through the Indian Islands to Australia and New Zealand. Australia has the peculiar genera Megascolidcs, Notoscolex, and Crijptodrilus, but the greatest number of peculiar genera are found in the Neotropical region. In Europe the most characteristic genera are Lumbricus and Allolobophora; these are found in most other parts of the world, but it is possible that they have been accidentally imported. 2 These figures refer to the &quot;Literature,&quot; p. 684. o ment ; these are similar to certain structures described in Anachteta by Vejdovsky (15) as abortive sete ; their presence in Urochxta and Eudrilus may indicate that the number of setae in hese worms has been reduced from a continuous circle round each .egment, such as exists in the family Perichsstidas (fig. 1, C). Imong the Naidomorpha there are delicate hair-like setae which iass by numerous intermediate forms into setae with a bifurcate xtremity ; in many limicolous forms, as in earthworms, the setae xre simple in form, ending in a slightly curved extremity. The ete are developed in the interior of cells of ectodermic origin ; pecial muscles effect their movements. Ccelom. The Oligochasta, like other Annelids, have a ccelom Ccelom. vhich is formed by the excavation of paired mesoblastic somites ; .omites ; the dorsal and ventral mesenteries, which in the Archi- mnelida suspend the gut from the body-wall, are largely absent n the Oligochseta, the cavities of each pair of somites becoming
 * he intersegmental septa represent the walls of each two adjacent
 * ontinuous. Traces of the dorsal mesentery are met with in some

brms ; in almost all the ventral mesentery persists to a great extent 11 a sheet which suspends the ventral blood-vessel from the intestine. The ccelom communicates with the exterior by the nephridia nd ducts of the reproductive organs and by certain dorsally-placed lores. These latter are sometimes present only on the head seg- nent (Criodrilus and many aquatic genera), sometimes on the body segments also (Enchytrssida) ; .in the majority of earthworms they ippear to exist only on the body segments, and the first one does lot usually appear before the third and fourth segment. In Pon- odrilus and a few other species the dorsal pores are entirely absent. The ccelom is lined with a peritoneum, the cells of which exhibit different characters in different parts of the body. The ntestine is covered with a layer of large cells containing numerous granules ; this cellular investment was originally described as lepatic, but it is now known to have no relation to the alimentary tract. The investigations of Kiikenthal (16) show that these cells are concerned with the excretory function. Nervous System. This consists of (1) a pair of cerebral ganglia Nervous onnected by a circumoesophageal ring with a chain of ventral system, ganglia arranged in pairs a pair to each segment ; (2) a system of small ganglia and nerves arising from the cerebral ganglia and innervating the anterior part of the alimentary tract ; and (3) two lateral ganglionated cords, which have a special interest for those who believe that the segmented worms are the Invertebrate group from which the Chordata (including the Vertebrata] have sprung. The discovery of Eisig (&quot;Die Capitelliden,&quot; Naples Monographs), that this lateral cord is on both sides connected with segmentally arranged sense-organs in the Capitcllidte, is an additional argument for considering this lateral system as the homologue of the lateral line in fishes. The nervous system of ^Holosoma is probably degenerate ; it con sists merely of a pair of cerebral ganglia, which are situated in the procephalic lobe in connexion with the epidermis. In Ctcnodrilus the nervous system, like that of the Archiannelida (see below), is imbedded in the epidermis throughout its whole extent. In the higher forms, in fact in all the remaining Oligochasta, the central nervous system has lost its primitive connexion with the epidermis. Moreover, in these forms the cerebral ganglia, originally developed in the procephalic lobe, have moved back, and may lie as far back as in the fourth segment. Vascular System. All the Olicjoch-azta possess, in addition to the Vascular corpusculated fluid of the ccelom, a system of closed vessels which system. in the higher forms attains to a very highly developed condition. This vascular system contains in nearly all the Oligochasta, as in the Polychseta and Hirudinea, a red-coloured fluid, which has been proved to owe its coloration to haemoglobin, and in which arc suspended corpuscles. ^Eolosoma has a colourless pseudhaemal fluid. In the lower forms the walls of the blood-vessels are exces sively delicate, and contain no muscles ; in the higher forms (e.g., Lumbricus} the blood-vessels are furnished with muscular tissue as well as with an epithelial lining. The cells of the latter give rise to the corpuscles of the blood, which consist of little more than the nucleus. The simplest form of vascular system occurs in JEolosoma and Ctenodrilus. The alimentary tract is surrounded with a network of blood capillaries, which in the cesophageal region unite to form a dorsal vessel ; this passes along the oesophagus, but is situated between the walls of the intestine and its covering of peritoneal cells ; beneath the cerebral ganglia the dorsal vessel gives oft a branch on either side ; these unite with a ventral vessel, which passes beneath the intestine, and gives off branches to it, which are regularly arranged in pairs. In the Enchytrasidie the dorsal vessel is also restricted to the anterior segments of the body, and originates from a blood sinus in the walls of the alimentary tract. The dorsal vessel gives off anteriorly two branches which unite to form the ventral vessel ; three pairs of slender vessels, a pair to each segment, originate from the dorsal vessel, and are connected with these two branches. In all the higher Oligochxta there is reticulum of blood capillaries developed in the walls of the alimen-