Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/721

Rh 679 tary canal, but the dorsal vessel, although connected with this network, does not originate from it, but passes from end to end of tho body. The dorsal vessel is also connected with the ventral vessel by paired trunks, which are segmentally disposed, i.e., a pair to each segment. In the Naidomorpha and Lumbriculida, the vascular system consists only of these parts, together with some few branches which penetrate the layers of the body-wall and reach to the epidermis. Among earthworms the vascular system is more complex ; our knowledge of the details of the circulation in certain tropical genera ( Urochseta, Pontodrilus) is due to Terrier, and but little of importance has been added to his descriptions. The circulation of Lumbricus is known principally from the investi gations of D Udekem, Claparede, and recently of Horst. In Lum bricus there are three longitudinal trunks (fig. 3) which run from end to end of the body (1) dorsal, (2) supranervian, (3)subnervian. The dorsal vessel is connected with the supranervian by seven FIG. 3. Diagrammatic Transverse Section through one of the Posterior Segments of Lumbricus (partly after Marshall and Hurst), n, nephridium ; /, funnel of nephridium ; n.c, nerve cord ; e, epidermis ; m, transverse muscles ; m, longi tudinal muscles ; s, s, ventral and dorsal pairs of setas ; t, typhlosole ; d , dorsal blood-vessel, connected by a vertical branch with typhlosole and by branches with intestinal blood-plexus; s.n, supranervian vessel;, infranervian vessel. On the left side are indicated the chief vessels given off from the main trunk to the body-wall and nephridium. pairs of stout trunks in the outer part of the body ; from the dorsal vessel in front of the last of these originates on either side a lateral longitudinal vessel which passes along the sides of the oesophagus and gives off branches to it. In the intestinal region the dorsal vessel is connected by branches with the capillary network of the intestine, and also directly by lateral branches with the sub- nervian vessel ; the latter gives off branches to the body-walls. The majority of earthworms possess in addition a small vessel running above the alimentary canal, but below the dorsal vessel ; this is especially concerned with the blood supply of the alimen tary tract, and in the intestinal region it runs in the interior of the typhlosole ; the presence of this supra-intestinal trunk was first noted by Perrier, who also discovered that more or fewer of the large contractile &quot;hearts&quot; of the anterior segments are connected with this vessel as well as with the dorsal vessel. The subnervian vessel is absent in Pontodrilus and other genera. The immense development of the integumental capillary system is characteristic of earthworms, and is no doubt to be explained by the much greater thickness of the body -wall ; it has been already stated that among the lower Oligochxta the integumental capillary system is present though feebly developed. ^In some genera of earthworms the capillaries penetrate the epidermic layer, as in some of the aquatic genera and in many leeches. It has been recently stated by Sarasin that these epidermic capillaries open by pores on to the exterior. That the vascular system of Jlolosoma represents a primitive condition is shown by the investi gations of Vejdovsky (15) into the development of Rhynchclmis. In this worm the dorsal vessel is at first only visible in the anterior region of the body, where it lies upon the oesophagus below the peritoneal covering of the latter ; posteriorly it communicates with a blood sinus surrounding the intestine ; this stage exactly corresponds to the adult jEolosoma, and the blood is also colour less ; subsequently the dorsal vessel becomes connected with the ventral by a few lateral trunks ; this stage is retained in the Enchytreeidee. Moreover, in the Chlorasmidas a similar condition exists (Horst, ZooL Anzeig., viii. p. 12), and in the larval Terebclla. Excretory Organs. It hns been recently shown by Vejdovsky Excretor (15) that the Oligochieta, like Pohjgordius, many Potychseta, organs. Gcphyrca, amllfirudinta, possess temporary larval excretory tubules as well as the definitive ncphridia. These organs have been found in Rhynchclmis, Nais, Chsetogastcr, and ^Eolosoma. In the first mentioned type they appear as a pair of fine tubes with a ciliated lumen, without any apparent internal aperture ; they run on either side of the pharynx, and each opens by a pore placed at the side of the mouth. In the young asexually produced individual of Nais, Chsetogaster, and *&olosoma similar organs are to be seen. The permanent excretory organs consist in the majority of OligocJixta of three parts: (1) a funnel-shaped expansion furnished with cilia, opening into the body cavity ; (2) a coiled glandular tube ; and (3) a terminal vesicle furnished with muscular layers, and opening on to the exterior of the body. The latter section may be absent, and in the relative proportions of the different parts as well as in certain details of their structure there are great differences. As a general rule, the funnel lies in a segment anterior to that which bears the external pore ; but in Plulellus Perrier states that the whole organ lies in one segment. In the majority of forms there are not more than a single pair of nephridia to each segment (except in Perich&ta, &c. ). In the limicolous Oligochseta nephridia are generally wanting in the anterior segments of the body, and disappear in those which contain the generative ducts when the latter are developed ; in the terricolous forms, on the other hand, the nephridia usually commence in the second or third segment. Ctenodrilus is remarkable for the fact that it only possesses a single pair of nephridia, the funnel of which is situated on the anterior side of the first dissepiment ; but in no form are these organs entirely wanting. The glandular part of the organ consists, as in the Hirudinca and most Platyhelminths, of a row of cells placed end to end, which are perforated by the lumen ; the lumen of the tubule is therefore, as was first discovered by Claparede, intra- cellular; in this particular the nephridia of the Oligochasta differ from those of the Polychseia, where the walls of the duct are made up of rows of cells, the lumen therefore being inter -ceha.r. l While in the greater number of Oligochasta the lumen of the tubule is simple and unbranched, in Chsetogastcr fine branches are given off, which ramify in the substance of the cells ; this is an important point of resemblance to the nephridia of the Hirudinca, where Vejdovsky and Bourne (Q. J. M. S., 1884) have described a similar branching of the duct. The glandular part of the nephridium is often, as in Lumbricus, differentiated into two parts ; the anterior section is composed of more delicate cells, the posterior of larger and more glandular cells ; the lumen is furnished with cilia. The external surface of the organ is frequently covered with rounded cells of a glandular appearance, which are to be looked upon as modified peritoneal cells; in certain cases (e.g., Pontodrilus and many &quot; Lvnicolet&quot;) those cells form a solid mass in the interior of which are concealed the windings of the excretory tubule. Special dilatations of the tubule are occasionally met with, as in Rhynchclmis ; and, among leeches, Cosine, PontoMclla, &c., show a similar dilatation, which, as in Rhynchclmis, comes imme diately after the funnel. The terminal section of the nephridium, the &quot;contractile&quot; vesicle, is more marked among earthworms than in the &quot; Limicolse &quot; ; it is lined by a delicate layer of cells and furnished with muscular fibres; in Urdbcnus (Benham, 7), as well as in many other species, this region of the nephridium is very largely developed, and is furnished with a long sac-like diver- ticulum. The differences in structure between the various parts of the nephridium are due to their different origin : the funnel is formed independently of the glandular tubule, though both take their origin from the mesoblast ; the contractile vesicle is inva- ginated from the ectoderm. In Acanthodrilus multiporus Beddard found that the number of nephridial pores in the anterior region of the body to each segment was more than 100. In Tijplixus an almost identical arrangement exists, and in Perichssta (Beddard, 4). In the last-named genus, as well as in an Australian form, Mcgascolidcs (Spencer, 12), the nephridial system forms a continuous network of tubules, uninter rupted by the septa. In Acanthodrilus the network of each segment is independent. In this genus, as in Perichxta, there are numerous ciliated funnels in each segment. The relation of the nephridia of the Chtetopoda to those of the Platyhelminths, on the one hand, and to those of the Hirudinca and Gcphyrca are variously interpreted. It has been proved that in Polygordius, many Ch&topoda, and many Hirudinea and Gcphyrca, the larva?, like those of the Oligochasta, possess excretory organs, which are constructed on the type of the nephridia in Platyhel minths. This is at any rate the case with Polygordius, the Chaitopod larva, and the larva of Echiurus; in all these types the nephridia are paired branched tubes, which open separately on to the exterior; they have, however, no internal openings, the flagellate cells of the Platyhelminth, with their single ilagellum and funnel-shaped per- 1 Xussbaum has lately (Arch. Slav, de Biol., i.) found that in the leech a number of cells fuse to form a single drain-pipe cell. This would tend to prove that the intercellular lumen preceded the iutracellular.