358 OLIGOCHAETA 



themselves, thus forming a network passing through the septum 

 and from segment to segment, or whether each tube is isolated 

 from its fellows, and forms a distinct nephridium, of which there 

 are many in each segment and entirely separate. This is, how- 

 ever, certain, that the complex nephridial systems of at any rate 

 Octochaetus and Megascolides are derived from the multiplication 

 of a single pair of tubes which are alone present in the embryo. 

 In Perichaeta the minute nephridia are furnished with coelomic 

 funnels ; in Octocliaetus they are not, except in the case of 

 certain nephridia which open into the terminal section of the 

 intestine. 



Both at the anterior and at the posterior end the nephridia 

 occasionally open into the alimentary canal. In various genera 

 the first pair of nephridia are larger than the others, and open 

 into the buccal cavity ; it seems likely that they serve as salivary 

 glands. A somewhat similar condition of things exists in Peri- 

 patus (vol. v, p. 17). In Octochaetus multiporus, for example, 

 there is a large tuft of nephridial tubes in the anterior region of 

 the body, which opens by a long muscular duct into the buccal 

 cavity. In the same species a good many of the nephridial 

 tubes open into the posterior section of the intestine, reminding 

 one of the anal vesicles of the Gephyrea (p. 436) and of the 

 Malpighian tubes of the Arthropods. 



In many Eudrilidae the ducts of the paired nephridia form 

 a network in the body-wall, which opens on to the exterior by 

 many pores. 



Alimentary Canal. The digestive tube is perfectly straight 

 in nearly all Oligochaeta. Only in Plagiochaeta and a species 

 of Disaster is it twisted in the intestinal region in a corkscrew- 

 like fashion. The mouth is under the buccal lobe (where, as in 

 the majority of cases, this is present) ; the anus is mostly ter- 

 minal, or rarely, e.g. Criodrilus, a little in advance of the end of 

 the body on the ventral side. In the simpler forms three regions 

 can be distinguished, which are themselves simple in structure. 

 The mouth leads into a buccal cavity, which in its turn 

 opens into the pharynx ; the latter is muscular, with thick 

 walls. The narrower oesophagus opens into the wider intestine, 

 which opens posteriorly, as already stated. In the earthworms 

 there is as a rule some complication. The oesophagus bears 

 certain glandular appendages, the calciferous glands ; and a part of 



