THE ANATOMY. ALIMENTARY CANAL 53 



It is clear too from the fact that the diffuse and paired form of the excretory 

 system occur in forms which are so nearly related, for instance in Acanthodrilua 

 and Octochaetus, that there can be no profound gap between the two kinds of 

 organs. The ' Plectonephrica' of Benham I agree with Vejdovsky in considering 

 an artificial group. 



V. Alimentary Canal. 



The alimentary canal in all Oligochaeta consists of a straight tube running from 

 the mouth which opens on to the first segment and is overhung by the buccal lobe' 

 (when present) to the posteriorly situated anus ; with two exceptions the anus 

 is surrounded by the last segment of the body. These exceptions are Criodrilus and 

 Sparganophilus ; in the former worm Vejdovsky figures (24, PI. x, fig. ai), seven post- 

 anal segments, and the anus itself as dorsal in position. There is, however, no fiexure 

 of the intestine in CriodrUus ; it passes, as in other Oligochaeta, perfectly straight 

 from the mouth to the anus. Developmentally the alimentary canal of the Oligochaeta, 

 as of other animals, consists of three portions: (i) Mesenteron, hypoblastic in origin, 

 (3) StoTnodaeum, (3) Proctodaeum, both formed by later invaginations of the epiblast ; 

 of the two the proctodaeum is the later formation. The greater part of the alimentary 

 canal is of hypoblastic origin. The actual extent of the stomodaeum seems rather 

 doubtful. Vejdovsky, who at one time thought that the end of the pharynx m&rked 

 its posterior limit, was subsequently inclined to think that the buccal cavity only 

 was of epiblastic origin in Lumbricus and Rhynchelmis. 



In the adult worms the alimentary canal may be divided into the following 

 regions: mouth and buccal cavity, oesophagus, pharynx, and intestine. The tnouth 

 is nearly invariably ventral in position^ and it leads into the buccal cavity which 

 is of limited extent; the buccal cavity of the Enchytraeidae is often provided with 

 one or a pair of small tongue-like organs which spring from its floor. These are 

 probably sense organs. Michaelsbn at one time put them down as playing the 

 part of a sucker. These organs are furnished with minute hair-like processes, and 

 appear to be entirely cellular; they are in fact a product of the lining epithelium 

 of the buccal cavity; in some species they can be everted; in a few species the 

 buccal cavity has a dorsal diverticulum in which the cells are more glandular ; 

 this state of affairs occurs in Benhamia, in Microdrilus and a few more species 

 In the higher Oligochaeta the buccal cavity is separated from the ensuing pharynx 

 by a constriction on which lies the cerebral ganglia; in Aeolosoma there appears 



' A few exceptions where the intestine is spiral are noted under the description of that organ. 

 ^ Terminal where the prostomium is absent. 



