ANNELIDA 835 



probably mesodei'mal parts, is converted directly into the 

 oesophagus, pharynx, and proboscis-sheath of the adult 

 animal. The intestinal canal attains its final shape as the 

 result of the ingrowth toward it of the dissepiments, 

 which thus cause the caecal diverticula of the intestine. At 

 the same time the intestine is provided with its musculature. 

 In Clepsine there are six pairs of such diverticula ; the 

 seventh pair grows backward thi'ough five segments, and 

 consequently acquires constrictions similar to those of the 

 intestine itself. The terminal portion of the intestine 

 extends straight backwards and unites with the ectoderm to 

 form the anus. 



The conditions are not so simple in the Gnathobdellldae. 

 Here the pharynx first formed is of a provisional nature, and 

 functions only in the reception of the albuminous nourish- 

 ment. After this office is discharged it degenerates ; the 

 mouth closes as the result of the concrescence of the head 

 germ and trunk germ (Bergh). At the same point there is 

 formed an invagination of the united bead and trunk 

 germs, the fundament of the permanent pharynx, which 

 grows into the larval pharynx and unites with the entoderm, 

 while the tissue of the old pharynx is gradually absorbed. 

 The jaws arise in tbe pharynx as fold-like elevations covered 

 by a firm cuticula (Leuckart). The oral sucker is formed 

 as a circular elevation of the superficial layer of the body. 

 The development of the mid-gut takes place in a manner 

 similar to that already described above for Clepsine. On 

 the other hand, according to Bergh's observations, the hind- 

 gut is formed as a solid outgrowth of the tissue of the 

 " trunk germ," which subsequently becomes hollow, and 

 unites with the entoderm. Such a mode of origin agrees 

 with Bergh's entire theory, according to which the whole 

 body of the leech, with the single exception of the mid-gut, 

 is formed from the so-called head and trunk srerms. 



The degeneration and regeneration of the pharynx in the Gnathob- 

 dellidae recall the replacement of the larval stomodajum by a permanent 

 pharynx in Lopadorhynchus as described by Kleinenbeeg, even though 

 the metamorphosis takes place there in a different way. 



