STUDIES ON EAETHWORMS. 95 



suggested its generic name. Here then we have a condition 

 intermediate between Digaster (E. P.) and Moniligaster^ 

 (E. P.), but whereas the former is post-clitellian, and the latter 

 aclitellian, Trigaster has its male genital pores situated within 

 the clitellum. 



External Anatomy. — The size and number of somites is 

 necessarily unknown, as the only specimen that I have had 

 the opportunity of examining is incomplete. 



The setae are arranged in four couples in each somite 

 (fig. 31); the ventral and lateral couples on one side are 

 nearer to one another than are the ventral couples of the 

 two sides ; the eight setse are all on the ventral surface. The 

 setae are very small and difficult to see owing to the greatly 

 contracted state of the worm. As seen in a transverse section 

 they are '2 mm. long, and are less curved than in Lum- 

 bricus. The preclitellar somites are quadriannulated, and 

 the third annulus forms a very distinct ridge (a in fig. 32); 

 the setae are placed in the second annulus (fig. 32). 



The prostomium is broad, and occupies the whole dorsal 

 border of the anterior extremity ; it is not embedded in 

 somite i. The setae commence in somite ii. 



The clitellum commences behind somite xiii, and occupies 

 twenty-seven somites, extending as far as somite xl. This is 

 an exceptional length for the clitellum, the nearest approach 

 being Tit an us with fifteen somites. The clitellum is incomplete, 

 there being a groove in the mid-ventral line, in which the two 

 ventral couples of setae are placed (fig. 31). In the anterior 

 part of this groove is the fossa (g) above mentioned. 



There are no dorsal pores, nor could I find any nephridio- 

 pores. 



The male pores are four in number, and although not 

 visible externally are indicated by the fossa, and are easily 

 seen in transverse sections. This fossa appears to extend 

 through somites xvii, xviii, xix, and xx and part of somite xvi, 

 but the somites are so short in this region and the setse so 

 difficult to see that the actual number of somites may be less, 

 ' 'Nouvelles Arch, de Mus. d'hist. nat. de Paris,' viii, 1872. 



