AN ATTEMPT TO CLASSIFY BAETHWOEMS. 215 



the latter organ. For instance, most species of Perichseta 

 have the gizzard occupying three somites, viz. viii, ix, x. 

 Some species have been credited with a gizzard in Somite v 

 or VI. I believe that the former is the typical position for 

 Perichaeta, and that a further examination of the forms 

 with a forward position of the gizzard will lead to a separa- 

 tion of these species. 



The absence of a gizzard in Criodrilus and Pontodrilus, 

 in addition to the absence of the anterior nephridia, is a point 

 worthy of note, both negative characters being, no doubt, con- 

 nected with their habitat and characters of food. 



The fixity of the ovary and testes in the thirteenth and 

 tenth somites respectively — or, rather, their nearly constant 

 position — gives us a fixed point or centre from which to count 

 the variations in position of other organs. And I am greatly 

 tempted, with Rosa, to regard the ovary as always in the 

 thirteenth : notwithstanding the apparent exceptions — M i cr o - 

 chseta and Brachydrilus, where they appear to be in 

 Somite xii, the error may be due to fusion of two of the 

 anterior somites to form a single " peristomium.^' ^ However 

 this may be, the majority of earthworms possess an ovary in 

 Somite xiii — this may be taken as a fixed point, and we may 

 compare the position of other structures in regard to their 

 greater or less distance from this point : thus the gizzard 

 lies so many somites in front, or behind, in the various 

 genera. This fact seems to indicate that the gizzard of 

 Eudrilusin Somite vi is not homologous or homogenetic with 

 the gizzard in Lumbricus in Somites xvii and xviii ; but 

 that a similar modification of the wall of the gut has occurred 

 in different somites in different worms. 



The region following the gizzard, and before the typhlosole 

 commences, is in Lumbricus very short, occupying only a 



^ This is certainly the case in Microchseta bed dardi, where small setae 

 can be detected in the apparent peristomium: and as this somite never carries 

 setae, we have here one somite occupying the position of two morphological 

 somites. In M. rappi I can detect no setae on the peristomium ; the fusion 

 is here complete. 



