ANATOMY, HISTOLOGY, AND AFFINITIES OF PHREORYCTES. 633 



Clitellum. 



In the majority of my specimens the clitellum was developed. The position of the 

 clitellum in this genus has not been hitherto known ; and, as this organ is of some little 

 importance in the classification of Oligochseta, it is particularly desirable to have some 

 information upon the point. The clitellar region was obvious, in all the specimens which 

 had reached that degree of maturity, by its swollen distended appearance and whitish 

 colour. The swollen appearance and the white colour are, however, due not so much to 

 the modification of the integument in this region of the body, as to the mass of generative 

 products, principally spermatozoa, which are developed in these segments, and cause them 

 to be considerably distended. The comparatively slight increase of thickness in the 

 epidermis of the clitellum, as compared with the epidermis over the general body-surface, 

 is not sufficient to distinguish this part of the body when examined without the aid of a 

 microscope. When the body is slit open, and the integument examined microscopically, 

 the extent of the clitellum is quite obvious ; it extends over three complete segments 

 and the part of a fourth ; the posterior boundary of the clitellum coincides with the 

 furrow separating segments XIII.-XIV.; anteriorly the clitellum is not so sharply 

 defined ; it commences on segment X. at or near the setse. 



The clitellum of Phreoryctes Smithii, therefore, occupies three segments and a portion 

 of a fourth, commencing on the Xth, and ending at the posterior border of the XHIth 

 segment. 



It forms a complete girdle round these segments, i.e., there is no ventral space not 

 invaded by the glandular modification of the integument. 



The clitellum, therefore, includes all the apertures of the generative ducts, which are 

 thus in the strictest sense " intraclitellian." 



The minute structure of the clitellum was investigated by longitudinal sections as 

 well as by an inspection of the entire clitellar area mounted in Canada balsam unstained. 

 Preparations of the latter kind showed that the epidermic cells were greatly modified 

 upon the clitellum ; the cells were filled with rouDded, highly refractive bodies ; in 

 sections the cells were seen to be considerably longer than the epidermic cells elsewhere ; 

 but there was no trace in my preparations of more than a single layer of cells. It is 

 important to notice this fact, because, if anything could be said in favour of the old 

 division of Oligochseta into Oligochceta limicolce and Oligochce taterricolce, it is that the 

 structure of the clitellum is markedly different in the genera assigned to the one group 

 from what it is in genera assigned to the other group. In earthworms it appears to be the 

 rule, and as far as we know at present, without any exceptions, that the clitellar epidermis 

 consists of two more or less distinct strata ; on the contrary, in all the aquatic and other 

 genera which have been referred to the Liniicolse, the clitellar epidermis only consists of 

 one layer. I take this opportunity, however, of pointing out this structural difference 

 between the " Limicolse " and " Terricolse," as I have not noticed any prominent statement 

 of the facts as an argument in favour of the above mentioned classificatory scheme. 



