508 



ZOOLOGY 



ZRCT. 



pairs. But in Herpobdella (Nephelis) they are very numerous, and 

 are not arranged segmentally. In the Rhynchobdellida the 

 muscular penis is absent, its place being taken by an eversible 

 sac. The form of the ovary with its containing sac in Hirudo 

 is exceptional. As a rule, there is an elongated hollow ovary, 

 producing ova from its epithelial lining, and thus agreeing very 

 closely in structure with the testis. 



In Clepsine, a fresh-water Rhynchobdellid, copulation in the 

 ordinary sense of the word has never been observed, but one 

 individual has been seen to deposit one or more spermatophores on 

 any part of the body of another often on the back. The spermato- 

 phore, which is nearly 3| mm. long, apparently exerts a solvent 

 action on the skin, since, after a short interval, the spermatic 

 substance streams through the skin into the coeloniic spaces, 

 probably making its way at last to the ovaries. This extraordinary 



mi. 





FIG. 424. Six stages in the development of Clepsine. g. b. germinal bands ; mg. rnegameres ; 

 mi. micromeres ; mth. mouth. (After Whitman.) 



process of hypodermic impregnation probably takes place in other 

 genera, but has been most closely followed in Clepsine. 



It is in Clepsine that the early stages of development are 

 best known. Segmentation is unequal, the embryo consisting, in 

 the eight-celled stage (Fig. 424, A), of four large ventrally placed 

 megameres (mg.) and four dorsal micromeres (mi.). One of the 

 megameres, posterior in position, divides into two cells (B) : the 

 so-called neuronephroblast and mesoblast, the latter of which at 

 once divides into two. As shown by their subsequent history, the 

 neuronephroblast and the mesoblast correspond respectively to the 

 first and second somatoblasts of Nereis. The former divides and 

 subdivides to form two symmetrical groups of four cells each, 

 situated at the posterior pole. The number of micromeres 

 increases, at first apparently by division of the megameres. The 

 latter subsequently give oft' a number of small endoderm cells. 



