PHYLUM ANNULATA 



479 



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 indi- 

 vidual has been seen to deposit one 

 or more spermatophores on any part 

 of the body of another often on 

 the back. The spermatophore, which 

 is nearly 3| mm. long, apparently 

 exerts a solvent action on the skin, 

 since, after a short interval, the 

 spermatic fluid streams through the 

 skin into the coelomic spaces, prob- 

 ably making its way at last to the 

 ovaries. This extraordinary process 

 of hypodermic impregnation prob- 

 ably 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. 383, 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); 



FIG. 382. Nephridial system of Pon- 

 tobdella. <in. lit, gn. 17, ganglia 

 of nerve-cord ; np. nephridiopore ; 

 nph. nephridial network ; nst. ne- 

 phrostoiue. (After Bourne.) 



'.b 



mlh 



FIG. 383. Six stages in the development of Clepsine. ;/.'/. germinal bands; mg. megameres : 

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



one of these divides again, and its products of division give rise 

 to paired germinal bands (C, g.b.), like those of the Earthworm. 

 The micromeres increase in number, forming a cap of cells, the 

 commencement of the ectoderm, on the dorsal pole of the embryo, 



