520 



ZOOLOGY 



SECT. 



FIG. 416. Nephridial system of Pon- 

 tobdella. cm. lit, (in. 17, ganglia 

 of nerve-cord ; np. nephridiopore ; 

 nph. nephridial network ; nxt. cili- 

 ated funnel. (After Bourne.) 



tacles appear to be the organs in which new blood -corpuscles are 

 manufactured. The ciliated funnels of the Hirudinea correspond 

 more closely with the ccelomoducts or ciliated organs of the Poly- 

 chneta than with the nephrostomes ; they are to be compared 

 also with the " urns " of the Gephyrea. In the Rhynchobdellid 



Pontobdella a very interesting modi- 

 fication of the nephridial system 

 occurs. Instead of distinct nephridia, 

 there is found on the ventral sur- 

 face of the body a very complex 

 network (Fig. 416, nph.\ which 

 sends off on each side of each seg- 

 ment a short branch terminating in 

 a ciliated funnel, and a similar 

 branch which opens externally (np.). 

 A similar modification occurs in 

 Branchellion. 



The nervous system alwaj's 

 closely resembles that of Hiruclo, 

 as also do the sense-organs. The 

 number of eyes is subject to con- 

 siderable variation : the} 7 may be 

 developed on the posterior sucker, or may be absent altogether. 



Reproductive Organs. The testes usually have the segmental 

 arrangement found in Hirudo, their number varying from five to 

 twelve pairs. But in Herpobdella (Nephelis) they are very 

 numerous, and are not arranged segmentally. In the Rhynchob- 

 dellida the muscular penis is absent, its place being taken by an 

 eversible sac or lursa copulatrix. 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 indi- 

 vidual 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 3J mm. long, apparently exerts a solvent 

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

 substance streams through the skin into the ccelomic spaces, 

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

 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. 417, A), of four large ventrally placed 

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



