420 EDWIN S. GOODRICH. 



segments, in the female they open ventrally as in the genital 

 segments. 



It will be seen that the general relations of the genital 

 organs is really much the same in both sexes ; that the 

 sperm-sac can be compared to the spermatheca; and that if 

 an invagination similar to that which has given rise to the 

 penis took place in the female at the mouth of the sperma- 

 theca, a condition would be brought about almost identical 

 with that which obtains in the male. Whether the sacs 

 themselves are formed from invaginations of the skin can 

 only be decided by a study of their development ; on the 

 whole, it seems probable that this is the case. 



As to the morphological value of the excretory organ, 

 whether it be a true nepliridium wath a nephridiostome, or a 

 nephromixium formed of a coeloraostome grafted on to a 

 nepliridium (5), only a knowledge of development can here 

 again help us to decide. The structure of the male organ 

 especially seems to lend itself readily to the latter interpre- 

 tation. The main part of the canal appears to be undoubt- 

 edly of nephridial nature, comparable to the very similar 

 nephridia of the Polygordiidas (2 and 5). On the other 

 hand, the funnel has very much the appearance of being 

 derived from the coelomic epithelium (figs. 9 and 12). 



Summary and Conclusion. 



The chief additions to our knowledge of the structure of 

 Saccocirrus recorded in the foregoing pages may be briefly 

 enumerated as follows : — the parapodia and chaetse are absent 

 from the last ten or twelve segments; in each bundle of 

 cha3t£e, besides the ordinary blunt bristles, is oue long 

 slender cheeta, ending in three prongs ; there is a stomato- 

 gastric nervous system, consisting of two nerves passing 

 backwards from the brain, and joining below the oesophagus ; 

 the buccal cavity is prolonged into a muscular diverticulum 

 below the oesophagus, which forms a ventral pharyngeal 

 pouch, lined with cuticle, and probably eversible ; the dorsal 

 blood-vessel divides below the brain into two branches. 



