INTRODUCTION TO THE METAZOA 



75 



Reproductive System. — In some Metazoa, the gonads or 

 essential organs of reproduction are the only organs involved in 

 the reproductive process. Thus, in the coelenterates, the germ 

 cells are dehisced directly from the body surface or into the 

 gastro vascular cavity. Ducts which lead from the gonads to the 

 exterior are present in most other Metazoa, These, and other 

 accessory organs, comprise the reproductive system. The gonads 

 may be single, paired, or multiple. Frequently, a gonad becomes 

 subdivided- into follicles, each of which has more or less the 

 appearance of an independent organ. 



Fig. 47. — Different relations of nophridia and sexual ducts in chaetopods 

 (after Goodrich). I, hypothetical primitive condition, gonoducts (g) and 

 protonephridium (p) independent; II, ciliated grooves discharge into duct of 

 protonephridium as in Phyllodoceids and Goniads; III, ciliated grooves and met- 

 anephridium (n) open independently as in Dasybranchus; IV, ciliated grooves 

 open into canal of metanephridium as in Syllids, Spionids, etc. (From Hertwig's 

 Manual of Zoology by Kingdey, courtesy of Henry Holt and Co.). 



When each individual bears the organs of but one sex, the 

 species is said to be dioecious. Very frequently among the 

 invertebrates a single individual may produce both eggs and 

 sperms and is then said to be hermaphroditic or monoecious. 

 Hermaphroditism commonly results from the occurrence of a 

 full set of gonads and accessory organs for each sex in the body 

 of the same individual but in some snails both eggs and sperms 

 are formed in the same gonad, which is therefore designated as a 

 hermaphroditic gonad. 



