LESSON XXV 



PoLYGORDius {Continued) 



Asexual reproduction is unknown in Polygordius, and 

 the organs of sexual reproduction are very simple. The 

 animal is dioecious, gonads of one sex only being found in 

 each individual. 



In the species which has been most thoroughly investi- 

 gated {P. neapolitanus) the reproductive products are formed 

 in each metamere from the fourth to the last. Crossing 

 these segments obliquely are narrow bands of muscle (Fig. 

 70, A, O.M) and certain of the cells of cojomic epithelium 

 covering these bands multiply by fission and form little 

 heaps of cells {Spy), each of which is to be looked upon as a 

 gonad. There is thus a pair of gonads to each segment with 

 the exception of the prostomium, the peristomium, the first 

 three metameres, and the anal segment, the reproductive 

 organs exhibiting the same simple metameric arrangement 

 as the digestive, excretory, and circulatory organs. It will 

 be noticed that the primitive sex-cells, arising as they do 

 from coelomic epithelium, are mesodermal structures, not 

 ectodermal as in hydroids (pp. 231 and 245). 



In the male the primitive sex-cells divide and sub-divide, 

 the ultimate products being converted into sperms (Fig. 70, 



