470 



ZOOLOGY 



SECT. 



Phosphorescence, the production of light rendering the 

 animal brilliantly luminous in the dark, occurs in a few cases 

 (various Polynoids, Chcetopterus, &c.). In some of these (Poly- 

 ophthalmus) the light is produced by eye-like organs in most of the 

 segments. 



In the arrangement of the reproductive organs in the 

 Chaetopoda there is an essential difference between the two sub- 

 classes, the Oligochseta being hermaphrodite, and the Polychseta, 

 with only a very few exceptions, unisexual. In the latter the 

 gonads, ovaries or testes as the case may be, are masses of cells 

 which are developed as the result of a proliferation of the crelornic 



pcrib 



p&rtf: 



ficrib 



J. 



v erit. vcas 



vcrti. vess 



FIG. 384. Diagram to illustrate the development of a gonad from the peritoneal (coclomic) 

 epithelium in one of the Polychseta. peril, peritoneal membrane ; rcpr. yl. gonad (repro- 

 ductive organ) ; vent. ress. ventral vessel. (After E. Meyer.) 



epithelium in certain positions (Fig. 384). Usually these organs, 

 which are only conspicuous about the breeding season, occur 

 in the great majority of the segments of the body ; sometimes they 

 are confined to a certain region. The exact place which they 

 occupy in the interior of the segment varies in different cases : 

 sometimes they surround one of the principal blood-vessels, 

 sometimes they are situated laterally, in the bases of the para- 

 podia. The sperms frequently undergo the final stages of their 

 development after they have become detached from the testes, 

 while floating in the coalomic fluid, and the same sometimes holds 

 good of the ova. Both sperms and ova appear to reach the 

 exterior, in the majority of cases, through the " segniental organs," 



