PSEUDOCOELOMATE AND MINOR EUCOELOMATE PHYLA 



Fig. 12.2. Sequence of 

 events in the life cycle of 

 the rotifer, Epiphanes senta. 



functions of ovaries and yolk glands. A short oviduct, continuous with the 

 wall of this organ, leads posteriorly and opens into the cloaca. In the male 

 there is usually a single, sac-like testis,- spermatozoa reach the exterior 

 through a simple ductus deferens which opens on the tail or posterior sur- 

 face. A cuticular spine or penis may be developed. It appears that copula- 

 tion does not usually occur; male gametes are injected into the pseudocoel of 

 the female, and there is no regular site for insertion of the penis. 



The life histories of rotifers have been the subject of much investigation. 

 In some species, as mentioned previously, onlv females exist, and reproduc- 

 tion is exclusively parthenogenetic. In others numerous generations of 

 females arise through parthenogenesis; eventually, as a result of some environ- 

 mental change, eggs are produced that give rise, parthenogenetically, to either 

 males or females. The eggs developed by these females require fertilization, 

 and thus zygotes are formed. These zygotes, or "winter eggs," become sur- 

 rounded by protective cysts and may remain dormant through long periods 



347 



