PHYLUM TROCHELMINTHES 141 



the food ready for digestion, when it passes on into the stomach. 

 A pair of digestive glands is usually associated with the stomach. 

 Both the stomach and intestine are lined with cilia. In the 

 genus Asplanchna, the stomach ends blindly, for there is no 

 intestine. 



Excretory System. — Coiled nephridial tubes lying in the 

 body cavity bear flame cells at the ends of their lateral branches. 

 These excretory tubules discharge their waste into a urinary 

 bladder and thence into the cloaca. 



Nervous System. — A single ganglion, usually located in the 

 anterior dorsal region of the body, gives off nerves to the sur- 

 rounding organs and from it a pair of lateral nerve trunks pass 

 posteriorly to the tail. One or more simple pigment spots, and 

 sometimes more complicated eyes, are often associated with the 

 brain and, aside from tactile hairs, represent about the only 

 development of sensory apparatus. 



Reproduction. — Rotifers are bisexual, though usually the male 

 is much reduced and in some instances has never been observed. 

 At times, the male lives as a parasite on the female. Since they 

 have no digestive organs, many males are very short lived. In 

 describing rotifer structure, the body of the female is considered 

 as typical. Most rotifers are oviparous or ovoviviparous, 

 though some (Asplancha, Philodina) are viviparous. Partheno- 

 genetic development, as commonly found here, frequently 

 involves two different sizes of eggs, of which the larger produce 

 only females and the smaller only males. Insemination of the 

 female to produce fertilized eggs seems to be accomplished by per- 

 foration of the body wall at any point to introduce spermatozoa 

 into the body cavity. Fertilized eggs thus produced differ from 

 the parthenogenetic eggs in the possession of heavy, resistant 

 shells and are designated as winter eggs. 



Sexual Organs. — The female organs usually consist of a single 

 ovary (two in Philodina) and a vitellarium of highly variable 

 form, though in a few cases there are two gonads with no dis- 

 tinction between ovary and vitellarium. In parthenogenetic 

 development, the young are produced within the body of the 

 female and are usually liberated by rupture of the body wall of 

 the parent. Winter eggs are usually carried inside the body some 

 time before they are discharged through the oviduct and then lie 

 dormant for a period before the young are hatched. Organiza- 

 tion of the male is frequently simple, due to the degeneration of 



