MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 



191 



This group of animals is bisexual, and dimorphism (striking dif- 

 ferences in form of the two sexes) is present. The males are usually 

 much smaller and may even live as a parasite on the female. The 

 males lack a well-developed digestive system and are therefore very 

 short lived. In the female of most species there is one ovary which 

 produces the eggs. Connected with this gonad is a yolk gland or 

 vitellarium. In a few forms there are two ovaries with no distinct 

 yolk gland. Rotifers may be oviparous (lay eggs), ovo viviparous, 

 or even a few are viviparous. The eggs produced during the sum- 

 mer are thin-shelled, of two sizes, and develop parthenogenetically. 



Large Egg 

 Females 



Female - 



/ 

 Small Egg 



/ 



Males 

 Sperm 



Winter Eggs 

 (Fertilized) 



Late SuKiner 



Pass Winter 

 in thick shell 



Females Females 



/ ^ 

 Eggs'" Eggs 



/ ^ 



Parthenogenesis Parthenogenesis k Svunner 



Many Generations Manv Generations f season 



I "i 



Females Females 



i i ^ 



Large Eggs Small Egg 



Fig. 97. — Life cycle of the rotifer, Hydatina. 



The smaller type produce males. The eggs produced during the 

 winter are thick-shelled, produce females only, and require fertili- 

 zation. The eggs when mature, or the young if born alive, are 

 carried by the tubular oviduct to the cloaca and are discharged to 

 the exterior through the anus. The less highly developed males 

 possess a single testis in which spermatozoa are produced. In some 

 there is a peculiar type of copulation during which the special 

 copulatory organ composed of a protrusible cirrus seems to per- 

 forate the body wall of the female. At this time the eggs of the 

 female are fertilized. In oviparous forms, the fertilized eggs are 

 usually carried in the body for a time and then discharged by way 



