410 



GENERAL ZOOLOGY 



ous system consists of a cephalic (cerebral) ganglion from which 

 nerves extend to the periphery. There are one or two dorsal 

 antennae, located over the ganglion, and two lateral antennae, 

 one on each side of the trunk. One or more rudimentary eye- 

 spots are usually present. They are dioecious, though there 



are a few species in which males are 

 unknown. Both oviparous and 

 viviparous types of reproduction 

 occur. 



The life history of a typical rotifer 

 is interesting. There are two kinds 

 of eggs: (1) a thick-shelled "winter" 

 or "resting" egg, which can survive 

 drought or cold; and (2) a thin- 

 shelled "summer" egg. There are 

 two types of females, externally 

 alike, but differing in the kinds of 

 eggs they produce. Amictic females 

 produce eggs that cannot be ferti- 

 lized. Midic females produce eggs 

 that may develop parthenogenet- 

 ically but are also capable of fertili- 

 zation. The fertilized winter eggs 

 develop into amictic females. 

 These in turn produce eggs that 

 develop parthenogenetically into 

 females, both mictic and amictic. 

 The eggs of mictic females develop 

 parthenogenetically into males, 

 which produce sperm that may 

 fertilize other eggs of mictic 

 females. Thus the same eggs that 

 produce males, if unfertilized, pro- 

 duce females if fertilized. 

 Whitney has shown that feeding Hydatina senta with 

 Polytoma, a colorless flagellated protozoan, results in the pro- 

 duction of a larger number of female-producing (amictic) 

 daughters; while feeding with Chlamydomonas, a green flagel- 

 late, increases the number of male-producing (mictic) daughters. 

 It will be noted that the effect of diet on sex potencies does not 

 appear until the second generation following special feeding. 



Fig. 238. — Diagram of a Roti- 

 fer, a, anus, b, brain; d, ovi- 

 duct; df, dorsal feeler; f, flame 

 cell; g, cement gland, by the 

 secretion of which the animal 

 attaches itself temporarily; e, 

 eyespot; i, intestine; m, mouth; 

 n, nephridial tube; o, ovary; p, 

 pharynx, containing masticat- 

 ing organs; R, retractor muscles; 

 s, stomach; v, contractile vesicle 

 (bladder). (After Parker and 

 Has well.) 



