186 



METAZOAN PHYLA 



the posterior end of the body. The term cloaca is apphed to any cavity 

 into which open both the ahmentary canal and ducts belonging to the 

 excretory and reproductive systems, and which in turn opens externally. 

 A ganglion lies against the wall of the body dorsal to the pharynx and a 

 second one may be present on the ventral side. Sense organs are also 

 present in the form of antennae, or feelers, and eyespots. The food of 

 rotifers consists of protozoans and other small organisms. 



o'//af^^ t^/'sk 



Mouth 

 Uasfax 



Sfomach 



Yolk gr/ar!cf± 



Ovary 



Ov/c/ucf 



Contfacf/'/e 

 ves/c/e 



Foot 



-Dor^(^^ fee/er 

 Eye spof 

 Musc/es 



F/a/ve cell 

 Cuf/cu/a 



Fxcrefory 

 c?luct 



Jnfesf/yje 



C/oaca 

 C/oaca/ openm^ 



•Toes 



Fig. 93. Fig. 94. 



Fig. 93. — Philodina roseola Ehrenberg. Not a typical rotifer, but one of the first types 

 studied by microscopists; the ciliated disks on the head of the organism suggested the 

 name rotifer. It both creeps and swims. A common American species. {From Jennings, 

 in Ward and Whipple, "Fresh-water Biology,'' after Weber, by courtesy of John Wiley & 

 Sons, Inc.) X 300. 



Fig. 94. — Diagram of a rotifer in section to show internal structure. The specimen 

 is a female. {From Parker and Haswell, ''Textbook of Zoology," by the courtesy of The 

 Alacmillan Company.) 



Rotifers are diecious and also polymorphic. The males are usually 

 smaller than the females and frequently exhibit degenerafion. This is a 

 condition involving simplification of structure and loss of organs. It is 

 exhibited by parasites, as in the platyhelminths, but it also occurs apart 

 from parasitism. The eggs are carried by the oviduct into the cloaca 

 and thus reach the outside. Rotifers produce three types of eggs — 

 female-producing summer eggs, smaller male-producing summer eggs, 

 and female-producing winter eggs. Both types of summer eggs have 



