ROTATORIA. 197 



internal ciliated funnels, and open on the tail. It is inter- 

 esting to know that two reproductive cells are set apart at a 

 very early stage, and that each divides into the rudiment of 

 an ovary and of a testis. 



The development is very regular. The eggs undergo 

 complete segmentation ; a gastrula is formed by the invag- 

 ination of a hollow ball of cells ; the body-cavity arises in the 

 form of two pockets from the gastrula cavity or archenteron. 



Appendix (2) to Annelid Series. 

 Class RoTATORiA^Rotifers. 



Rotifers are beautiful minute animals, abundant in fresh 

 water, also found in damp moss, and in the sea. They owe 

 their name and the old-fashioned title of wheel- animalcules, 

 to the fact that the rapid movements of cilia on their anterior 

 end produce the appearance of a rotating wheel. The food 

 seems to consist of small organisms and particles caught in 

 the whirlpool made by the lashing cilia. The little animals 

 are tenacious of life, and can survive desiccation, though 

 some instances of this are to be explained by the survival of 

 the eggs, not of the individuals. 



' The body is usually microscopic, and is sometimes {e.g., in 

 Melicerta and Floscularid) sheltered within an external tube. 

 There is no internal segmentation, but there are sometimes 

 external rings, and a ventral outgrowth or " foot " is some- 

 times segmented. The anterior end bears, on a retractile 

 ridge, the ciliated rings or "trochal apparatus." 



The nervous system is a single dorsal ganglion with a few 

 nerves. An unpaired eye and some tufts of sensory hairs 

 are usually present. 



The food-canal extends along the body in a well-developed 

 ccelome, and the fore-gut contains a mill in which two 

 complex hammers beat upon an anvil. The canal ends 

 posteriorly on the dorsal surface between the body and the 

 foot, and as the terminal portion also receives the excretory 

 canals and the oviduct, it is called a cloaca. 



There is no vascular system, but a nephridial tube of a 

 primitive type lies on each side of the body, and opens 

 posteriorly into the cloaca. 



