ROTATORIA. 



233 



of sensitive cells. The food canal is complete and simple, and lies in a 

 spacious ciliated body cavity. Corresponding to the external divisions, 

 the cavities of the head, body, and tail are distinct, being separated from 

 one another by septa ; a longitudinal mesentery supports the gut and 

 divides the cavities into lateral halves. 



There is no vascular system, nor are there any certain nephridia. It 

 is possible that the latter may be represented by the genital ducts. 



The animals are hermaphrodite, and the simple reproductive organs 

 lie near one another posteriorly. The two ovaries project into the body 

 cavity, and their ducts open laterally where body and tail meet. The 

 two testes project into the cavity of the tail ; and their ducts have 

 internal ciliated funnels, and open on the tail. Two reproductive cells 

 are set apart at a very early stage, and each divides into the rudi- 

 ment of an ovary and of a testis. The eggs undergo complete segmenta- 

 tion ; a gastrula is formed by the invagination of the blastula ; the 



/; : 



m 



4 f( - >~^~>- , ' X~-& t * : 



| 



l;^v^f fe-}^ 



I 



FIG. in. Development of Sagitta. After O. Hertwig. 

 Illustrating formation of a body cavity by pockets 

 from the archenteron ; also the early separation of 

 reproductive cells. 



EC., Ectoderm: En., endoderm ; ac.. archenteron; R., repro- 

 ductive cells; bl. , blastopore ; c.p., coelom pouches; ;//.. 

 mouth ; i. section of gastrula ; -2 and 3. origin of coelom 

 pouches. 



body cavity arises, in enteroccelic fashion, as two pockets from the 

 archenteron. The young forms are like the adults. 



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 move- 

 ments 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 prolonged drought. If they are 

 left dry for long, however, they die, though the ova may survive and 

 subsequently develop. 



The body is usually microscopic, and is sometimes (e.g. in Melicerta 



