ROTATOR TA. 245 



oesophageal about the middle of the body, long commissures between 

 them, and numerous nerves from both ; it retains its primitive con- 

 nection with the epidermis. There are two eyes and various patches 

 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 





3 



FIG. 126. 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 ; etc., archenteron; ./?., repro- 

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

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

 'pouches. 



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

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

 of an ovary and of a testis. The eggs undergo complete segmentation ; 

 a gastrula is formed by the imagination of the blastula ; the body 

 cavity arises, in enteroccelic fashion, as two pockets from the arch- 

 enteron. 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- 



