ARROW-WORMS 



281 



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 



,<^:S 





Fig. 151. — Development of Sagitia. — 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 ; m., 

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

 pouches. 



internal cihated 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 invagination of the blastula ; the body 

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

 enteron. The young forms are like the adults. 



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 

 and Floscularia) sheltered within an external tube. There is no 

 internal segmentation, but there are sometimes external rings, and the 

 attaching outgrowth or " foot " is sometimes segmented. The anterior 



