INVERTEBRATES OF UNCERTAIN POSITION 



181 



'%■ 



~b 



food. The members of the group are hermaphroditic, possessing 

 both male and female reproductive organs. 

 The Ch^etognatha are included under 

 the Nemathelminthes by some authori- 

 ties and are placed in a separate phylum 

 by others. 



6. Rotifera (Rotatoria) 



The Rotifera (Lat. rota, a wheel ; fero, 

 I carry) (Fig. 122), commonly known as 

 wheel animalcules, are extremely small 

 Metazoa. They were at one time con- 

 sidered Infusoria. Most of them are 

 inhabitants of fresh water, but some are 

 marine and a few parasitic. The anatomy 

 of a Rotifer is shown in Figure 123. 

 The head is provided with cilia (c 1 , c 2 ) 

 which aid in locomotion and draw food 

 into the mouth (mth). The tail or foot is 

 bifurcated and adheres to objects by means 

 of a secretion from a cement gland (c.gl). 

 The body is usually cylindrical and is 

 covered by a shell-like cuticle (cu). 



The Protozoa and other minute 

 organisms used as food are swept by the 

 cilia through the mouth (mth) into the 

 pharynx (ph), also called the mastax or 

 chewing stomach. Here chitinous jaws, 

 which are constantly at work, break up 

 the food. The movements of these jaws 

 easily distinguish a living rotifer from 

 other organisms. The food is digested in 

 the glandular stomach (st). Undigested 

 particles pass through the intestine (ini) 

 into the cloaca (cl) and out of the anus (a). 



, i.V 



-fc 



Fig. 121. — The arrow- 

 worm, Sagitta hexaptera 

 (of the group CHiETOG- 

 natha), ventral view. 

 a, mouth; b, intestine; 

 c, anus; d, ventral gang- 

 lion; e, movable bristles 

 on the head ; /, spines on 

 the head ; g, ovary ; //, ovi- 

 duct ; i, vas deferens ; 

 j, testis; k, seminal vesicle. 

 (From Shipley and Mac- 

 Bride, after Hertwig.) 



