116 



INVERTEBRATE ANIMALS. 



anus. A nervous system is also present, consisting of gan- 

 glia placed near the anterior extremity of the body and send- 

 ing filaments backward. There is, finally, a well-developed 

 water-vascular system. 



Most of the JRotifera are free-swimming, active little ani- 

 mals (Fig. 43, A), but some are permanently fixed, as in 

 Melicerta (Fig. 43, B), or in the crown-animalcule StepJiano- 

 ceros). They are usually simple, but they are sometimes com- 

 posite, forming colonies. As a rule, the male and female 

 Jtotifera diflFer greatly from one another, the males being 

 smaller than the females, devoid of any masticatory or diges- 



FiG. 43. — ^Eotifera. A. Diagrammatic representation of Hydatina senta (generalized from 

 Pritchard); ct Depression in tlie cilmted disk leading to tlae digestive canal; & Mouth; 

 c Pharyngeal bnlb with masticatory apparatus; d Stomach; 6 Cloaca; / Contractile 

 bladder ; g g Respiratory or water- vascular tubes ; h Nerve-ganglion, giving filament to 

 ciliated pit (A); o Ovary. B. Mt^iceHa ringens {aX^v Go&&q), 



tive apparatus, and more or less closely resembling the young 

 forms of the species. The males, in fact, merely lead a tran- 

 sient existence, and die as soon as they have succeeded in 

 fertilizing the females. The body in most cases is very dis- 

 tinctly ringed or annulated (Fig. 43, A), but is not composed 

 of distinct rings separated by partitions. The integument is 

 usually provided with bundles of muscular fibres taking a 

 longitudinal and transverse direction. In the free forms the 



