BRANCH TROCHELMIN'THES 



The animals associated together in this group may have de- 

 veloped independently from trochosphere-like ancestors, but 

 since they agree in general character- 

 istics, they have been regarded by some 

 as constituting a well-marked phylum. 

 On account of their size they were 

 formerly regarded as protozoans, but 

 they are multicellular and possess well- 

 defined digestive, excretory, nervous, and 

 reproductive systems. They have no 

 circulatory system. Respiration takes 

 place through the surface of the body. 



CLASS I. ROTIFERA 



The Rotifera (Fig. 37), or "wheel 

 animacules," are many-celled, micro- 

 scopic, unsegmented animals, most of 

 which are worldwide inhabitants of fresh- 

 water ponds and streams, or even of mud- 

 puddles and water-troughs. A numljer 

 of forms are marine. 



The anterior end is a retractile disk 

 surrounded by cilia, which are locomotive 

 organs as well as aids to securing food. 

 The mobile tail is often composed of tele- 

 scopic rings, rendering it retractile into 

 the trunk. The posterior ring of the tail 

 frequently has a pair of pincer-like stylets. 

 These and the adhesive glands enable 

 the rotifer to attach itself to objects. 

 There is a crelom. The alimentary tube consists of a ventral 

 mouth, an esophagus, a chewing apparatus (m.astax), a glandular 

 stomach, and an intestine which ends in a dorsal anal opening. 



46 



Fig. 37.— A rotifer, 

 highly magnified (Hy- 

 datina senia): A, cilia; 

 a, anus; b, contractile 

 vesicle; c, water-ves- 

 sels; e, ovary; /, gang- 

 lion. (From Holder*s 

 " Elements of Zool- 

 ogy," American Book 

 Co., Publishers.) 



