PHYLUM TROCHELMINTHES 143 



line, by the action of which locomotion is accomplished. The 

 body proper may be either smooth or covered with plates, spines, 

 or bristles. The mouth, which is borne at the anterior extremity of 

 the head, is usually surrounded by a circle of delicate oral bristles. 

 In addition, there are frequently lobes on the sides of the head 

 from which groups of sensory hairs protrude. 



The internal organization (Fig. 73) is relatively simple. The 

 digestive tract runs as a straight tube through the axis of the 

 body. In the body musculature, only a few longitudinal strands 

 of muscle but no circular muscles have been demonstrated. The 

 brain occupies much of the head region. The excretory organs 

 are protonephridia. 



Only females are known, yet it is uncertain whether these are 

 truly parthenogenetic or are hermaphroditic, and the male 

 gonads have never been observed. The ovary occupies the 

 posterior region of the body cavity and as fully formed eggs 

 push anteriorly in the body cavity, they frequently distort the 

 shape of the gravid female. 



Chaetonotus and Lepidoderma are the most representative 

 genera in the North American fauna. 



Outline of Classification 



Phylum Trochelminthes. — Microscopic, triploblastic, unsegmented Meta- 

 zoa; as adults usually resembling trochophore; mouth usually surrounded 

 by cilia; aquatic. 



I. Class Rotifera. — A crown of cilia (corona) typically at anterior 

 extremity; posterior extremity usually terminating in a foot or jointed 

 tail; pharynx bears mastax. 



a. Subclass Monogononta. — Ovary single. 



1. Order Notommatida. — Mouth not near center of corona. 

 Proales, Notommata, Synchaeta, Polyarthra, Distyla, Monostyla, 

 Rattulus, Diurella, Hydatina, Anuraea, Notholca, Brachionus, 

 Schizocerca, Asplanchna. 



2. Order Floscularida. — Mouth near center of corona. Flos- 

 cularia, Microdon, Apsilus, Stephanoceros. 



3. Order Melicertida. — Two parallel wreaths of cilia with a furrow 

 between; outer wreath always shorter. Pterodina, Pompholyx, 

 Pedalion, Triarthra, Trochosphaera, Melicerta, Conochilus. 



b. Subclass Digononta. — Two ovaries. 



1. Order Bdelloida. — Fresl) water. Rotifer, Philodina. 



2. Order Seisonida. — Marine; parasitic. Seison, Paraseison. 



II. Class Gastrotricha.^Body spindle-shaped; flattened ventral surface 

 bearing two rows of cilia; cuticular spines on back; ring of cilia (?) 

 around mouth. Chaetonotus, Lepidoderma. 



