MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 189 



Rotifers are plentiful in fresh water, and a few of them inhabit 

 the sea. They are microscopic in size, and they are often associated 

 with Protozoa. They are very resistant to adverse conditions pro- 

 duced by drought and may be distributed in dry form. 



The body of a rotifer is bilaterally symmetrical and can be di- 

 vided into head, trunk, and foot. It is covered externally by a 

 cuticle. The so-called head is rather largely a troclial disc com- 

 posed of various modifications of two bands of cilia over the anterior 

 end and around the mouth. These cilia are in active motion, often 

 creating two sets of water currents so as to resemble two rotating 

 wheels. They are responsible for obtaining food and for locomo- 

 tion. The mouth is located in an anteroventral position. The trunk 

 tapers toward the posterior and contains numerous organs. At the 

 posterior end is the tail or foot Avhich is forked or toelike in many 

 species. Here, too, in many forms, are located some cement or adhe- 

 sive glands which assist the animal in adhering to most surfaces. 

 The foot as a whole serves in locomotion, pushing the animal along. 



The internal organs include several systems which lie in the rather 

 extensive body cavity or false coelom. The digestive system begins 

 anteriorly at the mouth which receives other small organisms as 

 food. It is a cavity leading to the pharynx. Inside the pharynx is 

 a mill-like organ or mastax, composed of chitinous jaws, which mas- 

 ticates the particles of food. The movements of these jaws may be 

 observed in certain rotifers when alive. A short tubular esophagus 

 leads to the pouchlike stomach, and extending posteriorly is the 

 smaller cylindrical intestine which leads by way of the cloaca to 

 the anus. Nearly the entire internal surface of the alimentary 

 canal is lined with cilia which aid the movement of the food mate- 

 rial through it. The stomach and intestine are lined internally 

 with endoderm. 



The excretory system is well developed and consists of a number 

 of flame cells, similar to those of flatworms; and two winding 

 nephridial ducts which lead posteriorly to a contractile bladder. 

 This bladder is pouchlike and empties into the cloaca. (The name 

 cloaca is applied to any cavity which serves as the posterior portion 

 of the alimentary canal and also receives products of the urino- 

 genital system. It opens externally by way of the anus.) The flame 

 cells are distributed in the body wall from the anterior, posteriorly. 

 Some authors believe that the bladder functions also to assist in 



