470 FRESH-WATER BIOLOGY 



cation. The anal muscles are muscular strands passing from trie 

 transverse slit-like anus to the body walls near the lateral fields. 



There is no vascular circulatory system. These organisms are so 

 small that the colorless "blood" is aerated without the need of special 

 vessels. The movements of the body serve to propel the body-fluid 

 irregularly about through the body cavity and among the organs. 



The main locomotive movements of nematodes are due to the 

 alternate action of two antagonistic sets of muscle, dorsal and 

 ventral, extending nearly the full length of the body, and acting on 

 the lateral thickening of the cuticula as a fulcrum. The move- 

 ments are serpentine, but in a dorso- ventral plane. As the result- 

 ing body-curves are usually wider than the space between the 

 cover glass and the microscope slide, it follows that the micro- 

 scopical view of these nematodes is usually a lateral view. 



Locomotion is accomplished by the aid of friction on surrounding 

 solid objects, such as the stems or roots of plants, grains of sand 

 or other particles. Comparatively few of the aquatic species can 

 swim, and even these seem uneasy and frightened when they find 

 themselves floating free in the water. Most of the aquatic species 

 are supplied with -three unicellular caudal glands and a terminal 

 spinneret, whose main, and probably sole, function is to cement 

 the tail temporarily to various objects. From this attachment 

 as a base the nematode moves its head in various directions in 

 search of food, or of its mates. Some species, for instance some 

 species of Chromadora, attach themselves alternately first by the 

 head by suction, and then by the spinneret, executing movements 

 like those of the common caterpillars known as "inch- worms." 



The excretory organ of the free-living nematodes consists of a uni- 

 cellular* gland, the renette, lying in the body cavity, not far from the 

 junction of the intestine and esophagus. It empties through a duct 

 leading forward to a ventral excretory pore, usually located some- 

 where between the lip region and the intestine. There are a number 

 of genera in which the renette has not yet been seen. Its homologue 

 in the large parasitic species is renal in nature, at least in one case. 



Through the study of the free-living species the supposed excre- 

 tory function of the lateral fields, long believed in, has been dis- 

 * Rarely two to many-celled and double . 



