Phylum Nemathelminthes and Related Phyla 439 



crawl about like leeches. Some have developed protective cases of 

 their own secretion or by cementing sand particles together. Many 

 forms have the cuticle thickened into a lorica which may be provided 

 with bizarre spines and projections. Many of the species are cosmo- 

 politan in their distribution, for their resistant, tiny eggs are easily 

 carried by winds, dust particles, or animals from one place to another. 

 They may be found in similar situations in all parts of the world. 

 On the other hand, so sensitive are they to environmental conditions 

 that neighboring ponds may harbor totally different faunas. 



Fig, 142, — Anatomy of the rotifer, Philodina. 



In size, the rotifers are little larger than the protozoans, ranging 

 from 0.04 to 2 mm, in length, with most being about 0.5 mm. Despite 

 their miniature size, these are true metazoans with an organ level of 

 organization. All have bilateral symmetry, three germ layers, a pseudo- 

 coel, cephalization, a digestive tract which is usually complete and 

 differentiated into definite parts, and separate sexes. Their charac- 

 teristics which set them apart from the nematodes are the anterior 

 circles of cilia, the corona; a protonephridial excretory system with 

 flame cells; and the possession of movable jaws in the pharynx. The 



