462 FRESH-WATER BIOLOGY 



elusion is that there must be hundreds of thousands of species of 

 nematodes. 



Nearly all the tissues of the fresh-water nematodes are compara- 

 tively colorless and transparent, and whatever decided color the 

 body possesses is usually confined to the intestinal region. The 

 cells of the intestine itself are sometimes colored by the presence 

 in them of granules of a faint yellowish or brownish tint, and 

 the middle portions of the body are thus rendered yellowish or 

 brownish. The color of the ingested food, showing through the 

 tissues of the body, is also sometimes a color factor. The food 

 varies in color from nearly black to colorless, and the body is 

 correspondingly tinted. Species feeding on the juices of plants 

 are usually nearly colorless, e.g., species of Tylenchus and Aphe- 

 lenchus. A few species possess colored eye-spots near the head. 

 In some species the esophagus contains yellowish or brownish 

 pigment. 



Most genera, and even some species, of fresh-water nematodes 

 have a world- wide distribution. The small size and the vitality 

 of the individuals favor their transportation in a great variety of 

 ways, one of the most efficient vehicles being the feet of flying 

 water-fowl. Possibly some of the aquatic species are as resistant 

 to dryness as are rotifers, and, as "dust," are blown about in the 

 same manner. Certain species of plant-infesting nematodes will 

 revive after many years of desiccation. 



Another cause of this wide distribution is the fact that fresh- 

 water nematodes adapt themselves to a great variety of depths and 

 temperatures. They are found as near the poles as are any other 

 organisms. They occur in practically every body of water where 

 extreme conditions do not preclude life of any kind. Few organ- 

 isms are so easy to find. 



The outer covering of a nematode is composed of a non-cellular 

 layer usually divided into two parts, the cuticula and the subcutic- 

 ula. These groups are not easily defined, but the natural division 

 line is probably between the outer layers that are to be shed at the 

 next moult, and all the other layers. Thus the subcuticula in turn 

 becomes the cuticula. The cuticula is composed of about three 

 layers and the subcuticula of about an equal number. Though 



