Phylum N emathelmhithes and Related Phyla 429 



canal. The pseudocoel separates the digestive tract from the body 

 wall ; normally it is filled with a liquid. Within the pseudocoel, sec- 

 tions of the reproductive tract are visible. 



The Biology of the Free-Living Nematodes. — The wide variety 

 of habitats in which these animals can be found can scarcely be imag- 

 ined. They are known to dwell in hot springs and in arctic waters. 

 Some live in the hot deserts, others survive only at cold altitudes of 

 high mountains. Many are world-wide in their distribution, while 

 others are so restricted as to seem unbelievable. 



Despite their comparative lack of locomotor apparatuses, the 

 nematodes are good travellers. They are carried about by vagaries 

 of the wind, by animals, in dust, and by seeds. Inasmuch as the cuticle 

 is very thick, the body of the nematodes is quite rigid. Locomotion is 

 further limited by the lack of circular muscles. Most move through 

 the water or over the substratum by means of serpentine motions, us- 

 ing the rings or roughened portions of the cuticula to gain purchase. 

 Some possess bristles which attach temporarily and allow the worms 

 to progress in a leechlike manner. 



The innumerably different species of nematodes are as varied in 

 their feeding habits as in their habitat preferences. Some are sapro- 

 phagoiis, that is, they fed on decaying plant or animal materials, yeast, 

 or bacteria. The vinegar eel, for example, feeds on the yeasts and 

 bacteria found in home-cultured vinegar. A number of aquatic forms 

 feed on small algae and other aquatic plants while some forms are pre- 

 daceous, attacking other microscopic organisms. Some of these latter 

 forms have buccal teeth for grasping their prey, while others have 

 complex stylets by which they pierce and suck out the body juices of 

 their victims. 



The Plant Nematodes. — Economically the plant nematodes are 

 of tremendous importance due to their destruction of food plants. All 

 degrees of parasitism are noted among these forms. Some, for ex- 

 ample, feed upon plant tissues that have been invaded by bacteria. 

 Others actually use their stylets to pierce the cells and suck out the 

 contents. Still others penetrate and live within the spaces of the 

 plant tissues. The damage done may be actual destruction of plant 

 tissue or opening the way for bacterial invasions. 



There are several notorious plant nematodes, one of the most 

 famous of which is the root-knot nematode, Heterodera marioni, which 



