210 



ZOOLOGY. 



be absent (some parasites), but this is exceptional. 

 There are no special organs for circulation and respira- 

 tion. Sexes distinct in the large majority. I will 

 deal with only one of the orders belonging here. 



Order : Nematoda (Thread Worms), 



Elongated, thread-shaped or spindle-shaped, un- 

 segmented. A gut, terminating in a ventral anus, is 

 present. Outer investment of the skin smooth, often 

 weakly ringed or striated. The muscle layer under- 

 lying the true skin is interrupted here and there, 

 where the skin itself stretches further inwards. In 

 this way are distinguished the lateral limes or fields 

 (Fig. 128, a), which run right along the sides of the 

 body, dividing it into a dorsal and a ventral region. 

 There are often present as well two smaller but similar 



Fig. 128. — Diagrammatic transverse Bection througli the body of a Thread Worm, 

 internal organs being omitted : i>, cuticle and skin ; q, muscle layer ; a, lateral 

 lines ; b, dorsal and ventral lines. 



dorsal and ventral lines, respectively situated above 

 and below (Fig. 128, b). Mouth usually surrounded 

 by lip-like folds, but more or less distinct jaws may 

 also be present. The anus either lies at the apex of 

 the tapering posterior end of the body (trichina), or 



