TYPE NEMATHELMINTHE8. 



173 



the group. In the class Nematoda many forms live freely in 

 the sea, fresh water, or damp earth, while others are parasitic 

 during a part of their lives, and others again are parasites 

 practically throughout their whole existence. The Acantho- 

 cephala are without exception parasitic. 



I. CLASS NEMATODA. 



The Nematodes are distinguished from the members of 

 the second class by the presence in nearly all cases of a dis- 

 tinct digestive tract, usually with mouth and anus, and by 

 the absence of a retractile proboscis furnished with hooks 

 at the anterior end of the body. The arrangement of the 

 muscles of the body-wall are also peculiar inasmuch as longi- 

 tudinal muscles only are present (Fig. 86, m), which instead of 

 forming a closed sheath are interrupted along four longitudi- 



:d 



FIG. 86. TRANSVERSE SECTION OF Ascaris lumbricoides AT THE LEVEL OP 



PHARYNX (from HERTWIG). 



c = cuticle. m = longitudinal muscles. 



d = dorsal line. s = lateral Hue. 



h = hypodermis. = ventral line. 



w = nephridiuin. 



nal lines (d, v and s), or in some cases along a single ventral 

 line, in the former case there being four longitudinal bundles 

 of muscles extending the length of the body. In the struc- 

 ture of most of the organs, however, considerable variation is 

 found, and it will be most convenient to describe them as 



