SPONGES, FLAT WORMS, AND ROUND WORMS 163 



of the last-named lies one of the longitudinal excretory tubes 

 (ex. v.). The nerve cords are also embedded in the body wall. 



The intestine consists of a single layer of columnar cells, the 

 entoderm, coated both within and without by a thin cuticle. 



The codom of Ascaris differs from that of the higher animals in 

 several respects. Typically the ccelom is a cavity in the meso- 

 derm lined by an epithelium; into it the excretory organs open, 

 and from its walls the reproductive cells originate. In Ascaris 

 the so-called ccelom is lined only by the mesoderm of the body 

 wall, there being no mesoderm surrounding the intestine. Fur- 

 thermore, the excretory organs open to the exterior through the 

 excretory pore, and the reproductive cells are 

 not derived from the coclomic epithelium. 

 The body cavity of Ascaris, therefore, dif- 

 fers structurally and functionally from that 

 of a true ccelom, but nevertheless is similar 

 in many respects. 



ROUND WORMS IN GENERAL 



The round worms belong to the Phylum 

 Nemathelminthes. They are mostly parasitic ; 

 only a comparatively small number are free- 

 living, inhabiting damp earth or fresh and 

 salt water. Among the most interesting 

 round worms is the parasite, Trichina spiralis 

 (Fig. 83), which is often found encysted in 

 the muscles of the pig. When insufficiently 

 cooked pork infested by Trichina is eaten by 

 man, the young parasites become mature in 

 the intestine, and burrow through its walls, 

 causing a disease known as Trichinosis. 



, 83. Trichina 

 spiralis encysted 

 among muscle 

 fibers. (From 

 Shipley and 

 MacBride after 

 Leuckart.) 



