246 



THE INVERTEBRATA 



be described, as it is seen alive as a transparent object under the 

 microscope. Most species are free-living. They are obtained by 

 allowing small pieces of meat to decay in moist earth. The larvae 

 vs^hich exist in an "encysted" condition in the soil are attracted by 

 the products of decay, and in a few days become sexually mature. 



m,co. 



g.c.n. 



oes. 



lat.l. 



cx.c. 



cut. 



m.n 



^v.n. 



Fig. 173. Diagrammatic transverse section through Ascaris in the region of 

 the oesophagus, showing the single large cell occupying the space between 

 the body wall and the gut. Original, ait. cuticle ; d.n., v.n. dorsal and ventral 

 nerves ; g.c.n. nucleus of giant cell, cytoplasm dotted, vacuoles {vac.) shown 

 as clear spaces; ex.c. excretory canal; hyp. hypodermis; lat.l. lateral line; 

 m.co. contractile part of muscle cells; in.t. tails of the muscle cells running 

 toward the nerves in the median lines ; oes. oesophagus with three gland 

 cells gl.c. and radiating muscles ni.r. which increase the lumen of the oeso- 

 phagus and cause suction. The number of muscle cells in each quadrant is 

 much greater than in the drawing. 



Great numbers of adults and young can then be scraped off the surface 

 of the meat in the liquefied matter formed by bacterial decomposition. 

 It will be seen that the animal progresses by alternate contractions 

 of the muscles on each side of the animal, which bend the animal into 

 S-shaped curves and enable it to wriggle slowly through thick hquid 

 or on soil. The cuticle which covers the body is thin, tenacious but 

 elastic. It enables the animal to keep an almost constant round cross- 



