2l6 



THE INVERTEBRATA 



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 liquid 

 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- 



mxo. 



laid 



ex.c. 



cut. 



rn,r. 



i\ n. 



Fig. i66. 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, cut. cuticle; d.n.^ v.n. dorsaland 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; 7n.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 m.r. which increase the lumen of the oeso- 

 phagus and cause suction. 



section and length : in the presence of such a cuticle and the absence 

 of circular muscles the peristaltic movements of a worm like 

 Lumhricus are impossible. A cross-section through Rhabditis shows 

 a similar structure to Ascaris, though the muscle cells are much less 

 numerous (only two to each quadrant) : each cell contains a number 

 of contractile fibrils arranged in a different way to those in the Ascaris 



