264 ELEMENTARY BIOLOGY. [CHAP. 



the cut ends of delicate nerve-fibres ; and of a 

 denser peripheral ventro-lateral portion, in which 

 large nerve-cells are lodged. 



Compare sections through the longitudinal 

 commissures and ganglionic enlargements. 

 Nerve-cells are present in both, and except 

 so far as their relative proportions are con- 

 cerned, there is no important structural differ- 

 ence between them. In sections through the 

 latter, look for the origins of the ganglionic 

 nerves, and the small blood-vessels arising from 

 the lateral neural trunks which accompany 

 them. 



H. The study of transverse sections. 



i. Wash out the contents of the alimentary canal of a 

 freshly killed worm with \ p. c. chromic acid solu- 

 tion. When all the earthy matter has been removed, 

 cut up into segments of an inch in length and trans- 

 fer to solutions of chromic acid and alcohol of in- 

 creasing strength (see appendix E). Stain with 

 borax-carmine and mount in the usual manner in 

 Canada balsam. 



Examine the thickest sections under a low power, 

 and note — 



a. The body wall. It is mainly composed of a thick 

 muscular layer, external to which is a thin epidermis 

 clad in a delicate cuticle. 



b. The intestinal wall ; its thick tunic of yellow- 

 brown tissue. Internally to this there is a thin 

 muscular layer, and the whole is lined by a well- 

 defined ciliated epithelium. Examine the minute 

 structure of the typhlosole. 



