294 



ZOOLOGY 



SECT. 



the form of a spindle, striated longitudinally, and produced on its 

 inner face (i.e., towards the body-cavity) into a large and almost 

 bladder-like mass of protoplasm (p.) containing a nucleus (nu.). 

 Apparently the whole of this structure is derived from a single cell, 

 part of which has become differentiated into contractile substance 

 (c), the rest remaining protoplasmic. In transverse section the 

 contractile portion (B, c.) has the form of a plate bent upon itself 

 so as to be, as it were, wrapped round the protoplasmic portion 

 (p). The protoplasmic processes project to a greater or less extent 

 into the body-cavity, sometimes practically obliterating it, and are 

 produced into delicate filaments (/.) which take a transverse 



loll 



ex v 



01 "i/ 



FIG. 243. Ascaris lumbricoides, transverse section, cu. cuticle ; d. 1. dorsal line ; der. 

 epthm. deric epithelium or epidermis ; ex. v. excretory vessel ; int. intestine ; lat. I. lateral 

 line ; m. muscular layer ; ovy. ovary ; ut. uterus ; v. v. ventral line. (After Vogt and 

 Yung.> 



direction, and are mostly inserted into the dorsal and ventral 

 lines. 



The muscular layer is not continuous, but is divided into four 

 longitudinal bands or quadrants, two dorso-lateral and two ventro- 

 lateral, owing to the fact that at the dorsal, ventral, and lateral 

 lines the ectoderm undergoes a great thickening and projects 

 inwards, between the muscles, in the form of four longitudinal 

 ridges (Fig. 243, d.l, v.v., lat. I.). The ridges are composed of fibres 

 continuous with the fibres of the ectoderm. It is this arrangement 

 that gives rise to the lines seen externally. The ridges forming 

 the lateral lines are much more prominent than the other two. 



Digestive Organs. The mouth leads into the anterior division 

 of the enteric canal, the pharynx, stomodceum or oesophagus (Fig. 245, 

 ph.) : its walls are very muscular, its cavity is three-rayed in cross- 



