VI 



PHYLUM NEMATHELMINTHES 



277 



arranged longitudinally, and bounding the body-cavity. The 

 structure of the muscles is very peculiar : each (Fig. 224, A) has 

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

 inner face (i.e. towards the ccelome) into a large and almost blad- 

 der-like mass of protoplasm (p) containing a nucleus (me.). Ap- 

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

 part of which has become differentiated into contractile substance 

 (e), 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 



iitt 



ovy 



v.w 



FIG. 223. Ascaris lumbricoides, transverse section, cu. cuticle ; d. 1. dorsal line ; der. epthm. 

 deric epithelium or epidermis ; ex. r. excretory vessel ; int. intestine ; hit. I. lateral line ; m. 

 muscular layer ; ory. ovary ; ut. uterus ; v. v. ventral line. (After Vogt and Yung.) 



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

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

 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. 223, d.L, v.v., lat. /.). It is this arrangement that gives 

 rise to the lines seen externally. The ridges forming the lateral 

 lines ^tre much more prominent than the other two. 



