VI 



PHYLUM NEMATHELMINTHES 



cuticle. It is wrinkled trans- 

 versely so as to give the animal 

 a segmented appearance. Be- 

 neath the cuticle is a proto- 

 plasmic layer containing scat- 

 tered nuclei and longitudinal 

 fibres, and representing a syn- 

 cytial ectoderm, i.e., an ecto- 

 derm in which the cell-bodies 

 are not differentiated, and its 

 cellular nature is recognisable 

 only by the nuclei. 



Beneath the ectoderm is a 

 single layer of muscular fibres 

 of peculiar structure, arranged 

 longitudinally, and bounding 

 the body-cavity. 



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 in- 

 wards, between the muscles, 

 in the form of four longitudi- 

 nal ridges. It is this arrange- 

 ment that gives rise to the 

 lines seen externally. 



The mouth leads into the 

 anterior division of the enteric 



