ROUND-WORMS 



237 



{a) Most externally there is a thick chitinoid cuticle, 

 perhaps of protective value. With its presence may be 

 associated the scarcity of cutaneous glands, and the absence 

 of cilia, (b) Beneath this is the sub-cuticula or epidermis, 

 thickened along four longitudinal lines — median, dorsal, 

 ventral, and lateral — and consisting of a protoplasmic matrix 

 without distinct cell-limits. Except at the tail-end the 

 nuclei are confined to the longitudinal lines, and are most 



Tl.C 



ovo 



Fig. 129. — Cross-section through Ascaris. — 

 From a speoimen. 



DN., Dorsal nerve; n.c, non-contractile portion of muscle cells; 

 C, cuticle: £., epidermis; LL., lateral line; EV., excretory 

 vessel ; A/.', contractile portion of muscle cells ; VN., ventral 

 nerve ; OV., ovary ; LT., uterus ; G., gut. 



numerous laterally. The epidermis makes and remakes 

 the cuticle, which is periodically moulted, (c) Beneath the 

 epidermis is a layer of remarkable muscle cells, lying in 

 groups defined by the lines mentioned above. Many of 

 the Nematodes are very agile. 



Around the pharynx there is a nerve-ring from which 

 six nerves run forwards and six backwards. One runs 

 along the median dorsal line — a unique position in an 



