THE NEMATODA. PARASITISM 



297 



consists of several layers and is shed at intervals, an ecto- 

 derm (hypodermis) which is without cell-limits and must 

 therefore be classed as a syncytium (p. 119), and a single 

 layer of peculiar muscle fibres. The nuclei of the ecto- 

 derm, except at the hinder end, are collected along the 

 mid-dorsal, ventral, and lateral lines. Along these lines 

 the protoplasm bulges towards the body cavity. A nerve 

 cord is embedded in the dorsal and ventral lines, and an 

 excretory canal in each lateral line. The excretory canals 

 are without flame cells, or internal openings, but are con- 

 nected with four large, branched cells in the forepart of 

 the body cavity. They unite in front to open by the 

 excretory pore. The nerve cords are connected by trans- 



Fig. 204. — Ascaris lumbricoides. — From Sedgwick, after Leuckart. 



a, Hind end of male ; 6, head, from above ; c, head, from below ; d, egg, in shell ; 

 2, excretory pore ; SJ>, penial setse. 



verse commissures in the ectoderm, and in front join a ring 

 a little way behind the mouth. From this ring four other 

 cords run back a short distance at the sides, and six forwards. 

 The nerve ring is slightly thickened above and rather more 

 below, and contains some nerve cells. The only other 

 ganglion is placed at the hinder end of the ventral cord. A 

 few cells are scattered among the fibrils of which the cords 

 are composed, but there is no sign of segmentation in 

 these or any other organ of the body. Each muscle fibre 

 consists of an outer longitudinally-fibrillated part and an 

 inner part of undifferentiated cytoplasm containing a 

 nucleus. Fine strands of protoplasm stretch from the 

 inner parts of the muscle fibres to the dorsal and ventral 

 nerves. The alimentary canal consists of three parts : a short 



