ASCARIS. 



203 



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 

 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 Invertebrate. Here and 

 there on the ring and on the 

 nerves there are ganglion ic 

 cells, but there is but little 

 aggregation of these into 

 ganglia. Sense organs are 



represented by the papillae 

 already mentioned. 



As the food consists of 

 juices from a living host, 

 it is not surprising to find 

 that the alimentary canal 

 has but a narrow cavity. 

 It consists of three parts 



fore-gut or oesophagus, 



FIG, 106. Cross-section through 

 A scar is. 



d.n., Dorsal nerve ; #.., non-contractile 

 portion of muscle cells; c., cuticle; e. t 

 epidermis; /./., lateral line; e.v., ex- 

 cretory vessel ; m., contractile portion ot 

 muscle cells; v.n., ventral nerve; oz>., 

 ovary; , uterus; ., gut. 



lined by the inturned cuticle, 



a mid-gut or mesenteron of 



endodermic origin, and a usually short hind-gut or rectum 



lined by the cuticle. When the external cuticle is shed, so 



is that of the fore-gut and hind-gut (cf. Crayfish). 



There is a distinct space between gut and body wall, but it is lined 

 externally by the muscle cells, internally by the endoderm of the gut, 

 which has no mesoblastic coat ; the space is therefore not strictly 

 ccelomic. It contains a clear fluid, which probably discharges some of 

 the functions of blood. There are no free amoeboid phagocytes. 



Embedded in each lateral line there is a longitudinal canal. These 



