NEMATODA. 175 



Type, Ascaris (e.g. Ascaris megalocephala, the Round-worm 

 of the horse). 



This round-worm occurs in the small intestine of the 

 horse, while other species similarly infest man, the ox, pig, 

 etc. The body is cylindrical in cross-section and tapering at 

 each end. The colour is dead-white, the absence of pigment 

 being very characteristic of Nematodes. At the anterior 

 end is the mouth, furnished with three lips bearing sense 

 papillae ; the anus is - posterior and ventral. The male is 

 smaller than the female, and has a recurved tail furnished 

 with two horny spines and numerous sense papillae. It is 

 usually about seven inches long, while the female may be 

 as much as seventeen. 



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

 perhaps of service in enabling the animals to resist the action 

 of the digestive juices. With its presence may be associated 

 the scarcity of cutaneous glands, and the entire absence of 

 cilia. (i>) Beneath this is the sub-cuticula or hypodermis, 

 thickened along four longitudinal lines — median dorsal, 

 ventral, and lateral, and consisting of a protoplasmic mass 

 without distinct cell-limits, (c) Beneath the hypodermis 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 of the 

 latter runs along the median dorsal line — a unique position 

 in an Invertebrate. Here and there on the ring and on the 

 nerves there are ganglionic cells, but any aggregation of 

 these into ganglia is rare. 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, a fore-gut or oesophagus, 

 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, 



