NEMATHELMIA. 49 



which the mouth opens. Its wall, starting from the inside, is 

 made up of (a) a thick cuticle, and (b) a layer of epithelium, 

 continuous respectively with cuticle and epidermis of body-wall, 

 while outside these is (c) a muscular layer, the cells of which are 

 radially arranged. The gullet is about J of an inch long, and is 

 sharply marked off by a constriction from the mid-gut. This, 

 consists of an intestine, somewhat flattened dorso-ventrally, and 

 with an exceedingly thin wall made up of a single layer of 

 columnar epithelial cells, lined by a thin cuticle and covered 

 externally by a structureless membrane. The intestine passes 

 into a short hind-gut or rectum, the walls of which are thickened 

 and resemble those of the gullet in structure. In the male, the 

 terminal part of the rectum is a cloaca, since it receives the 

 genital duct. 



A definite body-cavity is found outside the wall of the gut, but 

 it is for the most part reduced to a system of narrow spaces by 

 the projecting parts of the muscle-cells. The body-cavity contains 

 a clear albuminous perivisceral fluid. 



Ascaris lives, for the most part, in the small intestine, and is 

 consequently surrounded by food much of which is digested i.e., 

 in a dissolved or finely-divided state. The gullet acts as a kind 

 of suction-pump, its radial muscle-fibres causing enlargement of 

 its cavity, when food rushes into it, after which the elasticity of 

 the cuticular lining comes into play, and, the lips being approxi- 

 mated, drives the food into the intestine. Here the digested 

 part of it diffuses into the perivisceral fluid. There is no special 

 provision for setting up currents in this, but the general move- 

 ments of the body effect an indefinite kind of circulation. 



4. Excretory Organs are represented by a narrow tube, imbedded 

 in each lateral line (Fig. 15), and ending blindly behind, while the 

 two tubes unite anteriorly to form an unpaired portion which 

 opens by the excretory pore. 



5. Reproductive Organs (Fig. 15). (1) The male organs chiefly 

 consist of a much convoluted tube, some seven or eight times the 

 length of the body, ending blindly at one end and opening by 

 the other end into the ventral side of the cloaca. The greater 

 part of this tube is very slender and constitutes the spermary, which 

 merges into a much shorter thicker portion, the vesicula seminalis, 

 and this again into an extremely short and narrow ejaculatory 

 duct, the walls of which are very muscular. 



2 4 



