SPONGES, FLAT WORMS, AND ROUND WORMS 161 



through the walls of which the 

 nutriment is absorbed. Just be- 

 fore the anal opening is reached, 

 the intestine gradually becomes 

 smaller; this portion is known as 

 the rectum. 



The excretory system consists of 

 two longitudinal canals (Fig. 81, 

 7), one in each lateral line; these 

 open to the outside by a single 

 pore (8) situated near the anterior 

 end in the ventral body wall. 



A ring of nervous tissue sur- 

 rounds the pharynx and gives off 

 two large nerve cords, one dorsal, 

 the other ventral, and a number 

 of other smaller strands and 

 connections. 



The male reproductive organs 

 are a single coiled thread-like 

 testis, from which a vas deferens 

 leads to a wider tube, the seminal 

 vesicle; this is followed by the 

 short muscular ejaculatory duct 

 which opens into the rectum. 

 In the female lies a Y-shaped 

 reproductive system. Each 

 branch of the Y consists of a 

 coiled thread-like ovary (Fig. 81, 

 3) which is continuous with a 

 larger canal, the uterus (4). The 

 uteri of the two branches unite 

 into a short muscular tube, the 

 vagina (5), which opens to the out- 

 side through the genital aperture 

 (6). Fertilization takes place in 

 che uterus. The egg is then sur- 

 rounded by a shell of chitin, and 



