17 



muscular layers and incloses an internal cavity almost entirely filled 

 with the alimentary canal, which forms the very simple digestive sys- 

 tem. This begins at the head end, in the mouth opening (fig. 15, m.) 

 and runs back for about one-third the length of the body in a narrow 

 tube, the oesophagtis, which has a thick wall and two bulb like enlarge- 

 ments, one near the middle and the other at the posterior end, (fig. 

 15, X and b.). These parts are rather indistinct in the very young 

 worm, but become more prominent as it grows older. From the 

 oesophagus the alimentary canal broadens out into the intestine or 

 stomach (s) which occupies most of the remaining length of the body, 

 terminating in a narrow portion, the irctiim, which has its outlet at 

 the anus, near the posterior end. The whole body is filled more or 

 less with granular protoplasm and fat globules. The only other 

 organ distinguishable at this stage is the sexual, which originates in 

 both sexes in a little cluster of minute cells situated close to the 

 intestine, near or just posterior to the middle of the body. This is 

 shown more enlarged in fig. 14. As the worm approaches maturity 

 it increases in length and proportionally in width, the alimentary 

 canal becoming more distinct and the sexual organs developing. 

 The sexes now become distinguishable. In the female the sexual 

 organ becomes an ovary. The cells composing it increase rapidly in 

 number, extending toward both ends of the body. At the same time 

 an opening called the indva (v) is formed through the body wall on 

 one side, about one third the body length from the tail. The worm 

 has now reached the stage shown at plate I., fig. 17, or the somewhat 

 later stage at plate II., fig 1. The ovary extends almost the entire length 

 of the intestine, forming a long tube full of small, roundish cells, the 

 immature eggs, and connected with the vulva or opening in the 

 side of the body. Or we may regard it as two tubes, one extending 

 forward and the other backward from the opening. 



In the male, meantime, the sexual organ has also developed into 

 a long tube, which however has no special outlet of its own but 

 opens directly into the rectum just in front of its opening at the anus. 

 This male organ is the testis, and in it the small round spcrjiiatozoa 

 are developed. In the extremity of the intestine, just above the 

 anus, there develops in the male a two branched, curved, sharp 

 pointed spicule, which can be protruded from the anus and serves as 

 an aid to copvilation. For the same purpose there is also formed in 

 the male a hood like expansion of the tail called the bursa. . Plate 



