544 



FRESH-WATER BIOLOGY 



passing from it directly through the sheath and obhquely to the 

 body wall. 



No trace of an alimentary system has been found in the adult 

 or in any stage of development. Nutrition is thus provided for 

 entirely by absorption. 



The sexes are separate in all cases. The genital pore in both is 

 at or very near the posterior tip. The male is smaller and more 

 slender than the female and often distinguished externally by a 

 bell-shaped bursa that surrounds the genital pore. This is a mus- 

 cular fold which is held within the body except at coition and 

 may be forced out by the contraction accompanying the preser- 

 vation of the specimen. Two oval testes He usually in the center 

 of the body one behind the other. Farther back is a group usu- 

 ally of a few large cells, the cement glands. 



■( hrre - I ^ 'P 



Fig. 844. Acanlhocephalus ranae. Entire male, br, brain; bit, copulatory bursa; eg cement glands; 

 cr, cement receptacle; iMr, invertor of neck region; /, lemniscus; pr, proboscis receptacle; pt, posterior 

 testis; re, retinacula; rpr, retractors of proboscis receptacle; st, suspensory ligament; ta, anterior testis. 

 X 30. (After Van Cleave.) 



In the female a ligament extends through the center of the body 

 cavity from end to end. The ovary, which is present only in the 

 larval stage, produces great numbers of ova that later, surrounded 

 by a heavy covering of three distinct membranes, float free in the 

 body cavity. A compUcated apparatus known as the uterine bell, 

 located in the body cavity near the posterior end, performs rhyth- 

 mic contractions that discharge' from the body all well-developed 

 embryos and return to the body cavity all that are not sufficiently 

 matured. 



The life history of Acanthocephala is almost unknown. Those 

 parasitic in terrestrial hosts develop probably without any rela- 

 tion to the aquatic fauna as Gigantorhynchus hirudinaceus of the 



