354 University of California Publications in Zoology [Vol. 19 



which represents the actual condition. As shown graphically this 

 accessory collecting tubule is quite long, whereas in the actual speci- 

 men it is reduced to a mere bulb, to which the capillaries join. In 

 figure 47 we find that the accessory collecting tubule (3b), to which 

 are attached capillaries 4x, 5x, and 6x, has entirely disappeared or 

 perhaps better, is stretched out between these capillaries. The same 

 condition exists in the case of the accessory collecting tubule of capil- 

 laries lx, 2x, and 3x. Near the posterior end of the ascending tubular 

 division of the bladder (le), two collecting tubules (2b and 2c) are 

 attached, one on the left and the other on the right-hand side. The 

 one on the right side (2b) receives, as shown graphically, two acces- 

 sory collecting tubules (3d, 3e), which in figure 47 are entirely absent 

 or are stretched out between Wx, llx, 12x, and 13x, 14x, and 15x 

 respectively. The collecting tubule on the left (2c) merges directly 

 into accessory collecting tubule 3f as shown graphically ; in figure 47, 

 3f has disappeared unless it is to be found between the capillaries 

 (17x and 18x) . The last collecting tubule (2d), in the graph and in 

 the living specimen is short, and is joined by accessory collecting 

 tubules (3g and 3h) .■ Thus it seems that what I have suggested as the 

 primitive pattern has been considerably modified, possibly to meet the 

 shape and needs of the cercaria or of the adult echinostome. 



The study of the development of the bladder in the cercaria of 

 Echinostoma revolutum is of peculiar interest and yielded some 

 definite results. The first appearance of the bladder or any part of the 

 excretory system was seen in a germ ball 0.134 mm. long and 0.104 mm. 

 wide (fig. 37), which had just commenced to change from the typical 

 round germ ball. At this time two widely separated tubes extending 

 nearly the entire length of the body were present. At the anterior 

 end the tubes appeared to end in flame cells which could not be clearly 

 distinguished, due to the lack of movement of their cilia. Each tube 

 clearly must function independently of the other since the two 

 excretory pores are now 0.045 mm. apart. When the embryo reaches 

 a length of 0.280 mm. the suckers are distinct ; the digestive system is 

 fairly well developed; the tail has commenced to differentiate from 

 the body, and the excretory pores are closer together, being only 

 0.018 mm. apart (fig. 38). When the embryo is 0.360 mm. long the 

 digestive system is complete. At this time the inner surface of the 

 walls of the excretory tubes, a short distance above the excretory pores, 

 nearly touch each other (fig. 39). The tail is now quite decidedly 

 differentiated from the body. At about one-fourth of the distance from 



