372 H. s. PRATT, 



alkaline, and the action of the digestive juices surrounding it would 

 thus be neutralized. In other locations the reaction of the secretion 

 would be adapted to the nature of the fluid surrounding the worm. 

 The prevailing opinion of the function of the "Kopfdrüsen" is 

 that advocated by Leuckart (1886 b, p. 32) who holds that their 

 secretion acts as an irritant to the tissue of the host, and causes an 

 increased blood-flow, which is of benefit to the parasite. 



Excretory System. 



This system forms a prominent and very noticable feature of 

 appendiculate Distomes. The excretory pore is at the distal end or 

 apex of the appendix. From this point the excretory vesicle extends 

 forward, in the worm we are discussing, as a long, slender canal with 

 an average width of 0.018 mm, dorsad of the reproductive organs and 

 ventrad of the intestine, to the neighborhood of the acetabulum (PI. 25, 

 Figs. 2 and 3). Just back of this organ and in front of the testes, 

 it divides into two branches, each of which slightly exceeds it in width ; 

 these pass to the right and the left of the body and then to its ex- 

 treme forward end, where they turn abruptly towards the median line; 

 dorsad of the pharynx they meet, forming, thus, a loop. I speak of 

 these two branches as if they were a part of the vesicle proper. It 

 would be more in harmony with the custom of authors, perhaps, to 

 limit the application of this term to the impaired portion of the 

 system, and to call the branches and their anterior uniting ends, the 

 "collecting tubules". The entire structure, however, including both 

 unpaired and paired portions, forms a single organ and no part of it 

 diff'ers from any other part in structure or function ; I apply the same 

 name, therefore, to the whole. 



The terminal end of the excretory vesicle in Distomes is often 

 constricted to form a short vestibule, and in a number of cases a 

 sphincter muscle has been observed controlling the opening of the 

 excretory pore. An extremely delicate sphincter muscle is present in 

 this worm and also a short vestibule. This sphincter muscle, however, 

 is not to be confounded with that which controls the opening of the 

 appendicular vesicle (within pag. 355). The walls of the excretory vesicle 

 are very contractile, and the unpaired portion may assume a variety 

 of positions as the animal extends or contracts its body, or thrusts 

 its appendix in and out. The walls are apparently lined with a 

 structureless intima; I could detect no cells or nuclei or the remains 

 of them in it. Delicate muscle fibres encircle the outer surface of the 



