136 



THE ANIMAL PARASITES OF MAN 



The EXCRETORY SYSTEM 1 consists of several parts : (i) of the 

 more or less numerous terminal or infundibular cells (figs. 70, 71) ; 

 (2) of the capillaries joined to them ; (3) of larger vessels taking up 

 the capillaries, and (4) of the excretory bladder. Terminal cells and 

 capillaries may be compared to unicellular glands with long excretory 

 ducts ; the cellular body (fig. 71) is comparatively large, stretched 

 lengthways, more rarely transversely, and provided with numerous 



processes that are lost in the parenchyma ; 

 it contains within a conical alveolus (analogous 

 with the secretory space of unicellular glands) 

 which continues direct into the structureless 

 capillary ; at its blind end is a bunch of cilia 

 which reaches the open space, and which, 

 during life, shows a waving motion (ciliary 

 movement). In the protoplasm of the ter- 

 minal cell at its blind end the nucleus is 

 situated. 



The entire apparatus begins as a cul-de- 

 sac, i.e., enclosed within the terminal cells, 

 to which may be ascribed the capacity of 

 taking up from the fluid that permeates 



the parenchyma the part to be excreted, which is first collected 

 into their own cavities and thence carried out by means of the 

 capillaries and vessels. 



The vessels possess definite walls, but some authors state that 

 these canal-like spaces taking up the capillaries are only separated 

 by a homogenous layer belonging to the parenchyma. They may 

 repeatedly reunite on either side, and again give rise to other canals 

 (GATHERING TUBES), which finally, travelling towards the posterior 

 end, discharge into the excretory bladder (fig. 70). 



The form and size of the bladder varies much according to the 

 different species, but it always possesses its own epithelium, sur- 

 rounded by circular and longitudinal muscles, the circular muscles 

 forming a sphincter around the opening. Frequently also the 

 structure of the bladder extends to the tubules discharging into 

 it, which therefore are not to be regarded as separate " vessels," 

 but rather as tubular diverticula of the bladder, directed anteriorly. 

 In some few species the diverticula also branch off -and the 

 branches anastomose, so that a network of tubules ensues which 



FIG. 71. Terminal cells 

 of Cotylogonimus hetero- 

 phyes (v. Sieb.)- 700/1. 

 Left from the side, right 

 from the surface. (After 

 Looss.) 



1 The following description relates to the Fasciolidea. 



