TREMATODA 



21Q 



expelled by a definite tubular system (excretory apparatus, proto- 

 nephridia, formerly also termed the water-vascular system). This 

 system, which is distributed throughout the entire body (fig. 124), is 

 symmetrically developed, and, in the ectoparasitic Trematodes, it 

 opens, right and left, at the anterior end on the dorsal surface ; in all 

 other flukes, however, it opens singly into the excretory pore (foramen 

 caudale) at the centre of the posterior border ; in those cases, however, 

 where a sucker is present at the posterior end, as in the Amphistomata, 

 the excretory pore is situated on the dorsal surface close in front of 

 the sucker. 



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

 or less numerous terminal " flame" cells or funnel cells (figs. 124, 125); 

 (2) of the capillaries ending in them ; (3) of 

 larger vessels receiving 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. 125) is comparatively large, stretched 

 longitudinally, more rarely transversely, and 

 provided with numerous processes, that are 

 lost in the parenchyma ; within is a conical 

 cavity (analogous to the secretory cavity of 

 unicellular glands) which is continued directly 

 into the structureless capillary ; at its blind 

 end is a bunch of cilia projecting into the 

 cavity, and which, during life, shows a flicker- 

 ing motion (ciliary flame). The nucleus is 

 situated in the protoplasm of the terminal cell 

 at its blind end. 



The entire apparatus thus begins blindly 

 i.e. t within the terminal cells, to which must 

 be ascribed the capacity of taking up from the fluid that permeates 

 the parenchyma the products which are first collected into their own 

 cavities and thence excreted by means of the capillaries and vessels. 



The vessels possess definite walls, consisting of a membrane and 

 a nucleated protoplasmic layer. They unite at many points on either 

 side, and again pass into other canals (COLLECTING TUBES), which 

 finally, travelling towards the posterior end, discharge into the 

 excretory bladder (fig. 124). 



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

 different species, but it always possesses its own flattened epithelium, 

 surrounded by circular and longitudinal muscles, the circular muscles 



FIG. 125. Terminal flame 

 cell of the excretory system. 

 n., nucleus of cell ; c. , bundle 

 of cilia forming the "flame"; 

 /., processes of cell extending 

 into parenchyma; </., excre- 

 tory capillary. (Stephens.) 



The following description relates in the main to the Distomata. 



