UEIXAET OEOANS. 



75 



is mainly discharged by the special secretory organs. These have 

 the form of glands of a simple or complex structure which originate 

 from invaginations of the outer skin or of the intestinal wall, and 

 consist essentially of simple or branched tubes, or of racemo; e and 

 lobulated glands. 



Among the various substances which by the aid of the epithelial 

 lining of the walls of glands are removed from the blood and some- 

 times utilised further for the performance of various functions, the 

 nitrogenous excretory substances are especially important. The 

 organs by which the excretion of these ultimate products of meta- 

 bolism are effected are the kidneys. In 

 the Protozoa they are represented by 

 the contractile vacuoles ; in the Worms 

 they appear as the so-called water- 

 vascular vessels, and are constituted of 

 a system of branched canals which 

 take their origin in delicate internal 

 ciliated funnels, which open into the 

 spaces in the parenchymatous tissues or 

 i nto the body cavity. In the latter case 

 the ciliated funnels have a wide opening. 

 In the Platyelminthes (flat worms) the 

 efferent ducts of the system consist of 

 two main lateral trunks (fig. 68, Ex.\ 

 which frequently open together at the 

 hind end of the body by means of a 

 medium terminal contractile vesicle 

 (fig. 68, ep). 



In the segmented worms the paired 

 kidneys are repeated in every segment, 

 and are known as seymental organs 

 (figs. 69 and 70). The shett-ylands of 

 Crustacea are in all probability to be traced back to these segments I 

 organs : as are also the paired kidney (organ of Bojanus) of mussels, 

 and the unpaired renal sac of Snails, both of which communicate by 

 means of an internal opening with the pericardial division of the 

 body cavity. 



In the air-breathing Arthropods and some Crustacea (Orchestia) 

 the urinary organs are tubular appendages (Malpighian vessels) of 

 the hind gut. In the Vertebrata the urinary organs or kidneys 

 obtain a greater independence, and open to the exterior by special 



Fro. CS. Young Distomum (after 

 La Valette). Ex, main stems of 

 the excretory system ; Ep, ex- 

 cretory pore ; O, month with 

 sucker; <S, sucker in the middle 

 of the ventral surface ; P, pha 

 rynx ; D, alimentary canal. 



