242 PHYSIOLOGY OF THE INVERTEBRATA. 



the present chapter will be chicjlij confined to a description 

 of excretion by means of renal or urinary apparatuses. 



Besides the waste products there are several useful products 

 of katabolism termed secretions — such as saliva, digestive 

 juices, &c. The principal secreting organs* of the Invcrte- 

 hrata have already been described in the chapters on diges- 

 tion ; but it may be remarked that in reality the secreting 

 glands are at the same time organs of excretion. All of 

 them take from the blood, or nutritive fluid, water and salts, 

 substances to which they offer a passage without in any 

 respect changing them. In addition, however, they form, at 

 the expense of the sanguineous materials, a special nitro- 

 genous product, the principal agent of these chemical trans- 

 formations being the epithelial cells. Such a product is a 

 secretion. 



When neither the epithelial cells nor the walls of a gland 

 exercise any modifying action on the materials of the blood 

 or nutritive fluid, but simply act as a filter, offering a passage 

 to certain substances and refusing it to others, there is merely 

 excretion. 



The excretory organs are never closed, for they always 

 pour externally the humour which they filter. This humour 

 is a dead product — the residuum of nutrition — whose expul- 

 sion is necessary to preserve life. The excrementitious 

 humours, of which the urine is a typical example, are solely 

 constituted of water holding in solution certain salts, and 

 crystallisable nitrogenous substances, which, forrced in the 

 anatomical elements themselves by disassimilation, pass first 

 of all into the blood or its representative, whence they are 

 extracted and excreted by various mechanisms which in the 

 higher forms are known as glands. 



The organs specially concerned in the elimination or 

 excretion of nitrogenous substances are termed kidneys ; 

 and in the InverteJjrata the form of these organs varies 



* Special organs of this nature will be alluded to later in the present 

 chapter. 



