EXCRETION 281 



nitrogenous waste products from protein catabolism include unchanged 

 amino-acids, and urea, uric acid and trimethylamine oxide. Nucleic acid 

 metabolism yields purines, which may be excreted in this form, or undergo 

 further deamination leading to uric acid, urea or finally ammonia. Por- 

 phyrin metabolism yields still further nitrogenous end-products, which are 

 sometimes conspicuous because of their coloration. 



Nearly all animals show some degree of ionic regulation, and maintain 

 the internal milieu constant by processes of differential absorption and 

 excretion of specific ions. Most marine invertebrates are isosmotic with 

 sea water, but in estuarine environments species with hyperosmotic body 

 fluids may have to pump out excess water which tends to flow into the 

 organism, in addition to that produced by the oxidation of foodstuffs. 

 Marine vertebrates are usually hypo-osmotic to sea water (except 

 elasmobranchs), and can utilize some of this metabolic water to reduce 

 anisosmotic hazard. Animals are normally efficient in conserving 

 monosaccharides produced by the hydrolysis of more complex carbo- 

 hydrates. 



There are instances in which the animal casts off part of its body at 

 intervals. In the Polyzoa the polyp periodically degenerates, forming a 

 compact brown body which is evacuated through the anus when a new 

 polyp is regenerated. Although sometimes considered a device for getting 

 rid of accumulated excretory products, it must be confessed that the 

 significance of this behaviour is unknown. Crustacea moult at intervals, 

 casting off their exoskeleton, with resultant loss of chitin. 



NITROGEN EXCRETION 



In this chapter we shall be concerned chiefly with nitrogen excretion. The 

 principal nitrogenous end-product among marine invertebrates is ammonia, 

 a highly toxic substance which must be eliminated rapidly. In these 

 animals the problem of nitrogenous excretion is greatly simplified by the 

 existence of an abundant circumambient medium for carrying away waste 

 materials. In littoral invertebrates which are partially terrestrial in habit, 

 and in fishes whose phyletic history has involved a return to salt water, 

 specialized mechanisms have developed in conjunction with osmotic 

 stress. 



In aquatic animals some part of the excretory nitrogen is lost across the 

 general body surface, especially through thin gill membranes where these 

 are present. Part is discharged through special excretory organs, kidneys 

 and nephridia, which often have other important functions as well as 

 nitrogen excretion, namely ionic regulation and osmoregulation. 



Nitrogen excretion presents three facets : the way in which end-products 

 are produced (biochemistry); the manner in which excretory products 

 are discharged to the exterior (physiology); and the relation between the 

 mode of nitrogen excretion and the environmental conditions in which the 

 animal lives (ecologic aspects). 



