RENAL FUNCTION 249 



A terminology has been developed by the vertebrate renal physiologists 

 which suffices for their needs but which is not helpful in dealing with ani- 

 mals which do not possess the same structures, even though the analogues 

 are there. At the beginning of his monographic work Smith ( 1951 ) clearly 

 identifies filtration with the glomeruli. Further terms are linked inseparably 

 with tubules, for example: "The tubules then 1. might absorb substances 

 from the glomerular filtrate and return them to the peritubular fluid 

 (tubular reabsorption of glucose, etc.), 2. they might remove substances 

 from the peritubular fluid and discharge them into the tubular urine 

 (tubular excretion of phenol red, diodrast, etc.), 3. they might elaborate 

 new substances that could be discharged into the tubular fluid (tubular 

 secretion)." 



If we cannot use these terms it appears wise to define the terms to be 

 used hereinafter. The meaning assigned to filtration will usually be quite 

 clear. Any process assigned this name will be understood to mean trans- 

 port under some kind of pressure head through a semipermeable cell or 

 membrane from blood, hemolymph, coelomic fluid, or intracellular fluid 

 into a structure leading out of the body. Diffusion processes will, of course, 

 accompany this and following processes. In most animals such a filtrate 

 might well carry away organic and inorganic compounds of use to the 

 animal. These useful compounds, having been filtered, may be reabsorbed 

 into the blood or hemolymph. The term reabsorption will be used consist- 

 ently in this sense. The material traverses cells and it is not intended that 

 the pump be thought to be dift'erent in principle from the pumps that work 

 in the other direction. Cells of the kidney may actively transport material 

 from the blood, etc. to the lumen of a structure leading to the outside of 

 the body. Active transport in this direction will hereafter be called secretion 

 without any consideration of chemical transformations being required or 

 not. These definitions leave the term excretion as a general, nondefinitive, 

 term to encompass the sum of all these activities. The nonmorphological 

 term kidney will be applied to those specialized excretory tissues, exclusive 

 of cells of the digestive tract, which discharge wastes to the outside of the 

 body, since we are not yet ready to distinguish on a functional basis the 

 dififerent kinds of nephridia which have been described. Where an author 

 has applied a specific term to a tissue being described, his term will be re- 

 peated without morphological implications. The term blood will be used 

 merely to indicate a circulating fluid unless there is a well-recognized 

 blood system independent of a coelomic fluid or circulating extracellular 

 fluid. 



Instead of using a phylogenetic approach, I wish to discuss each of the 

 three major mechanisms in turn, drawing on the materi^^^which is avail- 

 able from any invertebrate phylum in which forms have been studied in 



