152 THE VASCULAR SYSTEM 



Into each contractile vacuole a radiating series of drainage channels 

 lead. The channels are filled as the contractile vacuole discharges 

 its contents. In the higher animals tissues, selected for the secre- 

 tion of urates, form the nephridia. In the lowest Metazoa, so far 

 as is known, any surface cells may take on nephridial functions. The 

 higher forms of nephridial tissues are usually mesodermal structures. 

 These tissues are always epithelial ; one face of the epithelium is 

 directed towards the fluids from which waste products are being 

 taken, the other face forms the surface of a retaining or conducting 

 cavity. In the Ascidians the renal epithelium is a vestigial cuelomic 

 epithelium. Into this blind space waste products are excreted 

 and stored as solid particles. All other nephridial sacs or tubules 

 deliver the waste products to the exterior through nephridial pores 

 or ducts. In all the simpler forms where the nephridial tubules 

 have a small lumen, the latter is intracellular. In invertebrates 

 where the lumen becomes larger, and in all vertebrates it is inter- 

 cellular. 



The fluids from which the waste products are taken may be 

 intercellular fluid, ccelomic fluid or blood. Intercellular fluid and 

 ccelomic fluid when associated with nephridia bathe them on their 

 proximal surfaces. The blood supply is effected in two ways. In 

 a few types the nephridial tissues are merely bathed in the blood 

 (Insecta). Blood is usually supplied to the nephridial tissues 

 through the capillaries of a circulatory system. In the simplest 

 tissues there is but an ordinary supply. In the vertebrates there 

 is a general capillary supply as well as a terminal supply. The 

 terminal capillary structure is a more or less distorted plexus 

 which is supported upon a connective tissue framework at 

 definite terminal regions of the nenhridial tissues, forming a 

 "glomus." 



We may rest assured that the nephridial tissues act in 

 much the same way in every class of animal, viz. form vacuoles 

 containing granules of excretory substances and expel the con- 

 tents of these into tubules. Examination of the kidneys of 

 hibernating and thirsting animals show evidence of secretory 

 granules and vacuoles forming in the resting kidney. These 

 disappear when diuresis is established. There is no question of 

 filtration pressure in the excreting vacuole of Paramcccium, none 

 in the case of Insecta where the nephridia lie in a bath of blood, 

 and consideration of the structural conditions which pertain in 



