NEPHRIDIAL TISSUES 341 



covering the surfaces of the body. Arriving at the surface in a me- 

 dusa, for example, these materials are cast out into the surrounding 

 water through the layer of epithelial cells which cover the surfaces. In 

 this connection we must bear in mind that the intercellular fluid carries 

 food materials as well as waste materials. These and other valuable 

 materials are not taken up by the epithelial cells to be thrown out from 

 the body. We see, then, that these cells can discriminate and select only 

 the materials which must be removed from the internal tissues and fluid ; 

 with the possession of this power they become excretory or nephridial 

 cells. These comparatively simple forms of excretory cells have not 

 been differentiated into a distinct excretory tissue; they perform other 

 functions as well as that of excretion. 



In the more active Metazoa, where a greater differentiation has taken 

 place, tissues have been specialized that perform only the function of 

 taking up waste substances from collecting fluids. These tissues have 

 the power to take enough food from the body fluids for their own nour- 

 ishment. Except in disease they take no more of the food materials 

 than this. On the other hand, they select most of the waste products 

 from the collecting media and transfer them to the exterior. This trans- 

 ference is a vital and not a mechanical process. In this respect, it differs 

 from the transfer or exchange of gases in respiratory tissues. 



Tissues specialized for the selection of urates from collecting and dis- 

 tributing fluids form the nephridia. In the higher vertebrates, the ne- 

 phridial tissues are assembled and, together with their special blood and 

 nerve supply and connective-tissue elements, form an excretory organ, 

 the kidney. 



The collecting fluids are intercellular fluid, ccdomic fluid, and 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 of animals the nephridial tissues are merely 

 bathed in the blood. The Insecta furnish a good example of this. This 

 first mode of blood supply for nephridial tissues is very unusual. Blood 

 is usually supplied to the nephridial tissues through the capillaries of a 

 circulatory system. In the simplest tissues there is but an ordinary 

 capillary supply. This becomes more highly specialized in the higher 

 forms. 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 nephridial tissues. This latter capillary 

 structure is called a glomus. 



The nephridial tissues may have various origins. We have seen 

 that in the lowest Metazoa, so far as known, any surface cells may per- 

 form nephridial functions. The higher forms of nephridial tissues are 



