342 HISTOLOGY 



usually mesodermal structures. They all (except the ascidians) have 

 effected a subsequent and secondary relation to the ectoderm. 



These tissues are always epithelial. One face, the proximal surface, 

 of an excretory epithelium is directed toward the fluids from which 

 waste products are being taken; the other face forms the surface of a 

 retaining or conducting cavity. These tissues form, therefore, sac-like 

 or tubular organs. In the ascidians the renal epithelium is a vestigial 

 coelomic epithelium. Into this blind space waste products are excreted 

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

 liver the waste products to the exterior through nephridial pores and 

 ducts. In all the simpler forms where the nephridial tubules have a 

 small lumen, the latter is intracellular. In invertebrates where the lumen 

 becomes large, and in all vertebrates, it is intercellular. 



The character of the fluids with which nephridial tubules are func- 

 tionally associated, and the manner in which blood is brought to them, 

 has much to do with their structure. Tubules associated with inter- 

 cellular fluid, simple lacunar blood supply, or with certain ccelomic 

 fluids, have usually two distinct regions. These two regions are consti- 

 tuted by a system of excretory tubules and by terminal excretory cells, 

 of a peculiar type. In these forms the lumen is usually intracellular. 

 In the flat-worms we have such a nephridial system associated with a 

 simple collecting medium. In the nemerteans the nephridia, though 

 associated with a circulating blood, are fundamentally like those of the 

 flat-worms In the rotifers similar nephridia are associated with a 

 ccelomic fluid. On the other hand, there are no specialized end cells 

 in the simple nephridia of the nematods. This latter may represent 

 a case of retrogression. We shall later examine, as an example of these 

 simple nephridia associated with the simpler collecting fluids, the ne- 

 phridia of the tapeworm commonly found in the intestine of the robin. 



Tubules bathed in a blood supply are blind and have a uniform struc- 

 ture throughout their extent. We shall take as an example of this type 

 of nephridia the tubule of the insect. 



Certain forms in which the nephridia have a coelomic fluid from which 

 to take excretory fluids and a general distribution of capillaries over their 

 walls have uniform structure throughout their extent and end blindly. 

 In connection with the work they have to do on the ccelomic fluid 

 their blind ends bear a group of peculiar terminal cells similar to the 

 terminal cells that in lower animals act upon ccelomic and other simple 

 collecting fluids. Page, Goodrich, and others described such nephridia 

 in various polychaetes. We shall take Eulalia viridis Mull, for an ex- 

 ample. 



Certain tubules with a general capillary supply, and having no par- 

 ticular direct relation to the ccelomic fluid, are simple and undifferen- 



