NEPHRIDIAL TISSUES 343 



tiated throughout their course. Such an example we have in the mollusk, 

 Sycotypus. 



Tubules associated with a more complex supply tend to become 

 differentiated into distinct regions. In the highest forms where the 

 capillaries form terminal glomi, we have developed the tubule charac- 

 teristic of all vertebrates. We shall take as examples the various com- 

 plex nephridial tubules of Lumbricus, Homarus, and Iguana. 



In certain vertebrates a group of tubules have in common one 

 glomus. In these cases the tubules end with more or less expanded 

 walls which open into the ccelom. These open, expanded ends always 

 have strong cilia which are directed from the ccelom toward and into 

 the lumen of the tubule. Such a terminal structure is known as a ne- 

 phrostome and it may remove solid as well as fluid substances from the 

 ccelomic fluid. Among the invertebrates we shall take as examples 

 of nephrostomes of simple, intermediate, and complex structure those 

 of Polygordias neapolitanus, Perichceta malamaniensis Benh., and Lum- 

 bricus herculeus, respectively. The nephrostome of Ammocotes will 

 be taken as an example of the vertebrate nephrostome. 



In connection with the nephrostomes, especially among the inverte- 

 brates, interesting accessory structures are developed. The general 

 ccelomic epithelium may be considered in some forms as excretory in 

 function. Such is clearly the case, according to Schaeppe ('93), in the 

 ccelom of the annelid, Ophelia. The peritoneum of this worm elaborates 

 chloragogen in the form of granules of guanin. In the earthworm the 

 peritoneal epithelium gives off cells which become amoeboid and have 

 the power to elaborate excretion granules. Among certain mollusks 

 the pericardial epithelium, which is a ccelomic structure, becomes in- 

 tensified by the formation of glands known as the pericardial glands. 

 These give off wandering cells or amcebocytes, which, after they have 

 elaborated their concretions, find their way out through a canal known 

 as the reno-pericardial canal. This latter canal, therefore, is functionally, 

 if not morphologically, a nephrostome. As examples of such accessory 

 renal structures we shall take the ccelomic epithelium and the wandering 

 cells of Lumbricus and the wandering cells of the pericardial gland of 

 Unio. 



The function of highly modified ccelomic cells called chloragogenic 

 cells which are found investing parts of the alimentary canal of many 

 invertebrates is yet to be satisfactorily demonstrated. According to 

 Ladreyt ('04) these cells in Sipunculus modus Linn, excrete uric acid. 

 For convenience they will be described in this connection. As an ex- 

 ample we shall take those found about the intestine and large blood 

 vessels of Lumbricus. 



Also as a matter of convenience we shall here describe as a type of 



