I4O GENERAL PRINCIPLES OF ZOOLOGY. 



the fine network of vessels is provided with blind sacs, 

 the ends of which bear an actively-moving bunch of cilia, 

 the Flimmerlappchen, for driving onwards the contents 

 (Fig. 65). 



Nephridia. The scgmcntal organs (nephridia), on the 

 other hand, are simple tubes open at both ends ; one 

 opening leads to the exterior, the other communicates 

 with the body-cavity; between the two openings runs a 

 glandular tortuous canal. The opening into the body-cavity 

 is made through a trumpetlike ciliated widening of the 

 canal, called the ciliated funnel or nephrostome. The 

 excretory organs of the Crustacea and more certainly the 

 kidneys of the vertebrates, are to be traced back appar- 

 ently to such segmental organs (Fig. 67). The kidneys are 

 formed as a series of canals opening by the proximal ends 

 into the body-cavity, but with the distal ends emptying 

 into a single canal, the ureter. Only later does the primi- 

 tive kidney change into a compact glandular body, usually 

 with the disappearance of the peritoneal funnels. 



B. Sexual Organs. 



Sexual Glands and Outlets. In the sexual apparatus 

 of animals, there must be distinguished the areas where the 

 germinal cells are produced, the sexual glands, and the 

 outlets for these. The former are, temporarily or perma- 

 nently, present in all multicellular animals; the latter, on 

 the contrary, may be completely absent. If the sexual 

 products arise in the skin or in the walls of the digestive 

 tract, as is usually the case in the coelenterates, then 

 special outlets are superfluous, since the ripe elements can 

 reach the exterior directly by rupture of their covering or 

 by means of the digestive tract. 



Germinal Epithelium and Germinal Glands. Male 

 and female sexual cells, as we have seen, originate from 

 an undifferentiated incipient organ which is called the ger- 

 minal epithelium. Its predilection is to form a part of 



