116 



GENERAL PRINCIPLES OF ZOOLOGY. 



fig. 67). 



The canals begin with closed tubes, which are provided 

 internally at the end with a bundle of 

 actively vibrating cilia, the ' flame ' (fig. 

 68). One or more main trunks lead from 

 the canal system to the exterior. A little 

 before the external opening (excretory 

 pore) there is frequently a contractile 

 enlargement, the urinary bladder. 



With the appearance of a coelom there 

 is a central place for the collection of 

 excreta. The nephridia or segmental 

 organs usually simple tubes (rarely 

 branched) open at both ends lead from 

 this to the exterior. One opening is ex- 

 ternal (fig. 69), the other communicates 

 with the co3lom by means of a ciliated 

 n 



FIG. 67. 



FIG. 68. 



FIG. 67. Distomum hepaticum with water-vascular system. (From Hatschek.) 



jp, porus excretorius ; o, mouth. 

 FIG. 68. Blind end of one of the finest water-vascular canals (k) of a Turbellarian. 



(From Lang.) n, nucleus ; /, processes of the terminal cell ; it 1 /, ' flame ' of the 



terminal cell ; u, vacuole. 



funnel, the nepJirostome, a wide mouth with active cilia which 

 connects with the canal of the tube. Through this the ex- 

 cretion (in annelids peritoneal cells laden with guanin the dis- 

 integrated ' chloragogue ' cells) is carried to the outside. 



The excretory organs (kidneys) of vertebrates are derived from 

 such nephridia. The fact that in the embryos (and frequently in 

 the adults) these open into the coelom by nephrostomes makes it 

 probable that also in the vertebrates the coelom was once important 

 in excretion (fig. 70). The increasing importance of the blood- 

 vessels which envelop the nephridial canals and bring to them 

 the waste matter taken from the tissues is probably the cause of 

 the loss of connexion of the kidneys with the coelom by degenera- 



