166 ZOOLOGY 



The execretory organs are little eoiled tubes of which tjqoi- 

 cally a pair hes in each segment (Fig. 161). The inner mouth 

 of the tube opens into the body-cavity, where it sucks in parti- 

 cles floating in the body-cayity. The wall of the tube, which 

 is richly supplied with blood-vessels, extracts from the blood 

 the waste products, thus purifying it. The inner end and 

 cavity of the tul^e is lined l)y little beating hairs (cilia) which 

 maintain a movement of fluids toward the outer end of the 

 tube. It is to be noted that the excretory tubes are repeated 

 in nearly every segment, for we have seen that in the higher 

 animals — Crustacea — tubes are found in various segments 

 but performing diverse functions. This is a law of evolution 

 of organs — at first a repetition of similar ones with a common 

 function and then a reduction of the total number with a special- 

 ization of function and structure to each. 



The reproductive organs of annehds are relativelj^ simple. 

 Although the sexes are separate in Polycha?tes and in the 

 earthworm and its allies (Oligochetes) both eggs and sperm 

 are formed in the same individual ; nevertheless any egg of 

 an individual is not fertilized hy a sperm-cell of the same indi- 

 vidual. The egg and sperm-cells develop in the lining of the 

 body-cavity and when ripe, fall into the liody fluid. They 

 are sometimes pickerl up by the excretory tubes and cast into 

 the sea, where fertilization occurs ; y)ut sometimes the ripe 

 individuals break in two in ortler to allow the sexual products 

 to escape. In certain annelids the escape of the egg and sperm 

 occurs in thousands of individuals at the same hour, so that 

 the sea swarms with eggs and sperm ; thus the chances of their 

 getting together are increased. The special case of Autoljius 

 is referred to on page 187. From the fertilized eggs embryos 

 covered with cilia arise, which swim free in the sea as they 



