306 SYNOPTIC COLLECTION. 



The body is segmented. Late in life 1 each segment 

 may become ringed, making it difficult to determine the 

 exact number of segments, but the secondary rings are 

 superficial, not being represented by muscular partitions 

 in the body cavity. 



The two ends of the body are similar, since the forward 

 end is not differentiated into a head and there are no 

 tentacles nor eyes. The mouth leads into a proboscis 

 which is not armed with teeth. 



The worm is well supplied with locomotive organs in 

 the form of hooked setae which extend, with the excep- 

 tion of a few segments, in four double rows from one end 

 of the body to the other. 



The structure of the earthworm is an illustration of 

 specialization by the suppression of parts. The cause 

 for this suppression is found in the habits of the animal. 

 No longer swimming freely in the sea like its early ances- 

 tors, it has for some reason or other taken to crawling 

 through the earth. In such a situation what use is there 

 for paddles, eyes, or tentacles ? Eating soft, partly de- 

 cayed organic matter in the earth, what use has the worm 

 for horny teeth like those of its marine relative, the 

 greedy Nereis ? 



The forward end of the body is pointed for tunneling, 

 and the setae with the supplementary muscles can easily 

 pull the long body through the earth. Thus the worm is 

 adapted to its environment and it is seen that the adap- 

 tive organs are secondarily acquired. 



The earthworm is an hermaphrodite, although self-fer- 

 tilization does not take place. The worms mate and the 

 breeding season is indicated by the swollen clitellum or 

 saddle (see No. 759 and also No. 760, specimen on the 

 right; these preparations were made according to Sem- 

 per's method). The development is accelerated as we 

 have shown, and in this way the Oligochaeta differ from 

 the Chaetopoda. 



1 Cambridge Nat. Hist., II, 1896, p. 394. 



