64 EARTHWORMS AND THEIR ALLIES [ch. 



in the clitellum is as I also think is the case with 

 the genital papillae to prevent hybridisation. That 

 the sense of touch is delicate in these animals 

 seems clear from the abundant development of 

 epidermal sense-organs. It may be that the feel of 

 the clitellum during union enables two individuals of 

 a given species to come together and prevents those 

 of different species from mating. In any case there 

 is no positive evidence that hybridisation does occur 

 in this group of animals. The astounding variability 

 and yet constancy in a given species of the genital 

 papillae is in favour of regarding these organs as 

 tactile recognition marks ; and it will be noted that 

 they are not by any means characteristic of some of 

 the older types of earthworms. Furthermore they 

 are particularly conspicuous in such genera as 

 Pheretima, Megascolex etc., which possess a large 

 number of species. In these of course mutual 

 recognition would otherwise be more difficult. 



CHAPTER IV 



SENSE ORGANS AND SENSES OF EARTHWORMS 



As this is not an anatomical treatise we shall not 

 attempt any detailed anatomical and histological 

 account of the sense organs in this group of worms. 



