On the other hand larger numbers seem to arrive at maturity in the cocoons of _Allolobophora_. The more extensive clitellum must produce a relatively larger cocoon, and it is interesting to note that the cocoon of the widely distributed genus _Criodrilus_ (Europe and South America) is very long although not of great diameter. However the facts are not sufficiently great to dogmatise much upon this subject. Another conceivable reason for differences 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. [Illustration: Fig. 12. Ventral view of _Pheretima solomonis_ to show papillae which are to be compared with those of fig. 13. (x 2.)] [Illustration: Fig. 13. Ventral view of _Pheretima sedgwickii_. (x 2.)] CHAPTER IV SENSE ORGANS AND SENSES OF EARTHWORMS