CHAPTER 9 



REPRODUCTION AND 

 DEVELOPMENT 



1 . Introduction 



The reproductive processes of leeches show interesting speciaHza- 

 tions when compared with those of earthworms and other anneUds. 

 Internal fertilization is general and in the glossiphoniids there is 

 a well developed pattern of brood care. The method by which 

 fertilization is achieved in most families is remarkable, for the 

 sperms are enclosed in a spermatophore which is then attached to 

 the body wall of another leech. The sperms make their way, by a 

 process not fully understood, through the body wall into the 

 coelomic sinuses and thence to the ovaries. Brood care often 

 involves the attachment of each of several dozens of embryos to 

 the ventral body wall of the parent by a curious ectodermal ball- 

 and-socket joint and the parent may hold the embryos under her 

 body for many weeks, passing a current of water over them by 

 gentle dorso-ventral undulations. 



The leeches about which we have most information, the fresh- 

 water leeches of temperate climates, usually begin to breed in 

 spring or early summer. Breeding condition is indicated by the 

 differentiation of the clitellum or by the fact that stores of eggs or 

 sperm are visible through the ventral body wall. In Britain 

 Glossiphonia complanata, Helohdella stagnalis and Erpobdella 

 testacea first come into breeding condition in March, E. octoculata 

 in May and Theromyzon tessulatum in lune (Mann, 1951; 1953b; 

 1957a; 1957b; 1961). There has been no experimental study of 

 the factors controlling the reproductive cycle but from field 

 observations it is fairly clear that there is a correlation between the 

 environmental temperature and the onset of breeding. Not only 

 is the breeding time of a particular species later in more northerly 



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