TUBIFEX. 387 



one or two could remain in the terminal portion of 

 the spermathecal duct a sufficient time to allow of their 

 being moulded to its shape. It is true that I have never 

 observed the posterior end of the spermatophore lying in 

 this position, but I have found it several times only a 

 short way along the duct and with the conical head 

 perfectly moulded. 



X. The Cocoon. 



It is common amongst all Oligochaeta for the ova and 

 spermatozoa to be deposited in cocoons, and Tubifex 

 rivulorum is no exception to the rule. The worms are 

 sexually mature in the autumn, and the cocoons are first 

 seen in November. They are deposited on the mud in 

 which the worms live, and very soon are completely 

 buried in it. The cocoons are made of a fibrous substance 

 secreted by the glandular cells of the clitellum, and have 

 a characteristic form. They are usually whitish or 

 greyish in colour and semi-transparent, but when viewed 

 with the naked eye they appear opaque, but this is due 

 to the eggs which they contain. They are usually oval in 

 shape, but sometimes become more nearly spherical, and 

 at either end they are drawn out into a short neck 

 through which, when the young worms are ready to hatch 

 out, they emerge (PI. VII, fig. 54). The neck is filled 

 with a plug of the same substance of which the cocoon is 

 formed, but it is probable that it is of a less resistant 

 nature. The number of eggs which may be present in a 

 cocoon is very variable. Sometimes there is only one, 

 more commonly from four to nine, but occasionally the 

 number is greater, and in a very few cases I have seen as 

 many as thirteen or fourteen. Although I have not 

 been able to find any spermatozoa even in a freshly-laid 



