388 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



cocoon, there can be no doubt that in Tubifex, as in other 

 Oligochaeta, the spermatophores are passed into the cocoon 

 with the eggs, and are there dissolved to set the sperma- 

 tozoa free. The rest of the cocoon is filled with a clear, 

 colourless fluid, probably albuminous in nature. It seems 

 to depend largely on the number of eggs in a cocoon as to 

 whether they all develop into embryos or not. I have 

 been able to count as many as nine young worms in a 

 cocoon, all of which hatched out, but never more than that 

 number ; so it is probable that, as a rule, all the eggs in a 

 cocoon develop, but when the number is unusually large 

 certain only of them develop, the others breaking down. 

 The young worm when hatched exactly resembles the adult 

 in external form, and, with the exception of the reproduc- 

 tive organs, all the organs of the body are well represented. 

 The newly-hatched worm is about a quarter of an inch 

 long, and consists of 30 to 35 segments. The alimentary 

 canal is complete with the exception of the anus, 

 which does not develop until later. The whole canal 

 contains a large number of yolk granules, the remains of 

 those found in the egg, and there are some also in the 

 coelomic cavity. The dorsal and ventral blood vessels 

 are visible, the former already slightly contractile. The 

 setae are perfectly developed, but small in size, and there 

 are not more than two or three in each bundle in the 

 anterior segments, and one more posteriorly. 



PARASITES. 



Tubifex rivulorum is a host for several internal 

 parasites, and in addition to these it often has attached to 

 the body wall externally Yorticellae and Fungi. 



The Yorticellae are not true parasites, for they all 

 possess an active circlet of cilia at the distal end and a 



