GENERATIVE ORGAXS OF ECHIXODEEMA. 227 



calcareous ring*. In each tube (in S. digitata) the sperm is developed 

 on the inner surface, while the ova are formed beneath it, and when 

 fully developed form a longitudinal band which projects into the 

 lumen of the tube. There is a common duct for both sets of 

 products. If this condition must necessarily be regarded as a lower 

 one, from which the ordinary dioecious arrangements have been 

 derived, we find an interesting phamomenon in the Synaptas ; for 

 while their generative gland has retained its primitive structure and 

 primitive function, great changes have taken place, for the glands 

 have been reduced in number and degree of complication, and have 

 only one excretory duct for the whole apparatus. 



Q 2 



