ONTOGENY OF THE ANNULUS VENTRALIS. 137 



The right or left handed symmetry of different adult annuli is 

 first visible in the fifth larval stage when the median groove has 

 bent to the right or to the left and the accompanying folds have 

 a reversed position in the two cases. Later other outgrowths 

 may also harmonize with the groove and folds to complete the 

 two adult forms ; mirror-images of one another. 



While the sperm receptacle is a necessary organ in Cainbants 

 it is phylogenetically a new one since Canibarns is the most 

 specialized genus of crayfish and other genera have no receptacle. 

 In C. affinis this new organ is not seen till the individual has 

 reached a third stage after leaving the egg. 



Besides beginning late in ontogeny this new sperm receptacle 

 is variable in all its stages of growth. In its early and simple 

 state in the larva it resembles the adult receptacle of a less spec- 

 ialized species, C. Clarkii. 



In some of its variations of lateral bendings in early larval 

 stages it suggests adult conditions in other species. 



BALTIMORE, 



December 12, 1905. 



