No. 12.—THE ANNULUS VENTRALIS. 
BY E. A. ANDREWS. 
Introduction. 
LIBi 
NEW 
BOT v 
Of the nine genera of crayfishes only one — Cambarus — is known 
to have the peculiar organ called by Hagen the annulus ventralis. In 
this genus there are some sixty-six species, as compared with about 
forty-four species in all the other genera, and each species is thought 
to have its peculiar specific form of annulus ventralis. 
The cravfishes of the genus Cambarus are widely scattered over 
North America east of the Rocky Mountains and occupy this large 
area to the exclusion of all other crayfish except for a very few species 
of Astacus which extend from the west of the above mountain range 
some distance to the east. 
The discovery of the annulus ventralis and its utilization as a speci¬ 
fic character was first made by Hagen (’ 70 ), but its physiological value 
escaped him, as was almost necessary, for he says: “I have not seen 
living specimens” (’ 70 , p. 16). The annulus is found in the female 
crayfish only and Hagen thought it fit for some sexual purpose and 
likely to be used for secretion, possibly, but not probably, for gluing 
the eggs to the abdominal appendages. 
The figures and descriptions given by Hagen and by subsequent 
authors who have made use of the annulus as a specific character 
(Faxon, ’ 85 , ’ 98 ; Hay, ’ 96 ), show only the form of the external pro¬ 
tuberances and depressions and sutures of a specialized area of the 
shell and give no idea of its internal structure, except in the first case, 
that of Hagen, whose figure of the internal side of the shell is inade¬ 
quate and whose description is but brief and fundamentally erroneous. 
When therefore it was observed (Andrews, ’ 95 ) that this organ is used 
as a sperm receptacle into which the male puts the sperm that is sub¬ 
sequently to fertilize the eggs, its great importance as a necessary 
link in the chain of reproductive organs, as an organ whose absence 
would lead to the extermination of all the Cambarus crayfish, seemed 
to demand a more thorough examination of its structure. 
In fact the annulus is here claimed to be one of those remarkable 
