ANDREWS: ANNULUS VENTRALIS. 
477 
12. The above five species were taken at random and in each were 
found specific differences in the direction of the pocket, whether more 
or less longitudinal or transverse, in the character and amount of its 
bendings, in the form (when present) of a vestibule and recess, as well 
as in the previously known characters of external sculpturing. In 
each species there are also individual differences in all these characters. 
13. The “lowest” species examined has a simple annulus and the 
“higher” species have more complex annuli. 
14. The employment of a sperm receptacle in Cambarus is a 
higher specialization than the mode of indirect sperm transfer that 
had been described in other crayfish, some European species of Astacus. 
We may suppose the use of the annulus as a sperm receptacle to have 
been gained during the evolution of Cambarus from Astacus in North 
America. 
15. With the evolution of some sixty specific forms of sperm¬ 
transferring organs in the males there seem to have been evolved as 
many corresponding forms of annuli. Whether the male organs are 
so closely adjusted to their corresponding annuli as to prevent crossing 
between species remains for experiment to determine. 
16. The nearest approximation to the annulus of Cambarus is 
found in the sperm receptacle of Homarus americanus ; and that 
would seem to have been acquired independently. 
17. In four of the five species examined, both right- and left-handed 
annuli were found. This inversion of symmetry in the annuli of 
different individuals may well be general in Cambarus. 
18. Assuming the right and left annuli to be comparable to the 
reversed spiral symmetry in some molluscs, which Conklin ascribes 
to changes in the egg, we may tentatively regard both right- and left- 
handed annuli of each species and also the special shape of annuli in 
all species as having arisen from sudden germinal changes affecting 
both sexes harmoniously. The forms of annuli and the annulus 
itself would then have appeared as mutations. Experiments in cross¬ 
breeding should aid in deciding if natural selection has been a factor 
here or not. 
