48 
Parasitism 
copulatory styles and by the possession of an abdomen which has not 
attained to the full female width. Nearly all such crabs show on 
dissection some vestige of genital gland which exhibits testicular 
structure. 
A large number of infected spider crabs occurs, however, which 
cannot be assigned without careful thought to either sex. They have 
an abdomen of the exact female proportion, copulatory styles and 
abdominal swimmerets. Moreover the closest search in the vast 
majority of cases fails to distinguish any remnant of reproductive 
gland. But in two or three of the two hundred crabs of this class 
examined by Smith there were slender ducts and attenuated glands 
of an unmistakably male character. This piece of evidence is the first 
argument for regarding the whole of the class described above as 
modified males. When in addition, we consider that while individuals 
rigidly identified as males by the character of their gonads assume 
known female characters, the reverse change never occurs, so that for 
instance a crab which on dissection showed purely female glands has 
never been found to have developed even a rudimentary copulatory 
style or to assume a swollen chela, it seems justifiable to conclude that 
these crabs of debateable sex are really the last link in the chain of 
male modification. 
In Eupagurus, a hermit crab, extreme modification also occurs as 
the result of parasitism by Peltogaster, another parasitic cirripede 
(Potts, 1906). This latter organism closely resembles Sacculina in its 
anatomy and life-history. 
The difference between the sexes of Eupagurus is shown only in 
a couple of external characters, the position of the generative apertures 
(as in all Decapods) and the character of the abdominal appendages. 
The abdomen of the hermit crab is furnished on one side only with 
a few appendages, insignificant, but with definite functions. It is in 
the female that we see the full development of the appendage as a 
swimmeret with two equal branches, the inner one provided with long 
hairs affording a secure anchorage for countless eggs while the outer 
one is of equal size in both sexes, and in both by its paddle-movement 
maintains respiration currents in the shell. No use has been found for 
the outer branch in the male and so has become quite rudimentary, 
but the effect of the parasite Peltogaster is to stimulate the growth 
of this rudiment. There is of course great variability of response to 
this stimulus but those individuals which experience the maximum 
amount of change possess swimmerets exactly similar to those of a 
