F. A. Potts 
49 
mature female, even in the assumption of the curious bunches of 
barbed hairs which in this case can never bear eggs. As in the spider- 
crabs so here, the female appeared incapable of the reverse change, 
and the large number of hermit crabs with typical female appendages 
and sealed genital apertures are undoubtedly to be regarded in part as 
modified males. 
1. Unmodified Male 
2. Modified Male 
3. Fully modified Male 
or normal Female 
Abdominal Appendages of Hermit Crab ( Eupagurus). 
Fig. 3. 
A protest will conceivably be uttered against the attribution of 
a special sexual significance to the development of typical swimmerets 
in the male in both spider crabs and hermit crabs. It is of course 
well-known that in the larval stages of these Crustacea biramous 
abdominal appendages are found in both sexes to be subsequently 
reduced or lost in the male. Lest this, then, be deemed a happy 
opportunity for applying the term “ reversion ” to this phenomenon 
I hasten once more to point out that when the male develops biramous 
abdominal swimmerets they are of the type associated with female 
maturity, and that the specialised nature of their nursing-hairs cannot 
well be associated with ancestral conditions. 
Both Sacculina and Peltogaster inflict sterility upon their host and 
apparently entire abortion of the gonad generally is the final conse¬ 
quence. On the external appearance of the parasite the eggs of the 
female shrink through absorption of their yolk, and the formation 
of spermatozoa is after a time suspended in the male. The testis of 
the spider crab dwindles and disappears without undergoing any 
Parasitology n 4 
