46 
Parasitism 
progressively greater in degree. In a hermit crab in which very 
similar phenomena are evoked by the presence of another parasitic 
cirripede Peltogaster, moulting is not prevented and it is easily 
observed that even those individuals in whose secondary sex characters 
little or no modification could be traced (a condition which might be 
ascribed to the shortness of the period of parasitism previous to the 
last moult), never showed at subsequent moults the least tendency to 
increase the degree of modification with the prolongation of the period 
of infection. So many similarities exist between the two cases, that 
we are probably justified in assuming that the maximum alteration of 
which the individual host is capable is established at an early stage 
of parasitism in the spider crab as well as in the hermit crab. 
Male Modified Male Female 
(swimmerets of one side only shown) 
Abdomen of Spider Crab (Inachus), under view. 
Fig. 2. 
The female spider crab possesses small flattened chelae or pincers, 
and can be also identified by the broad trough-shaped abdomen. The 
ventral surface of the thorax is much hollowed out so that between 
the thorax and the reflexed abdomen a spacious cavity exists where 
the eggs can securely run the course of their development. From the 
second, third, fourth and fifth abdominal segments spring pairs of long 
biramous appendages ( swimmerets ) the inner branches of which are 
richly provided with branching hairs, and it is to these last that the 
eggs are securely cemented. In the ways thus described the female is 
provided with such adaptations of structure as enable her to cope with 
the cares of motherhood, providing a safe attachment for the embryos 
and a brood-chamber for their protection. The male type of abdomen 
is much narrower and rather shorter and the thorax is not scooped out 
