24 
Guide to Crustacea. 
Wall- 
cases 
Nos. 1-f 
Wall- 
cases 
Nos. 1— ; 
the secondary sexual characters of the crabs. The details of the 
modifications are explained at length in the labels accompanying 
the specimens, and need not he recapitulated here ; but it may be 
said in general that the characters distinctive of either sex, e.g., 
the large chelipeds of the male, or the egg-carrying appendages of 
the abdomen in the female, become reduced in infected specimens, 
and that in some cases the male may even assume the characters 
of the female, although it would appear that females never take on 
distinctively male characters. 
ADAPTATION TO ENVIRONMENT. 
The remaining specimens in Wall-cases 1-6 will, for the most 
part, be referred to in describing the systematic series to which 
they properly belong. A number of exhibits, however, attempt 
to reconstruct the natural environment of the animals, and may 
conveniently be mentioned here. It is, of course, very hazardous 
to attempt to apply theories of “protective resemblance” to explain 
the characters of animals that are preyed upon by, and in turn 
prey upon, organisms, of which the sense-organs differ widely 
from our own ; but it is at all events certain that — to human eyes — 
the slender thread-like Caprellids are extremely hard to detect 
among the branches of the Hydroid zoophytes to which they cling 
(Wall-case No. 4), and that it is very difficult to sort out the little 
pebble-like Ebalia (Wall-case No. 6) from the gravel brought up 
by the dredge. Still more effective are the disguises assumed by 
certain crabs of the tribe Oxyrhyncha, and illustrated by the 
specimens of Macropodia, Maia, and Eyas in Wall-case No. 6. In 
these crabs the surface of the body and limbs is covered by a mass 
of living seaweeds, sponges, and zoophytes, which render the 
animals almost invisible when they crouch motionless at the 
bottom of a rock-pool. It has been found that when this covering 
is removed artificially, or when after moulting the surface of the 
body is clean, the crab actually plants little fragments of seaweed 
and the like on its own back. The fragments are held in place by 
hooked hairs on the surface of the body, and they continue to grow 
and thrive in their new position. 
