24 



Guide to Ci'Jistacea. 



Wall- 

 cases 

 Nos. 1-3. 



tlic secondary sexual charactei^s of the crabs. The details of the 

 modifications are explained at length in the labels accompanying 

 the specimens, and need not be recapitidated 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 ENVIEONMENT. 



Wall- 

 cases 

 Nos. 1-6. 



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 dififtcult to sort out the little 

 pebble-like Ehalia (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 Hyas 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. 



