120 A NATURALIST ON DESERT ISLANDS. 



(1) The fore-part carries certain sensory structures, 

 jaws and legs, by which the animal enters into relation 

 with the outer world. It also contains the central nervous 

 system and the stomach, both of which are intimately 

 connected with the external organs indicated above. 

 This is the only part which the animal habitually and 

 completely extrudes from the shell. 



(2) A mid-part which, when the fore-part is thrust out, 

 fills the mouth of the shell. This part is provided 

 with some modified limbs, which although helping 

 the animal to retain a grasp of the shell, are quite 

 subordinate in this respect as compared with the means 

 to be presently described. This mid-region is an important 

 one ; for it carries the main breathing apparatus, which 

 requires protection, and yet must be in free communication 

 with the air. It also contains the heart. 



(3) A hind-part, or abdomen, which contains a bulky 

 liver and the organs of generation, and also in the case of 

 the female, carries on its appendages the eggs ; which thus 

 obtain the shelter provided by the shell until the proper 

 time arrives for their further development. 



It is in this part of the body that we find an apparatus 

 by means of which in normal circumstances the hermit- 

 crab retains its hold upon its home. This apparatus 

 has been evolved through a curious claw-like modifica- 

 tion of the hindermost pair of abdominal appendages 

 (pleopods), which are situated on the last abdominal 

 segment of the animal. It may be regarded as a sort of 

 anchor ; and it is reinforced by a broad band of muscle 

 (the cable of the anchor), which is situated along the 

 ventral side of the abdomen. It serves exactly the same 

 purpose as the columella muscle of the original maker 

 of the shell, and serves it, we may add, remarkably well. 

 You can pull as long and as hard as you like on the visible 

 outside structures of the crab (always providing that you 

 take care of that awful claw) ; but you will find that your 

 efforts always result in pulling the animal in half and 

 leaving the part desired still inside the shell, before this 



