CARNIVOROUS CRUSTACEA AND KING-CRABS 137 



in function, though not in the details of structure, of the gizzards 

 of birds, crocodiles, some insects, and certain molluscs. 



The Common Prawn [Palcsmon serratus) is not unlike a 

 minute lobster in structure, and naturally hunts down smaller 

 prey. The Common Shrimp (jCrangon vulgaris) is still smaller 

 than the prawn, to which it has a general resemblance, differing, 

 however, in many details of structure. There are no pincers, 

 properly so-called, but the corresponding limbs are converted 

 into grasping organs by the device already exemplified among 

 various air-breathing Arthropods (pp. 114, 117); that is to 

 say, the end-joint can be folded back upon the rest of the limb. 

 Lobsters, Prawns, and Shrimps are all members of the Large- 

 tailed Decapods, this region of the body being used like a fin, 

 enabling the animal to dart rapidly backward through the water. 

 Related forms are also to be found in fresh water, of which the 

 most familiar is the Common Crayfish {Astacus fluviatilis), which 

 is a notable scavenger, not limited in its diet, however, to animal 

 matters, though it would seem to prefer food of that kind. It 

 is said to make foraging expeditions in the neighbourhood of 

 its native river, seizing worms, insects, snails, and any other 

 small animals that happen to be available. Tropical rivers are 

 the home of monstrous Prawns, such as the Jamaica Prawn 

 {Palcsmon Jamaicensis), found both in the West Indies and 

 Central America, and remarkable for the enormous development 

 of its pincers. 



Returning to marine Long-tailed Decapods, we find an in- 

 teresting species in the spiny-looking Rock- Lobster {Palinurus 

 vulgaris) (see vol. i, p. 412), in which the seizing limbs are con- 

 structed on the same type as those of the Shrimp. The value 

 of the tail as a swimming organ is here diminished by the fact 

 that the tail -fin is quite soft, and this creature is commonly in 

 the habit of "sitting" in wait for prey, with its soft fin pushed 

 into a crevice among the rocks for protection. This case leads on 

 naturally to the interesting forms known as Hermit- or Soldier- 

 Crabs, which are commonly included in the " Crabs " or Short- 

 tailed Decapods, but are better considered as intermediate between 

 these and the lobster-like forms. Taking such an example as the 

 Common Hermit-Crab [Pagurus Bernhardus) (fig. 403), we shall 

 find that it uses as a dwelling the shell of a gastropod mollusc, 

 and is markedly asymmetrical in accordance with the shape of its 



