CRUSTACEA 389 



CRUSTACEA 



The Sea-spiders and the King-crabs form connecting links 

 between the previous group and the Crustacea. The Crustaceans 

 of course belong to the kingdom of the Invertebrates, and to the 

 sub-kingdom of the Arthropoda. The class is named Crustacea, 

 because the species vs^hich it comprises are almost invariably- 

 encased in a hard unyielding "'crust" or shell, which covers body 

 and limbs alike, with the exception of the Hermit-crab group, 

 whose soft bodies have to be protected by the adoption, in place 

 of armour, of any mollusc shell that happens to be handy. They 

 have generally ten well-developed limbs, sometimes more; but as 

 these are variably feet, or feeders, or jaws, or claws, it is difficult 

 at times to say how many feet, or how many limbs or organs a 

 Crustacean possesses. 



It is easy, however, to follow one characteristic feature of classi- 

 fication, and that is as regards their tails. Some have long tails, 

 as, for instance, the lobsters; and some have tails scarcely worth 

 mentioning, as in our sea-side friends the crabs. So there is a 

 short-tailed group and a long-tailed group. 



The crabs proper include many species — the edible crab, land- 

 crabs, and a great host of others which much resemble each other 

 in general build. The lobster group embraces, besides true lobsters, 

 prawns, shrimps, hermit-crabs, crayfish, &c. 



Crustacea are mainly inhabitants of seas; but some, as for 

 instance crayfish, live in fresh water, while some have successfully 

 assumed a terrestrial life and make forest-land their home. Besides 

 those mentioned, common wood-lice, sand -hoppers, fresh -water 

 shrimps are all included in this class ; as are also barnacles, and the 

 almost microscopic water-fleas. 



We may take the common lobster as an excellent type of this 

 bony-clad group. 



