CHAPTER XVII. 



THE CRUSTACEA. 



A VERY slight acquaintance with the common objects of the 

 sea-shore, and the inhabitants of marine aquaria, suffices to 

 render us familiar with the varieties of the articulate creatures 

 which are classified as the Crustacea, Lobsters, spiny lobsters, 

 Cray fish, prawns, shrimps, crabs, spider crabs, hermit crabs, 

 sand hoppers, wood lice, water fleas, king crabs, and last, but 

 not the least interesting, the barnacles, are readily recognised 

 and distinguished by all who take the commonest interest in 

 natural history. 



The bodies of these animals are divided into segments ; the 

 skin is hard, calcareous, or horny, and the limbs are jointed, 

 attached to special rings, and are arranged in a double series. 

 The limbs, or rather the articulated extremities, are the antennae, 

 jaws, and mouth pieces, and the walking, swimming, and pre- 

 hensile legs. The nervous system is ganglionic, and is situated 

 below the digestive organs, and above and in front of them. The 

 respiration is almost invariably aquatic, and, although some spe- 

 cies can exist on dry land, still moisture is required for their 

 peculiar lung cavities. Some Crustacea have no special breathing 

 organs, but respire by means of their skin. These statements 

 recall to the mind the organisation of the true insects and the 

 spiders. The tracheae are wanting in the Crustacea, but never- 

 theless the classificatory anatomist combines these classes, otherwise 

 so united by their common structures, and with much truth, for 

 it is by no means improbable that they all had a common pa- 

 rentage soon after articulate life commenced on the crlobe. 



