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 globe. 
