THE CRUSTACEANS. 253 
exception of the land Onisci, respire in the water. The perfect insect 
undergoes no further change; the Crustacean, on the contrary, increases in 
size with every successive year. The Crustacean possesses a heart, which 
propels the blood after it has been aérated in the gills; in the insect the 
circulation of the blood is by no means so highly organized. No insect has 
more than six legs, no Crustacean less than ten, 
The centipedes respire air like the insects, and are distinguished by their 
elongated form of body, and the great number of their legs, far surpassing, 
in this respect, the most richly-endowed Crustacean. 
Spiders, and particularly scorpions, have undoubtedly the greatest out- 
ward resemblance to the Crustaceans ; but all spiders have only eight legs, 
and are generally provided with eight eyes; while the Crustaceans have 
only two of these organs of vision, which, in the higher species, are gen- 
erally fixed on stalks. The claws of the crab or lobster are properly fore 
feet, and serve for creeping, or the seizure of prey; while the claws of the 
scorpion are nothing but peculiarly-formed feelers, which do not in the least 
contribute to locomotion. Besides, the scorpion inhabits the dry land, while 
the Crustacea, with the exception of a few species that dwell in humid places, 
inhabit brooks and rivers, but principally the ocean, where their legions are 
found along the coasts, or people, far from any land, the deserts of the 
high seas. 
The respiratory apparatus of the Crustaceans exhibits many interesting 
particulars. In some of the lower orders it is seated in the legs, whose 
extremely thin and delicate teguments allow the complete aération of the 
blood. To move and to breathe are with these nimble animals one and _ the 
same thing. In others the branchiw appear in the form of floating feathery 
plumes, or as membranous vesicles attached to the basis of the fore feet. 
In the most developed Crustaceans, finally, the crabs and lobsters, they are 
enclosed in two chambers, situated one at each side of the under surface of 
the carapace, or broad, shelly plate, which covers the back of the animal. 
Each of these chambers is provided with two openings, one in the front, 
near the jaws, the other behind. In the long-tailed species, the posterior 
opening is a wide slit at the basis of the feet; in the short-tailed kinds, a 
small, transverse aperture before the first pair of feet. By means of this 
formation the short-tailed crabs, like those fishes that are provided with a 
narrow opening to their gill-covers, are enabled to exist much longer out of 
the water than the long-tailed lobsters. Some of them even spend most 
of their time on land, and, still better to adapt them for a terrestrial life, 
the internal surfaces of the branchial caverns are lined with a spongy tex- 
ture, and the gill-branches separated from each other by hard partitions, so 
NO. XVII. 85 
