178 THE HARMONIES OF NATURE. 



Decapoda the branchiae 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 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 which, from its limiting the amount of 

 evaporation, prevents the drying of the branchiae the 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 lobsters. Some of them even live habitually out of water ; 

 and, to fit them for this terrestrial life, their respiratory caverns 

 are provided with folds and lacunae, capable of serving as re- 

 servoirs of a certain quantity of water, or with a spongy mem- 

 brane equally well calculated to store up the fluid necessary to 

 keep the organs of respiration in the state of humidity neces- 

 sary to enable them to perform their functions. It is well 

 known, too, that the land-crabs never remove far from damp 

 and shady situations, where the moisture of the sultry air re- 

 duces evaporation to its lowest degree. While in fishes the 

 water that serves for respiration flows from the front backwards, 

 so as not to impede their motions, the stream of water traversing 

 the gills of the crustaceans is made to flow from behind forwards, 

 and thus harmonises perfectly with their retrograde or sidelong 

 movements. So wonderfully has the anatomical structure of 

 these animals, like that of all other living things, been suited to 

 their peculiar mode of life ! 



The same beautiful adaptation of means to end strikes us on 

 examining the locomotive apparatus of the various tribes of 

 crustaceans. Thus in the Grecarcini or Land-crabs, and particu- 

 larly in the Ocypoda or Sand-crabs, which inhabit the seashores of 

 warm climates in both hemispheres, the legs are extremely strong 

 in comparison to the weight of the body, and consequently able 

 to carry it along with great rapidity. In the burrowing Hippidae 

 they are short, thick, and awkward, but well formed for work- 

 ing in the sand, like those of the mole. In the Sea-spiders we 

 find them extremely elongated, so that the animal swims badly, 

 and is a very indifferent pedestrian. But its mode of life, which is 

 strictly confined to the shallow waters of the littoral zone where, 



