CRUSTACEA. 559 



CRUSTACEA. 



HAVING now completed our brief survey of the insects, we proceed to the CRUSTACEA, 

 a very large class, in which are included the lobsters, crabs, shrimps, water-fleas, and a 

 host of other familiar beings. Even the Cirrhipeds, popularly known under the name 

 of Barnacles, are members of this large class, and a number of curious animals, which 

 have until lately been classed with the spiders, are now ascertained to belong to the 

 Crustacea. 



These beings can be easily separated from the insects on account of their general 

 structure, the head and throat being fused into one mass, called technically the cepha- 

 lothorax ; the number of limbs exceeding the six legs of the insects ; and the mode of 

 breathing, which is by gills, and not by air-tubes. As a necessary consequence of the 

 last-mentioned structure, the Crustaceans possess no spiracles, such as are found in all 

 the stages of insect life, from the larva to the imago. They undergo a well-marked 

 metamorphosis, and in those creatures whose development is best known, the change 

 of shape is so entire as to have led the earlier zoologists to consider the undeveloped 

 Crustacea as separate species. They may be also distinguished from the spiders by 

 the presence of a series of feet, or rather of locomotive organs arranged under the 

 abdomen, as well as by the metamorphosis of their earlier stages, a phenomenon which 

 is not known to take place among the spiders. 



The name of Crustacea is sufficiently appropriate, and is given to these creatures on 

 account of the hard shelly crust with which their bodies and limbs are covered, a cov- 

 ering which, in some cases, is of such flinty hardness as to be, used for the purpose of 

 sharpening knives, and in others, attains a glossy polish which reminds the observer 

 of glazed porcelain. 



As our space is rapidly diminishing, we must proceed at once to the different families 

 and genera, simply noting the more important characteristics as we proceed through 

 the class. 



The first section of these creatures are called the Podopthalmata, or Stalk-eyed Crus- 

 taceans, because their eyes are set upon footstalks. The first order is that of the Ten- 

 legged Crustaceans, so called on account of the five pairs of legs that are set in each side. 

 These are exclusive of the complicated apparatus of the mouth, and the jaw-feet which 

 guard its entrance. The Crabs are placed first in the list of Crustaceans, and are techni- 

 cally called Brachyura, or Short-tailed Crustaceans, because their tails are of compara- 

 tively small size, and are tucked under the large shielded body. In the preliminary stages, 

 however, the Crabs have tails as proportionately long as those of a lobster or a cray fish. 



IN the accompanying illustration may be seen two examples of the first family of 

 these animals, being a group of Crustaceans distinguished chiefly by the singular form 

 of the carapax or upper shell, which is wide and abrupt at the base, but is prolonged 

 in front, so as to form a long and pointed beak. In all these creatures the legs are long 

 in comparison to the body, but in the LEPTOPODIA, which is seen in the lower part of 

 the engraving, they are of such inordinate length, as to remind the observer of the round- 

 bodied, long-legged harvest spider, which scuttles over the ground so rapidly when 

 disturbed. On account of this great length of limb and small size of body, these crabs 

 are often called Sea Spiders. The eyes of the Leptopodia are rather large, and not 

 retractile. It may here be mentioned that the eyes of Crustacea bear some resemblance 

 to those of insects, being compound organs, with a large number of facets, some square 

 and some hexagonal. The eyes of the common shore crab or the shrimp afford excel- 

 lent examples of this structure. 



The generic name of the Leptopodia is very appropriate, being composed of two 

 Greek words signifying " slender legs." It is a native of the West Indies. 



The upper figure in the illustration represents one of the British Crabs. In the 

 Stenorhynchus, the projecting beak is proportionately shorter than in the preceding 

 genus, is cleft at the tips, and very sharp. The fore limbs, which are furnished with 

 large claws, are stout and strong. 



