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 

 cephalothorax ; 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 Crus- 

 tacea 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 covering 

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

 ing 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 Crusta- 

 ceans, 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 technically called 

 Brachyura, or Short-tailed Crustaceans, because their tails are of comparatively 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 men- 

 tioned that the eyes of Crustacea bear some resemblance to those of insects, being com- 

 pound organs, with a large number of facets, some square and some hexagonal. The eyes 

 of the common shore crab or the shrimp afford excellent 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. - 



