LOBSTERS, CRABS, SHRIMPS, ETC.; 



CRUSTACEA. 



; AVING now completed our brief survey of the insects, we proceed to the CRUS- 

 TACEA, 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 

 cephalo-thorax ; 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 ur dergo 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 covering 

 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. 



TEN-LEGGED CRUSTACEANS; DECAPODA. 



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 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. 



