286 



Malacostraca.- 



-CRUSTACEA.- 



-Decapoda. 



these gaugliuiis the nerves are distrihuted to the differ- 

 ent organs and memhers of the body, and to the muscles 

 which move them. 



The sexes are distinct. The males have not as j'et 

 been seen in all ; but there is good reason to believe 

 that, with the exception of the cirripedes or barnacles, 

 the two sexes exist separately in all cases, though in 

 some the males differ very much in appearance from 

 the females. They always produce their young by 

 means of eggs, which, in most or all instances, are 

 fecundated within the body of the female before she 

 lays. In some genera the young are fully formed in 

 the internal ovary of the mother, and are extruded 

 alive. In many, the young at birth are like the parent, 



but in others they are very unlike, and undergo several 

 changes, amounting almost to a kind of real transforma- 

 tion, before they assume the perfect form. The Crus- 

 tacea differ from each other very much in form and 

 appearance ; they differ also very much in their habits. 

 Most of them live constantly in the water, but there are 

 some which during the gi'eater part of their life inhabit 

 the land, and are called Land-crabs. The larger num- 

 ber are carnivorous, and many even devour their own 

 species. 



For convenience sake, we divide the class Crustacea 

 into two great sub-classes, the Malacostraca and the 



E.NT0MOSTHAC.\. 



Sub-class I.— MALACOSTRACA. 



Tins name was originally applied by Aristofle to the 

 whole class in general, but by Latreille and others it 

 is used to designate only a particular division of it. It 

 contains the greater part of the large Crustaceans, and 

 more especially those which are useful to man. 



It has been divided into two great sections, the Stalk- 

 eyed Crustaceans or Podophthalma (those which have 

 their eyes placed at the tip of a footstalk), and the 

 Sessile-eyed or EDiaoPHTiiALMA (those in which the 

 eyes are not placed on footstalks) 



Section I.— STALK-EYED CEUSTACEA (roDOPHTHALMA.) 



The Stalk-ej'ed Crustacea are chiefly distinguished by 

 the possession of organs exclusively formed for the 

 purpose of respiration (branchise, or gills), and the 

 existence of eyes placed at the extremity of a movable 

 peduncle or footstalk — hence their name. A large 

 portion of the front of the body is covered by a buckler- 

 shaped shell or carapace, which extends more or less 

 beyond the thoi'ax, and this kind of covering generally 

 tends to distinguish the animals of this division of Crus- 

 tacea at first sight. 



They are divided into two orders, Decapoda, or 

 Ten-footed Crustacea ; and Stomapoda, or Sea- 

 Mantes. 



Order I.— DECAPODA {Tcn-foolcd 

 Crustaceans). 



This order contains the greatest number of species 

 of all the orders of the Crustacea. The animals which 

 belong to it are those which have the most eomi)licated 

 organization, and the most perfectly developed facul- 

 ties. The branchise, or gills by which they respire, 

 are fixed on the internal surface of the thorax, and are 

 lodged in a cavity formed by the prolongation of the 

 carapace beyond the sides. The head and thorax are 

 compactly united or soldered together, and are covered 

 by a large shield-shaped carapace, which stretches be- 

 yond the front of the body, descends on each side to 

 the base of the feet, and extends backwards as far as 

 to the origin of the abdomen. The feet, properly so 

 called, are five pairs ; the first of which are transformed 

 into arms, and are terminated by a pair of large claws 



or pincers, forming organs of prehension ; the four other 

 pairs being the true organs of motion. These animals 

 are slow of growth, live long, and some of them become 

 very large. 



They are divided into three large groups, which are 

 distinguished from each other by the comparative length 

 of the abdomen, or, as it is generally called, the tail — 

 the Short-tailed, orBracfiyura; theLong-tailed, oril/ac- 

 roura; and the anomalous-tailed, or Anomoura. 

 Brachyura. — The Short-tailed Crustaceans, or Crabs, 

 have their short abdomen generally bent under the 



Fig. 183. 



The Common Crab. 



body and lodged in a cavity there, so that it is of little 

 or no use to them in swimming. The species are very 



