CLASS CRUSTACEA. 



CHAPTER I. 

 ANATOMY OF CRABS AND LOBSTERS. 



Characteristics of the Crustacea Their Mode of Existence External Covering Body Segments Locomotory and other 

 Appendages Nervous System Digestive Organs Respiration and Circulation Reproduction and Development 

 Metamorphosis Exuviation Reparation Classification. 



THE Crustacea, represented by the Crab and Lobster, and a great A'ariety of other crust-clad animals 

 with jointed limbs, form the fourth class of the Arthropoda. Most of the members of this class seem 

 to be essentially fitted to live in water, being furnished with branchiae or gills. 



Taking the Common Lobster for an example, the entire body and legs are encased in a hard 

 structure which is called the shell, but it is quite different from that of a Whelk or of an Oyster. 

 Neither is it composed of the same material as our own bones, nor is it horny, but it is formed 

 of a nitrogenous substance, insoluble in alkalies, termed chitin, arranged in layers, between which 

 salts of lime (mainly the carbonate) are deposited.* 



This shell serves the double purpose of a defensive covering to the softer parts of the animal, and 

 also, by means of its overlappings, iiifoldings, projections, and rugosities, of giving attachment and 

 support to the muscles which move the limbs and also to those of the internal organs, as the stomach, 

 itc. In the common Prawn and Shrimp this shelly envelope is quite thin and translucent, and its 

 structure can be seen under the microscope without preparation ; but in the Crab the shell is often 

 very dense and thick, and needs to be cut into thin vertical sections, or rubbed down, before its 

 structure can be clearly made out with the microscope. If a thin vertical slice be prepared, three 

 distinct layers or strata will be seen, namely, first, a horny structureless layer covering the exterior ; 

 second, a cellular stratum ; and third, a laminated tubular layer. The innermost and even the middle 

 layers may, however, be altogether wanting, as in some larval forms (e.g., Phyllosoma), or as in the 

 delicate covering of the Shrimp, in which only the cellular and horny layers are present. 



Dr. Carpenter remarks : " In the Common Edible Crab (Cancer pagurus) we can readily separate 

 the structureless; homy outer layer, after a short maceration in dilute acid, thus leaving the middle 

 cellular layer exposed, in the cells of which the pigment, or colouring matter, of the shell is contained. 

 The thick inner layer may be best seen by means of a section perpendicxilar to the surface of the shell, 

 when we can, with a magnifying power of 250 diameters, observe the parallel laminae of which it is 

 composed, and through which straight non-branching tubuli are seen to rise up at intervals through 

 the cellular stratum forming little papillary elevations. It is from the thinness of the pigment matter 

 in this layer, at these spots, that the coloured portion of the shell derives its minutely speckled 

 appearance. In the shell of the Prawn we may notice the large star-shaped pigment or colour cells 

 distributed over the surface, which, by concentration or diffusion of the colouring matter contained in 

 them, tend to render them more like the sea-bottom which they frequent In the Shrimps the cellular 

 layer is not distinctly seen, whilst the calcareous portion forms concentric rings similar in structure 

 to that seen in the papillae of the surface of the deepest layer of the Crab's shell." 



If any common Crustacean, such as a Lobster, Prawn, cr Shrimp, be examined, it will 

 readily be perceived that its body-covering is made up of a number of rings or segments jointed 

 together, to which the feelers, claws, and legs are imited by means of movable sockets. To give 

 greater protection to the soft parts of their body, it often occurs, as in the Crab and Lobster, that a 

 considerable number of these body-rings are soldered together into one piece, which may be compared 

 to the back and breast-plate of a knight's armour. It was, however, discovered by naturalists long ago 



* An analysis of the shell of the common Crab gives : 



Animal matter 28'6 



Phosphate of lime G'O 



Carbonate of lime ... ..... . .... 02 "8 



Phosphate of Magnesia ..... ? I'O 



Soda, salts, &c. . , , , .,,..... 1'6 



100-0 



