278 THE MICROSCOPE. 



We may here remark, that Dr. Carpenter divides the articulate sub- 

 kingdom into eight classes, viz. Entozoa, Rotifera, Annelida, Myriapoda, 

 Insecta, Crustacea, Cirrhipoda, and Arachnida. The immense number 

 and variety of this portion of the animal kingdom necessitates a corre- 

 sponding multiplicity of subordinate divisions, which it would be quite 

 impossible for us to attempt a detailed description of. Our space will 

 only permit of a few remarks upon the Artlvropoda, or True Articulata ; 

 the first of which is the Crustacea. 



The skeletons of Crustacea* are external to the soft parts ; in a 

 great number of species it is thin and membranous, in others it is of 

 a horny material, thickened with calcareous matter, having a distinct 

 series of pigment cells of a stellate figure, all supplying beautiful ob- 

 jects for microscopic examination. 



A crustaceous animal consists of three parts : the head, the body, 

 and the carapace, which is covered with one entire shell, and is popu- 

 larly called the tail, consisting of seven rings, or joints. There are 

 properly fourteen rings in that part of the body which is called the 

 carapace; but they are only used when the animal changes its shell. 

 The joints in the tail are to enable the animal to spring forward, which 

 it does frequently when it wishes to change its position. It can also 

 crawl ; but it moves in this manner awkwardly, and in an oblique di- 

 rection. The river crawfish belongs to the same genus (Astacus) as 

 the lobster, and both have long tails, which are spread out when they 

 crawl, and numerous legs and claws, with which they can pinch se- 

 verely when they wish to defend themselves. The crab has a short 

 tail, and belongs to the genus Cancer. The shrimp, though it has no 

 claws, properly so called, has two feet larger than the others, each of 

 which has a hooked jointed nail. The prawn, which is quite different 

 from the shrimp, is nearly allied to the crawfish, or thorny lobster. All 

 the Crustacea have the power of renewing their claws if they are torn 

 off at a joint, and they change their shells every year. 



Dr. Carpenter describes the shell of the crab and lobster as being 

 composed of three layers, viz. the epidermis or cuticle, the rete-muco- 

 sum or pigment, and the corium. The epidermis is of a horny nature, 

 being generally more or less brown in colour, and under the highest 

 magnifying powers presenting no trace of structure (Plate VI. No. 7) ; 

 it invests all the outer parts of the shell, and has in many instances 

 large cylindrical or feather-like hairs developed from certain portions of 

 its surface. The rete-mucosum, or pigment-cells, consist of either a 

 series of hexagonal cells, forming a distinct stratum, or of pigmental 

 * Criistacea (from crusta, a shell). 



