CLASSIFICATION OF ANIMAL PREPARATIONS. 429 



Besides the structure of the shells of the molluscous animals 

 before enumerated, there are others belonging to the classes 

 of Echinodermata, Crustacea, and Cephalopoda, that require a 

 separate mention ; thin sections of these, in different directions, 

 are prepared in the same manner as those of shell, which they 

 have been said somewhat to resemble in their minute struc- 

 tural arrangements. The most interesting varieties are men- 

 tioned in the following list: — 



Belemnite, Cuttle-bone, Pentacrinite, 



Cidaris, spine, EcUnus, spine, Prawn, 



Crab, red part, shell, Slirimp, 



— black part, Encrinite, Spatangus, 



Cray-fish, Lobster, Star-fish. 



The structure of the spines of the Cidaris, and many other 

 species of Echinodermata, form some of the most beautiful 

 objects that have yet been exhibited by the microscope ; they 

 are so very brittle, that the greatest care is required in grinding 

 them down ; the method described in page 332 for delicate 

 specimens of bone and teeth, should be the one adopted, and 

 to preserve them from injury, and at the same time to display 

 aU the peculiarities in their arrangement, they should be 

 mounted in Canada balsam; some of the very minute coloured 

 spines from small species of Echini are interesting subjects 

 for examination when laid flat in balsam, without any 

 previous preparation. Transverse sections are the best for 

 exhibiting the cellular arrangement ; the longitudinal do not 

 show much more by the microscope than can be seen by the 

 naked eye. 



In aU the shell structures, in order to understand the 

 arrangement of the animal matter, one or more sections in each 

 direction should be subjected to the decalcifying process, as 

 described in page 332, the acid employed being the hydro- 

 chloric, diluted with forty times its bulk of water. 



Scales of Fish. — These dermal appendages may be divided 

 into two classes; first, those that are made up of a horny 

 material, such as in the salmon and carp; and, secondly, 

 those whose structure is true bone. The scales of the majority 

 of fishes belong to the first class, but very few species now 



