14 Marine Shells of the Western Coast of Florida 



ance and anatomical structure. These differences form the basis of 

 classification of all animal forms — first into two principal groups — 

 mvertebrates and vertebrates, or those forms without a backbone and 

 those possessing one. These groups are divided into phyla (Gr., 

 phylon, meaning race or tribe). Each phylum is subdivided on the 

 basis of anatomical structure into groups which show the increas- 

 ingly closer relationships which establish class, order, family, genus, 

 and species. 



The phylum Mollusca (Lat., mollis, soft, having reference to the 

 soft body) is fifteenth on the scale of increasing complexity of 

 structure among the invertebrates. It has perhaps, more than 100,- 

 000 species, a greater number than any other group of animals ex- 

 cepting the Arthropoda which includes the insects and crustaceans. 



Mollusks have no supporting skeleton, but with few exceptions 

 they have developed a protective calcareous structure, the shell-, 

 which has evolved into a number of highly differentiated forms. The 

 shell IS a part of the animal, and failure to consider this relation- 

 ship and the relationship of mollusks to other animal species, de- 

 prives the student of one of the great advantages and pleasures of 

 his avocation. 



The shell is secreted by a specialized part of the mollusk's body 

 called the mantle, which envelops the soft structures like a closely 

 wrapped cape, and whose external, secreting surface is in intimate 

 contact with the internal surface of the shell. The secreted shell sub- 

 stance consists of about five per cent of an organic matrix called 

 conchyolin impregnated with mineral salts taken from the sea water, 

 principally carbonate of lime and small amounts of magnesium. The 

 conchyolin, which is elaborated and secreted by the mantle, pro- 

 vides a framework for deposition of the mineral salts, and in an unim- 

 pregnated state forms an external covering for the shell which affords 

 protection against chemical injury from corrosive substances which 

 the water may hold in solution; it is called the epidermis or 

 periostracum. Differences in structural arrangement of the shell 



- Dr. Samuel Johnson's Dictionary gives eight meanings for the word 

 "shell". The second is "The covering of a testaceous or crustaceous animal". The 

 word is of Saxon origin — scyll or scell — meaning rough, homespun. 



