THE SHELL. 15 



chambered portion (phragmocone), which is properly a visceral 

 skeleton ; in Spirula the shell is reduced to this part. 



The shell is so characteristic of the mollusca that they have 

 been commonly called " testacea " (from teda^ "a shell") in 

 scientific books ; and the popular name of " shell-fish," though 

 not quite accurate, cannot be replaced by any other epithet in 

 common use. In one whole class, however, and in several 

 families, there is nothing that would be popularly recognized as 

 a shell. 



The study of the shell is of great zoological importance, as 

 its form and composition vary characteristically in the different 

 genera ; and it becomes still more important geologically, inas- 

 much as it is almost the only portion of the vast number of 

 fossil species which has been preserved to us ; and by the study 

 of it in comparison with recent species, we are enabled not only 

 to distinguish the species and genera of these extinct forms, but 

 even to predicate the external appearance, the anatomy, the 

 physiolog}^ of the animals, with nearly the accuracy with which 

 the vertebrate palseontologist reconstructs a mammal or a reptile 

 from its osseous fragments. 



Shells are said to be external when the animal is contained in 

 them, and internal when they are concealed in the mantle ; the 

 latter, as well as the shell-less species, being called naked 

 mollusks. 



Three-fourths of the mollusca are univalve^ or have but one 

 shell; the others are mostly bivalve, or have two shells; the 

 Pholads have accessory plates, and the shell of Chiton consists of 

 eight pieces. Most of the multivalves of old authors were 

 articulate animals (cirripedes), erroneously included with the 

 mollusca, which they resemble only in outward appearance. 



All, except the Argonaut, acquire a rudimental shell before 

 they are hatched, which becomes the nucleus of the adult shell ; 

 it is often differently shaped and colored from the rest of the 

 shell, and hence the fry are apt to be mistaken for distinct 

 species from their parents. 



In Cymba (i, 15), the nucleus is large and irregular ; in Fusus 

 antiquus it is cylindrical ; in the Pyramidellidffi it is oblique ; 

 and it is spiral in Carinaria, Atlanta, and many limpets, which 

 are symmetrical when adult. 



The rudimentary shell of the nudibranchs is shed at an early 

 age, and never replaced. In this respect the molluscau shell 

 differs entirely from the shell of the crab and other articulate 

 animals, which is periodically cast oflfand renewed. 



In the bivalves the embryonic shell forms the umbo of each 

 valve (ii, 31) ; it is often very unlike the after-growth, as in TJnio, 

 Cyclas, and Hinnites. In attached shells, like the oyster and 



