36 MANUAL OF THE MOLLUSCA. 



being essentially a calcified portion of the mantle, of which the 

 breathing-organ is at most a specialised part.* j 



The shell is so characteristic of the mollusca that they have i 

 been commonly called "testacea" (from testa "a shell"), in 

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

 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 recognised as ;| 

 a shell. 



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 naJced moUusks. 



Thi-ee-fourths of the 7nollusca are univalve, or have but one i 

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

 pholads have accessory plates, and the shell of cliiton consists of ; 

 eight pieces. Most of the multivalves of old authors were arti- 

 culate animals {cirripedes), erroneously included with the mollusca, \ 

 which they resemble only in outward appearance. 



All, except the argonaut^ acquii-e a rudimental shell before 

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

 it is often difterently shaped and coloured from the rest of the \ 

 shell, and hence the/r^ are apt to be mistaken for distinct species i 

 from their parents. I 



In cymha (fig. 20) the nucleus is large and iiTcgular; 'mfusus\ 

 antiquus it is cylindrical ; in the pyramidellida it is oblique ; and 

 it is spii-al in carinaria, atlanta, and many limpets, which are 

 symmetrical when adult. 



The rudimentary shell of the nudihrancJis is shed at an early 



* In its most reduced form the shell is only a hollow cone, or plate, pro- 

 tecting the breathing organ and heart, as in Umax, testacella, carinaria. Its 

 peculiar features always relate to the condition of the breathing-organ ; and in 

 terebratuJa and pelonaia it becomes identified with the giU. In the nudi- 

 branchs the vascular mantle perforais wholly or in pai't the respiratory office. 

 In the cephalopods the shell becomes complicated by the addition of a distinct, 

 internal, chambered portion {phragmocone), which is properly a visceral 

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



