36 MANUAL OF THE MOLLUSCA. 



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

 breathing- organ is at most a specialised part.* 



The shell is so characteristic of the mollusca that they have 

 been commonly called "testacea" (from testa "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 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 7iaked mollusks. 



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

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

 pliolads have accessory plates, and the shell of chiton 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^ acquire a rudimental shell before 

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

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

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

 from their parents. 



In cymha (fig. 20) the nucleus is large and irregular ; mfusus 

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

 it is spiral in carinaria, atlanta, and many limpets, which are 

 symmetrical when adult. 



The rudimentary shell of the nudibrancTis 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 

 terehratula ani pelonaia it becomes identified with the gill. In the nudi- 

 brauchs the vascular mantle perfomis wholly or in part the respiratory office. 

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

 internal, chambered portion (phrapnocone) , which is properly a visceral 

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



