28 MANUAL OF THE JMOLLUSCA. 



ENCEPHALA 



Orders. Classes. 



Dibranchiata. Owen. . 



Tetrabrancliiata. Owen. j- Cephalopoda. 



NucleoLranchiata. Bl. n 



Prosobranchiata. M. Eclw. ( 

 Pulmonifera. Cuv. >CtASTEropoda. 



Opistliobranchiata. M. Eclw. ^ 

 Aporobranchiata. Bl. Pteeopoda. 



j'Palliobranchiata. Bl. Bkachiopoda. 



ACEPHALA } Lamellibranchiata. Bl, Conchifera. 



[Heterobrancliiata. Bl. Tunicata. 



The Shell. The relation of tlie shell to the breathing- organ is 

 very intimate : indeed, it may be regarded as a 2^^^umo- skeleton, 

 being essentially a calcified portion of the mantle, of which 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 ^es^a, "a shell") in 

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

 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 j^opularly 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 

 molluscs. 



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

 shell; the others are mostly hivalvc, 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 

 articiilate animals {cirripedes), erroneously included with the 

 mollusca, which they resemble only in outward appearance. 



All, except the argonaut, acquire a rudimental shell beforo 

 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 fry are apt to be mistaken for distinct 

 species from their parents. 



In cymba (Fig. 20) the nucleus is large and irregular; in 



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

 breathing organ and heart, as in Umax, testacella, carinaria. Its peculiar features 

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

 pelonaia it becomes identified with the gill. In the nudibranchs the vascular mantle 

 performs wholly or in part the respiratory office. In the cephalopods the shell becomes 

 complicated by the addition of a distinct, internal, chambered portion {phragynocone), 

 which is properly a visceral skeleton ; in spirula the shell is reduced to this part. 



