28 MANUAL OF THE MOLLUSCA. 
ORDERS, CLASSES. 
Dibranchiata. Owen. 
Tetrabranchiata. Owen. 
| Nucleobranchiata. Bl. 
; CEPHALOPODA. 
HENC BPEVATGA 40 ).s 1 Prosobranchiata. M. Edw. 
Pulmonifera. Cuv. GASTEROPODA. 
Opisthobranchiata. M, Edw. 
| Aporobranchiata. Bl. PTEROPODA. 
Palliobranchiata. Bl. BRACHIOPODA, 
ACEPHALA.... . .< Lamellibranchiata. Bl. CONCHIFERA. 
Heterobranchiata. Bl. TUNICATA. 
The Shell. The relation of the shell to the breathing-organ is 
very intimate: indeed, it may be regarded as a pnewmo-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 festa, ‘‘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 ag 
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 naked 
molluses. 
Three-fourths of the mollusca are wnivalve, 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 muiltivalves of old authors were 
articnlate animals (cirripedes), erroneously included with the 
mollusca, which they resemble only in outward appearance. 
All, except the argonaut, acquire a rudimental shell beforu 
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 limar, testacella, carinaria. Its peculiar features 
always relate to the condition of the breathing-organ; and in terebratula and 
pelonaa 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 (phragmocone), 
whioh is properly a visceral skeleton ; in spiruda the shell is reduced to this part, 
