STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 29 
fusus antiquus it 1s cylindrical; in the pyramidellide 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 molluscan shell 
differs entirely from the shell of the crab 
and other articulate animals, which is 
periodically cast off and renewed. 
In the bivalves the embryonic shell forms 
the wmbo of each valve; it is often very un- 
lke the after-growth, as in wnio pictorwm, 
eyclas henslowiana, and pecten pusio. In 
attached shells, like the oyster and anomia, 
the umbo frequently presents an exact imita- 
tion of the surface to which the young shell 
orignally adhered. 
Shells are composed of carbonate of hme, 
with a small proportion of animal matter. 
The source of this lime is to be looked for in 
their food. Modern inquiries into organic 
chemistry have shown that vegetables derive 
their elements from the mineral kingdom _ : 
(air, water, and the soil), and animals theirs *'% ?° Conte: 
from the vegetable. The sea-weed filters the salt water, and 
separates lime as well as organic elements; and lime is one of 
the most abundant mineral matters in land plants. From this 
source the mollusca obtain lime in abundance, and, indeed, 
we find frequent instances of shells becoming unnaturally 
thickened through the superabundance of this earth in their 
systems. On the other hand, instances occur of thin and 
delicate-shelled varieties in still, deep water, or on clay bottoms; 
whilst in those districts which are wholly destitute of lime, 
like the Lizard in Cornwall, and similar tracts of magnesian- 
silicate in Asia Minor, there are no mollusca.—(orbves.) 
_ The texture of shells is various and characteristic. Some, 
when broken, present a dull lustre like marble or china, and 
are termed porcellanous ; others are pearly or nacreous ; some 
have a fibrous structure ; some are horny, and others glassy and 
translucent. 
The nacreous shells are formed by alternate layers of very 
thin membrane and carbonate of lime, but this alone does not 
* Fig. 20. Cymba proboscidalis, Lam., from avery young specimen in the cabinet 
of Hugh Cuming, Esq., from Western Africa. 
