50 ZOOLOGY. 
sense, as it included radiata, like Actinia, Medusa, and Echinus; crustacea 
like Lernzea, as well as Ascidia, Limax, and the Cephalopoda, and even 
some annelida; whilst his order (Vermes) Testacea included most of the 
univalve and bivalve shell-fish. The “class Mollusca” of Lamarck, follows 
his class Conchifera, and includes the five “orders,” as he terms them, of 
Pteropoda, Gastropoda (excluding the spiral univalves), Trachelipoda 
(including the spiral univalves), Cephalopoda, and Heteropoda. Pallas and 
Cuvier were the first to use the term Mollusca in its modern sense; and the 
latter, in 1798, recognised three sections, subsequently provided with the 
systematic names of Cephalopoda, Gastropoda, and Acephala. The term 
_ Mollusca having by these means acquired a somewhat indefinite meaning, — 
Blainville proposed that of Malacozoa, which is the preferable term, if the © 
term Malacology supersedes that of Conchology. 
The shell of the Malacozoa can have one, two, or more pieces; it may 
be external or internal, and it varies much in size, being in some cases 
capable of containing all the soft parts, with room to spare, and in others a 
mere excrescence upon some part of the animal. It is secreted by the 
mantle, a fold of the integument which appears as a flap, lining the shell in 
bivalves, and a continuation of it appears as a collar around the neck, and 
lining the aperture of the shell in the spiral univalves. In the order 
Tunicata the body is inclosed in a kind of coriaceous purse instead of a shell. 
The mantle usually secretes two kinds of material, the nacre, or pearly 
portion of the shell, and the epidermis, or perzostraca ; but in some gencra 
the latter is absent, as in Oliva (pl. 75, figs. 122, 125, 126) and Cyprea 
( pl. 76, figs. 5-7), in which the wide mantle is turned up on each side, so 
as to inclose the shell. When the shell is broken by accident, the crevice 
is closed by layers of nacre; and if a pebble gets into the shell by accident, 
and cannot be got rid of, it is covered in like manner. When the nacre 
or “ mother-of-pearl” is of a fine quality, the tubercular masses which arise 
from injuries have a peculiar lustre, which causes them to be admired under 
the name of pearls. Shells are aPieh found repaired, which had been 
crushed and distorted to such an extent as to lead one to believe it scarcely 
possible for an animal to survive after so much mutilation. 
The varied spots and lines which ornament so many marine shells, are 
distributed by the periodical action and inaction of the secreting process. 
The species of the genus Conus, although covered by a thick epidermis, are 
usually marked with bands, spots, and reticulations of brilliant colors, which 
make them a favorite ornament to collections. The epidermis, however, 
should not be removed, or it should at least be preserved upon a specimen 
of each species, as it varies considerably according to the species. The 
former unscientific custom of polishing such shells as have a homely 
exterior is now discontinued. 
The microscopic structure of shells presents many curious features, which 
have’ been thoroughly studied by Carpenter, who has illustrated the subject 
with figures. In some shells the structure resembles that of minerals, and 
seems to be formed of minute crystals of sasilepie © of lime, or of fibres 
resembling arragonite. 
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