760 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [8] 
THE NATURAL HISTORY OF MOLLUSKS AND CRUSTA- 
CEANS. 
MOLLUSKS. 
The third division of the animal kingdom, that of Mollusks, contains 
animals having a generally symmetrical body, that is a body with simi- 
lar parts upon either side of a median line, and which never presents 
an internal skeleton, as in vertebrates, or external one, as in the crus- 
taceans, and never becomes divided into segments, aS with the annelids. 
The body is soft, and is generally inclosed in a calcareous test or shell, 
which may be univalve or bivalve. 
The nervous system of mollusks, especially the ganglionic, does not 
present a median longitudinal disposition as in the vertebrates and 
articulates, but consists simply of symmetrically disposed little no- 
dules or ganglions of nerve matter, united together by nerve cords, and 
located throughout the body in the neighborhood of the principal or- 
gans. Mollusks breathe in the same manner as fish, by means of gills, 
which are either superposed lamin or branched filaments, and which 
act by separating from the water and absorbing those gaseous elements 
held in solution. Some few species present an internal respiratory 
cavity, to which is given by analogy the name of lung. These excep- 
tional species are always terrestrial. . Those mollusks having the most 
perfect organization are ranked in the class of Cephalopods (yegaiy, 
head, zovs, foot ; head-fogted animals), so named because of the tactile 
or prehensile appendages, which are located upon the head end of the 
body in a complete circle about the mouth, and called arms, feet, or 
tentacles. The only useful species is the cuttle-fish (Sepia officinalis), 
which produces the color called sepia or India ink, and the bone sold 
in commerce under the name of sea-biscuit or cuttle-fish bone. The 
poulpe or squid also belongs to this class, and is likewise called cuttle- 
fish; it serves as food for some of the poorer classes along the coast of 
Italy. 
The second class of mollusks is that of the Gasteropods (yast7p, belly, 
zovs, foot; belly-footed), which owe their name to a fleshy, contractile 
base serving as an organ of locomotion. Their shell, when it exists, is 
always univalve. As examples may be mentioned the slug, the snail, 
and those animals which furnish the helmet and porcelain shells. 
The last class and the least specialized group of mollusks, which, how- 
ever, contains species of great importance in a commercial point of view, 
is that of the acephalous mollusks (4, without, yegai7, head; headless). 
This class contains all of those mollusks having a bivalve shell, such 
as the oyster and the mussel, which we propose to study. All are 
without a distinct head, hence the name. The body has the form of a 
flattened, oval disk, and is pierced at one extremity with a mouth orifice 
