102 ZOOLOGY. 
pieces, followed by two smaller pieces with a fifth dorsal piece. Sometimes: 
there are accessory pieces at the base, and the whole are in some cases so 
much reduced in size as to be rudimentary like the shell in CAitonellus (p. 
81). Sowerby’s genus, Lethotrya, occupies holes in rocks; but it is not 
known whether it forms them, or takes possession of those previously made 
by boring mollusea. 
Orver 2. Acamprosomata. In this order the animal is short and conical, 
without a peduncle, the shell solid and conical, sometimes sub-cylindrical, 
with the base or attached portion open or closed, the aperture provided with 
a two-valved or four-valved operculum. Those of the order known to the 
ancients were named Lalanus (pl. 76, fig. 54), on account of their 
resemblance to an acorn, a name which is still retained. They were a 
favorite article of food with the ancients. The natives of Chili eat a very 
large species, Balanus psittacus, which is five and a half inches high by 
three and a half in diameter; it has much the taste of a crab. The young 
of this species are attached to the adults, and in turn support their desendants, 
so that they occur in large masses of fifty or a hundred individuals. 
Some genera, as Pyrgoma, are buried in coral; Acasta inhabits sponges ; 
Chelonobia is attached to turtles; and Zubecenella and Coronula (pl. 76, 
fig. 58) are imbedded in the skin of whales. 
Class 3. Crustacea. 
In this class the sexes are separate; the body and limbs are distinctly 
articulated; the breathing is by means of gills, or more rarely (in some of 
the lower forms) by the external surface. The larger forms, as the lobsters 
and crabs, and the great majority of the smaller ones, inhabit the sea, where 
they take the place of the insects which are so abundant on land. Some 
species inhabit the fresh waters, and a few the land. The larger species 
are many times the bulk of the largest insects, from which they decrease to 
forms of microscopic size. In some of them the characters of the class are 
so obscure that they have been placed with the parasitic worms. Named 
from the hard integument, this affords a prominent characteristic, being a 
calcareous exterior skeleton of considerable thickness and strength in the 
larger species; becoming more delicate, and often transparent in the 
smaller ones, to disappear, or to escape observation in some of the obscure 
forms. This covering is periodically cast off and renewed, like the integu- 
ments of certain reptiles, and the larvee of insects. 
In comparing various members of this class (pl. 78), the number of 
segments, and the consequent ability to bend the body, will be found to be 
very variable, so that whilst the body of some (figs. 1-9) is a solid box 
incapable of flexure, that of others is composed of a number of loosely 
connected segments moving freely, and chiefly downwards from the 
horizontal position. This allows some of the members of the class to roll 
themselves into a ball by approximating the head and tail. 
The normal number of the segments in the body of the Crustacea is 
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