II1TRODUCTION. XIX 
There are nine orders ; but the common snail will give an 
idea of the habits of the class. 
4. Acephala, or Headless Mottusca. These animals have two shells 
united by a hinge, and no distinct head, like the oyster. There 
are two orders. 
5. Brachlopoda, or arm-footed Mottusca. These animals have two 
shells not united by a hinge. There are two orders. 
6. Cirrhopodes, or Fringed-footed Mollusca. These animals are soft 
and headless, like the oyster, but they are furnished with fringe 
or hair like tentacula ; as for example, the lepas or barnacle. 
It must be observed, that the system of Cuvier is not generally 
adopted in classing shells, as his classes are formed from the animals 
alone, and we have manyshells, the animals of which are unknown; and 
that of Lamarck is preferred, as he founds his classes partly on the 
animals, and partly on the shells. Lamarck divides the species 
into three classes, viz., Cirrhipeda, Conchiferee, and Mollusca. The 
first is the same as the sixth class of Cuvier ; the Conchifera are 
acephalous, or headless, and two-shelled or bivalves, like the oyster; 
and the Mollusca are univalves and cephalous, that is, they have 
only one shell, and a kind of head like the snail. 
THE ARTICULATED ANIMALS 
Have no back-bone. The covering of the body is sometimes hard 
and sometimes soft; but it is always divided into segments by a num- 
ber of transverse incisions. The limbs, when the body is provided 
with any, are jointed. The senses of tasting and seeing are more 
perfect than those of the Mollusca, though that of feeling seems 
much less acute. In other respects the four classes differ consider- 
ably from each other. 
I. TheAnnelides, or Red Blooded Wbms, have no heart, properly so 
called, but they have sometimes one or more fleshy ventricles. 
They breathe through branchiae. Their bodies are soft, and 
more or less elongated, being divided into numerous rings or 
segments. The head, which is at one extremity of the body, 
can scarcely be distinguished from the tail, except by having a 
mouth. These animals have no feet, properly so called, but 
they are furnished with little fleshy projections, bearing tufts 
of hairs or bristles, which enable them to move. They are gene- 
rally of carnivorous habits, andlive principally on the blood of 
other creatures. They lay eggs, but the young are frequently 
hatched before exclusion, and hence these creatures are said 
to be oviviporous. Their study is called Helminthology. 
As examples of the three orders of this class may be men- 
tioned the serpula or worms often found on shells, the com- 
mon earth-worm, and the leech family. 
II. The Crustacea comprise the shell-fish commonly called crabs, 
lobsters, shrimps, and prawns. They have a distinct head, 
