ORDER V. HETEROPODA.—ORDER VI. PECTINIBRANCHIATA. 251 
hollow on the right side, covered by a thin, transparent, horny shell. 
These mollusks resemble a great naked snail. They dwell in every sea, 
frequenting chiefly rocky shores, where they creep along, feeding upon the 
alow. Some species, however, make use of their mantle folds for swim- 
ming. A peculiar gland pours out, through an orifice near the vulva, a 
limpid humor, which is said to be very aerid, if not absolutely poisonous, 
in some species. A purple liquid also oozes from the edges of the cloak, 
when they are alarmed, which discolors the water, and conceals them from 
their foes. 
ORDER V. THE HETEROPODA. 
The Heteropods have the foot compressed into a vertical muscular lami- 
na, which they use as a fin, and on the edge of which, in several species, 
is a sucker, in the form of a hollow cone, that represents the disk of the 
other orders. The body, which is a transparent, gelatinous substance, is 
elongate, sheathed with a muscular layer, and terminated with a compressed 
tail. The mouth has a muscular mass, and a tongue garnished with little 
hooks. They have the power to inflate the body with water, the object of 
which is not known; and they swim in a reversed position. The genera 
are Frrota, ATLANTA, and CARINARIA. 
The Cartinarie are very curiously formed animals, carrying on their back 
a shell fastened to a stalk, under which the fringed branchiw project. On 
the under side of the body the foot forms a round disk, furnished with a 
sucking-cup. The whole animal seems to be made up of disjointed parts. 
The species live far away from shore, and are generally found swimming 
about, or attached by the foot to some floating objects. The most beautiful 
species inhabits the Indian Ocean, and produces a shell worth from two to 
three hundred dollars. 
ORDER VI. THE PECTINIBRANCHIATA. 
This order is, beyond comparison, the most numerous of the class, since 
it comprehends almost all the univalve spiral shells, and several which are 
simply conical. The branchiw, composed of numerous leaflets or fringes, 
ranged parallelly like the teeth of a comb, are affixed in one, two, or three 
lines, according to the genera, to the floor of the pulmonary cavity, which 
occupies the last whorl of the shell, and which communicates outwards by « 
wide gape between the margin of the cloak and the body. Two genera 
