932 DIVISION II. MOLLUSCOUS ANIMALS.—CLASS III. GASTEROPODS. 
only — Cyelostoma and Helicina —have, instead of branchiw, a vascular 
network, clothing the ceiling of a cavity in all respects the same as that 
of the order; and they are the only ones which respire the atmosphere, 
water being the medium of respiration to all the rest. 
All the Pectinibranchiata have two tentacula and two eyes, raised some- 
times on pedicles ; a mouth in the form of a proboscis, more or less length- 
ened; and separate sexes. 
Cuvier divides the order into four families: the Trochoides, which have 
a shell with an entire aperture, without sinus, or canal for siphon, and 
furnished with an operculum, or some organ as its substitute; the Capu- 
loides, which have a widely open shell, without an operculum or emargi- 
native canal; the Luccinoides, distinguished by a spiral shell, the mouth 
of which has, near the end of the columella, a sinus or canal, for the pas- 
sage of the siphon, which is formed by an elongated fold of the cloak; and 
the Strombuside, which comprise the shells, with a canal either straight, or 
bent to the right, the external lid of the aperture becoming, at its maturity, 
more or less dilated, and always marked with a sinus near the siphonal 
canal, whence the head issues when the animal comes out. 
In the first family we find several fine shells, —as the Trochus turritus, 
Turbo, Ampulonia, and Nerita. To the third family belong those splen- 
did specimens, known as Cones, Volutes, Buccinuim, and Murex, all 
magnificent shells, beautifully colored. The last is remarkable for its 
elongate canal, and the numerous spines which cover the whole, giving it 
something of the appearance of a chevawa-de-frise. The fourth family 
contains the Pteroceras scorpio, a shell highly valued by conchologists. 
ORDER VII. TUBULIBRANCHIATA. 
These mollusks have a shell formed more or less like an irregular tube, 
spiral only at its apex, and fixed permanently to other bodies. There are 
three genera: Vermetus, which has a tubular shell, whose whorls, at an 
early age, form a kind of spine, and centinued on in a more or less irregu- 
larly bent or twisted tube, like the tubes of. the Serpula; Macarius, with 
a tube at first quite regularly spinal, and then extended in nearly a straight 
line. It is common in the coral rocks of the Isle of France, and its tube 
sometimes reaches the length of three feet; and SmiQquorta, which has the 
irregular tube of the Vermetus, but there is a fissure on the whole length 
of the shell. 
