99 
EAST. ZOOL. GAL,] NATURAL HISTORY. 
form of laminae, radiating from a centre, and extending 
all over the back of the mantle, the edges of which are 
rolled over towards the centre, so that they form a de¬ 
pressed tube on the back of the animal for the water to 
pass through. The head is produced, and furnished with 
two tentacles. From the positions these animals assume 
they have been called Sea Cats. They feed on green 
algae, and are themselves generally of a deep green colour. 
The other families have the gills in the form of plates 
on the edge of the under side of the mantle. 
4. The family of Phyllidiadoe (Case 22) are destitute of 
any shell; they have two retractile tentacles, and the vent 
on the side of the body. In Phyllidia the mantle is hard, 
convex, and tubercular, like the Sea Lemons; in Dipliyl - 
lidia it is soft, with the edges turned up. 
5. The family of Limpets ( Patellidce , Case 22) have a 
simply conical shell, with the apex directed towards the 
head of the animal, contrary to what prevails in almost 
all other shells. The animal has two tentacles; a short 
snout, with a very long cartilaginous tongue, armed with 
cross rows of bent back spines; the heart is on the left 
side of the neck, and the vent is on the right side of 
the body, near the head, 
6. The family of Sea Woodlice (Chitonidce , Case 22) are 
as anomalous as the above, since instead of having a single 
shell, they have a row of shelly valves, like plate armour, 
arranged in a regular series down the middle of the back. 
The animal has no tentacles, but a membranaceous veil 
over the mouth ; the heart is on the rectum behind, and 
the vent is in the hinder margin. The Tonichia has the 
upper surface of the mantle bald and cartilaginous, and 
the under covered with a very hard striated skin. Acan- 
tliochetes is peculiar for having a bundle of bristles placed 
on each side of the valves ; and Chitonellus and Amicula 
only differ in having the valves nearly hidden in the man¬ 
tle of the animals. 
The remaining Gasteropodous 31olluscahave been called 
Pneumobranchiata, (Case 23—26,) from their respiring 
free air, which is received into a cavity between the mantle 
and the back, lined internally with numerous reticulated 
vessels, like the lungs of beasts and birds. They are mostly 
terrestrial, and when aquatic they come to the surface of 
