126 
NATURAL HISTORY. [EAST. ZOOL. 
in lines acros the back; in Scyllcea, they are tree-like., 
and placed on fin-like processes on the edge of the back ; 
their body is compressed, and the foot narrow, which en¬ 
ables it to clasp the stems of the sea-weed on which it is 
almost constantly found. They have no jaws, while the 
mouth of the Tritonia is armed with two horny, sharp- 
edged, lateral jaws, like sheep-shears. 
The Tetliyce have their head enveloped in a large hood. 
The family of Placobrancliidcz have the gills in the 
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 co¬ 
lour. 
The other families have the gills in the form of plates 
on the edge of the under side of the mantle. 
The family of Phyllidiadce 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, con¬ 
vex, and tubercular, like the Sea Lemons; in Diphyllidia 
it is soft, with the edges turned up. 
The family of Limpets ( Patellidce ) 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. 
The family of Sea Woodlice ( Chitonidce ) are as anoma¬ 
lous as the above, since instead of having a single shell, 
they have a row of shelly valves, like plate armour, ar¬ 
ranged 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 - 
