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NATURAL HISTORY. [EAST. ZOOL. 
The order Gymnobranchiata have naked gills of 
various forms placed on different parts of the back., or a 
series of plates placed round the edge of the mantle of the 
animal. Many of these animals are destitute of any shell. 
The family of Sea Lemons,, or Doridcz, have the gills 
placed round the vent, which is situated in the hinder 
part of the back. In some* as Doris and Onchidoris, the 
whole body is protected by a hard convex skin, so as to 
resemble half a lemon; others, which are only covered with 
a soft skin, are generally angular; some of these have 
several pairs of filiform tentacles. 
The family of Tritons ( Tritoniadce ) have their gills of 
various forms, scattered on different parts of the back or 
sides, and the vent is placed on the middle of the side. 
These genera, which are all destitute of shells, are chiefly 
distinguished by the form of the gills; in Glaucus , they 
are in the form of a tuft of simple processes on each side 
of the body; in Eolida , they consist of similar tufts placed 
in lines acros the back; in Scyllcea , they are tree-like, 
and placed on fin-like processes on the edge of the back. 
The Tethyce have their head enveloped in a large hood. 
The family of Placohranchidce 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- 
GUte 
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 har'd, con¬ 
vex, and tubercular, like the Sea Lemons; in Dipkyllidia 
it is soft, w T ith the edges turned up. 
The family of Limpets ( Patellidce ) have a simple 
conical shell, w r ith the apex directed towards the head 
of the animal, contrary to what prevails in almost 
all other shells. The animal has two tentacles. The gills 
