66 NATURAL HISTORY. [EAST. ZOOL. 
round the edge of the mantle of the animal. Many of these animals 
are destitute of any shell. They are all marine, and frequently swim 
in a reversed position, with the concave foot on the surface like a boat, 
using the margin of their mantle and their tentacles as oars. 
The young animals of this order, and of Aplysia , have an external 
shell which soon falls off. The genus Cirroptera may have been 
established from young animals of this kind. 
The families of the first section of this order have the gills placed on 
the back and exposed. 
These animals having no shells, the greater part of the specimens 
will be found among the other animals in spirits in the Northern 
Zoological Gallery. 
1. The family of Sea Lemons, or Dorid^:, 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. Their eggs form gelatinous bands on stones, shells, 
sea-weed, &c. 
2. The family of Tritons (Tritoniadje) 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 des¬ 
titute 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 
across the back ; in ScyJlcea , 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 enables 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 Tethyce have their head enveloped in a large hood. 
Briar eus, which has been described as a genus of this family, is a 
Crustacea near Phyllosoma. 
3. The family of Placobranchid^: have the gills in th^ 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 depressed 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 alga;, 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 PHYLLiDiADiE (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 Diphyllidia it is soft, with the edges turned up. 
The family of Limpets (Patellid^e, 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 
