98 
NATURAL HISTORY. |^NEW BUILDING. 
consist of a series of pyramids formed of triangular plates, 
are placed between them ; the head is produced, and fur¬ 
nished with two or four tentacles. 
The family of the Umbrellidce (Case 21) have a suborbi- 
cular, flat, hard, external shell. The genus Umbrella was 
formerly supposed to have its shell placed on its foot, and 
was called Gastroplax , but this has been proved to be an 
error. Tylodina differs in the head being produced and 
bifid; while in Umbrella it is sunk into a deep cavity in 
the front of the foot. 
The order Gymnobranchiata (Case 22) 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. They are all marine, and frequently swim in a re¬ 
versed position, with the concave foot on the surface like 
a boat, using the margin of their mantle and their tentacles 
as oars. 
1. The family of Sea Lemons, or Doi'idce, have the gills 
placed round the vent, which is situated in the hinder 
part of the back. In some, as Doris and Onckidotis , 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 ge¬ 
latinous bands on stones, shells, sea-weed, &c. 
2. The family of Tritons ( Tritoniadce) have thfeir 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; 
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 Tethyee have 
their head enveloped in a large hood. 
3. The family of Placobranchidce have the gills in the 
