GALLERY.] NATURAL HISTORY. 125 
seen on the apex of the true Argonaut* we should expect 
the animal which formed that shell* to have a large egg. 
The other animals of this order have their branchiae 
placed on the right side of the body, in the groove between 
the edge of the mantle and the foot. They comprehend 
the two following families. 
The family of Pleurobranchidce * have none or a very 
thin membranaceous shell inclosed in the mantle. The 
edge of the mantle and the foot project; the gills* which 
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 have a suborbicular* 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. Tglo- 
dina 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 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 reversed 
position* with the concave foot on the surface like a boat, 
using the margin of their mantle and their tentacles as 
oars. 
The family of Sea Lemons* or Doridte, 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 ge¬ 
latinous bands on stones* shells* sea-weed* &c. 
The family of Tritons ( TritoniadrB ) 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 
