GASTEROPODA. GYMNOBRANCHIA. 15 
niadee; or covered by a dilatation of the mantle on the right 
side, and protected by the shell, in Bullade. In the order 
Cyclobranchia the branchie are foliaceous, and are situated on 
each side, between the base of the foot and the mantle, Pl. II. 
ae. 11. 
The shell is wanting entirely in the order Gymnobranchia ; 
it is much depressed and very obscurely spiral in the three first 
families of the order Stegobranchia, and involutely spiral in the 
Bulladze, which form the fourth family of the same order. 
In the order Saccobranchia the shell is altogether wanting, 
is represented by fragments of calcareous matter placed under 
the mantle, or by a depressed shell, in the family Limacidee ; 
or produced into a depressed or highly elevated spire, with every 
intermediate state, in the families Helicidee, Carychiadee, and 
Limnadee ; whilst in the last family Ancylidee, the shell is sim- 
ply conic, with the most obscure rudiment of a spire. 
In the orders Aspidobranchia and Cyclobranchia, the shell 
is conical, or auriform, with every modification that can exist 
between these forms; or is elongate, and composed of eight 
pieces, which are either external or enveloped in the mantle. 
In the orders Phyllobranchia and Antrobranchia, the shell 
is always spiral, and has for the most part its aperture closed 
by an operculum, which is vesiculose in the family Janthinade, 
wanting in Cypreeadze, and horny or testaceous in Purpuradee 
and 'Turbinidee. 
The food of the Gasteropoda consists of vegetable, animal, 
and excrementitious matters. 
The sexes are hermaphrodite in the orders Gymnobranchia, 
Stegobranchia, and Saccobranchia; distinct in the orders Phyl- 
lobranchia and Antrobranchia ; androgynous in the orders As- 
pidobranchia and Cyclobranchia. 
ORDER I. GYMNOBRANCHIA. 
Nudibranches, Cuvier. 
Branchie nullo modo tectze, dorsales. Testa nulla. Sezrus 
hermaphroditus. 
