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ORDER I. THE PULMONEA.— ORDER II. NUDIBRANCHIATA. 229 
intermediate and gradually-progressive forms, between the naked Gastero- 
pods and those that are covered with a perfect spiral shell. First, there is 
a rudimentary internal or external shell, nearly covering and protecting the 
most important organs ; by degrees it expands and shields the whole animal, 
and the first signs of a spiral development make their appearance ; and at 
last the snail’s palace appears in all its perfection and beauty. 
ORDER I. THE PULMONEA. 
These mollusks breathe the atmosphere through a hole which opens under 
the margin of their cloak, and which they can dilate or contract at pleasure. 
They have no branchi, but only a network of pulmonary vessels, which 
creep upon the walls, and more particularly upon the ceiling of their respira- 
tory cavity. Some are terrestrial, and others aquatic; the latter are com- 
pelled, at intervals, to come to the surface to receive within their pulmonary 
cavity the air for respiration. They are all hermaphrodites. 
The terrestrial Pulmoneans are separated into several genera, the best 
known of which are the following : — 
Lroiax.— The Limaces have no apparent shell. The group comprises 
the family of slugs, one species of which, Z. rufus, was once thought a 
valuable remedy for diseases of the chest, taken in the form of a broth. 
Vacrnutus, Herrx, CrausmiA, and Acnatina comprehend the common 
snails. Of the last, Somerby remarks, that they are, for the most part, 
African and West Indian species. Two species, A. zebra and A. vir- 
ginea, are distinguished for their beautiful shells. 
The aquatic Pulmoneans, as they are obliged to come to the surface to 
breathe, live in fresh waters, or near the shores and mouths of rivers. The 
genera are OncuipiuM, PLaNnorpis, Limnaus, Puysm, ScaraBes, Avu- 
ricuLA, and MELAMpES, the last two of which are noted for their magnifi- 
cent shells. 
ORDER II. NUDIBRANCHIATA. 
Cuvier describes this family as having neither a shell nor pulmonary 
cavity, but their branchiw exposed naked upon some part of the back. 
They are all hermaphroditical and marine: they swim in a reversed posi- 
tion, the foot applied against the surface, and made concave like a boat, and 
use the edges of the cloak and the tentacula as oars to assist their progres- 
sion. The principal genera are Dorts, found on the shores of all seas; 
TRITONIA, a curious group, which has two rows of tufted branchie along 
NO. XVII. 82 
