GASTEROPODA TECTIBRANCIIIATA. 
355 
Thk Bursatelles, Blainv., — 
The lateral crests are united in front, so as only to leave 
an oval opening for the water to pass to the branchiae 
which are also destitute of a covering cloak. It is, how- 
ever, probable that this genus should be allowed to lapse 
into the Notarchus.* 
The Aceres, {Akera, Muller)— 
Have the branchiae covered like the preceding genera, but 
their tentacula are so much shortened, widened, and sepa- 
rated, that there seems to be none at all, or rather they 
form together a large, fleshy, and nearly square buckler^ 
under which the eyes are placed. Moreover, their her- 
maphroditism, the position of their sexual organs, the 
complexity and structureof the stomach, the purple liquid 
which several of them shed, all approximate them to the 
Aplysiae. The shell, in such as have one, is more or less 
convolute, with a slight obliquity, without a visible spire, 
and the mouth has neither sinus nor canal ; but as the 
columella is convex and protuberant, the mouth has a 
crescent-like shape, and the part opposite to the spire is always widest and rounded. When the shell 
is buried in the cloak, M. de Lamarck names the genus Bullaea. The shell has few whorls, and is too 
small to contain the animal. 
The Bull<ea aperta, Lam., is an example which is found in almost every sea, where 
it lives on oozy bottoms. When the shell is [external], covered with a thin epidermis 
and sufficiently roomy, M. de Lamarck allows them to retain the old name Bulla. 
The Bulla lignaria, ampulla, and liydatis are examples, [distinguished not only by the 
characters of the shells, but by peculiarities in the armature of the stomach, which 
consists of two or three comparatively large osseous pieces or jaws of different shapes 
Of those of B. lignaria, Gioeni constituted a genus to which he assigned L'o.—Buiiiea aperta. 
his own name ; it is the Tricla of Retzius, the Char of 
Bruguifere, and disfigured our systems until the cheat 
was detected by Braparnaud.] I restrict the term Accra 
to such species as have no shell whatever, or merely a 
vestige of it behind, although the cloak has the external 
form of one. The genus is the Doridium of Meckel 
and Lobaria, Blainv. There is a small species in the 
Fif^. ]/i -Bulla lignaria. ( Fig. 172 .-Bra.npulla; Mediterranean (Bulla carnosa, Cuv.), whose stomach 
is as destitute of any armature as its cloak is of a shell, but the oesophagus is fleshy and very thick. 
Fig. 169. — Bursatella Leachii. 
in each. 
The Gasteroptbron, Meckel, — 
Appears to be only an Aceres with the sides of the foot expanded into broad fins, by whose aid it is 
enabled to swim, which it does in a reversed position. It also has no shell, and no stony apparatus 
in the stomach. A very slight fold of the skin is the sole vestige of a branchial cover to be observed. 
The one species known (G. Mechelii) is a Mediterranean Mollusk, about an inch long by two in breadth, when 
its wings are spread out. 
Until a more ample anatomy has been made of it, we believe that it is in this order, and near to the 
Pleurobranchus, that the singular genus 
Umbrella, Lam. {Gastroplax, Blainv.) — 
Should be placed. The animal is a great circular Mollusk, whose foot exceeds by much the cloak, and 
has its upper surface roughened with tubercles. The viscera are in a superior and central rounded 
part. The cloak is only visible by its slightly projecting sharp edge along the entire front, and on the 
right side. Under this slight edging of the cloak are the branchiae, in lamellated pyramids, like those 
of Pleurobranchus ; and behind them is a tubular anus. Under this same margin, in front, are two 
* -^pfysUi virtdis, Montug., raised to a genus by Oken under the 
name of Actann, and which is at least nearly allied to the Elysia timida 
of Risso, has been considered as a near ally of Aplysia, but from want 
of a knowledge of the branclniE, 1 cannot classify it. [The branchite 
cover the back and the superior surface of the lobes under the form of 
avascular network, so that the true position of the Elysia is next to 
Placubranchus.] 
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