MOLLUSCA. 
352 
branous laminae to cover them in time of danger : and besides the two conoid tentacula in front, similar to those 
of Doris, they have four, or sometimes six others, which are simply pointed. 
The Tritonies {Tritonia, Cuv.), — 
Have a body, superior tentacula, and generative organs, as in the Doris ; but the anus and the vent of 
the peculiar secretion are on the right side, behind the vulva : 
the arbuscular branchiae are arranged along each “side of the 
back, and the mouth, guarded by broad membranous lips, is 
armed within with two lateral horny and cutting jaws, in shape 
somewhat like to the scissors for shearing sheep. 
We have a large species (Tritonia Hombergii, Cuv.) on our coasts ; and 
there are many others, some of them very small, which exhibit great variety in the size and figure of their branchiae. 
[Melibea, Rang, differs in having filiform simple tentacula issuing from a wide sheath, and two series of ovate 
muricated or tuberculated branchiae on the back, which readily fall off when the animal is handled. M. rosea, 
which lives on floating sea- weeds near the Cape of Good Hope, is the type ; but there are some European Mollusca, 
of small size, which are also referable to it.] 
The Thethys, Linn., — • 
Have along the back two row^s of tufted 
branchiae ; and upon the head a very large 
membranous fringed veil, which curves, in its 
contraction, under the mouth. The mouth is 
a membranous proboscis without jaws ; there 
is at the base of the veil two compressed 
tentacula, from the margin of which issues a 
small conical point. The orifices of generation, 
of the intestine, and of the peculiar secretion, 
are as in Tritonia. The stomach is mem- 
branous, and the intestine very short. 
There is, in the Mediterranean, a beautiful spe- 
cies of a greyish colour, spotted with white (Thetis 
fimbria, Linn.). 
The SCYLL.3EA, Linn. 
In this genus the body is compressed ; the 
foot narrow and furrowed, to enable it to embrace the stems of sea- 
weed ; no veil ; the mouth forming a small proboscis ; the exterior 
orifices as in Thethys ; the tentacula compressed, terminating in a 
cavity from which a little point, with an uneqnal surface, can be 
protruded ; and upon the back are two pairs of membranous crests, 
carrying, on their inner aspect, some pencils of branched filaments. 
The middle of the stomach is covered with a fleshy ring, armed 
wfith horny laminae as sharp as a knife. The common species is found on Fucus natans, or gulf-weed, 
wherever this appears. 
The Glaucus, Forster, 
Have the elongate body and the vents as in the preceding ; four minute conical tentacula ; and on each 
side [two or] three branchiae, each formed of long fringes ar- 1| 
ranged like a fan, and by whose means they swim. They are M 
little charming Molluscs of the Mediterranean and Indian a 
Ocean, agreeably painted with azure-blue and silver, and swim ® 
with great quickness on their backs. Their anatomy closely f 
resembles that of Tritonia. The species have not, as yet, been | 
satisfactorily distinguished. ^ 
The Laniogerus, Blainv., has, on each side, two series of little 
plates, finely divided in a pectinate manner, which are the branchiae. 
The body is shorter and thicker than in Glaucus, but they have its 
four little tentacula. 
The Eolidia, Cuv., 
Resemble little slugs, with four tentacula above, and tw^o on 
Fi(f. 167-— Scyllaea pelagica. 
Fig:. 1C6.— Thethys leporina, upper and under sides. 
Fig-. 165. — Tritonia. 
