352 



MOLLUSCA. 



branous laminae to cover them in time of dang-er: 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 {Triionia, Cuv.), — 



Have a body, superior tentacula, and generative organs, as in the Doris ; but tlie 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. 31. rosea, 



which lives on floating sea-weeds near the Cape of Good Hope, is the type ; but there are some European MoUusca, 



of small size, which are also referable to it.] 



The Thethvs, Linn., — 



Have along the back two rows 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 vf^iite (Thetis 

 fimbria, Linn.). 



The Scyll^a, Linn. 



Fig. 166.-Thethys leporina, upper .nd under Bide.. j^^ ^j^jg ggj^„g ^]^^ ^^^y J^ cOmprCSSCd ; 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 unequal 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 



with 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- 

 ranged like a fan, and by whose means they swim. They are 

 little charming Molluscs of the Mediterranean and Indian 

 Ocean, agreeably painted with azure-blue and silver, and swim 

 with great quickness on their backs. Their anatomy closely 

 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 thebranchia;. 

 The body is shorter and thicker than in Glaucus, but they have its 

 four little tentacula. 



The Eomdia, Cuv., 

 Resemble little slugs, with four tentacula above, and two on 



F.B. ir,r.-Scyll.x.a (.elagi, 



Fig. 168.- 



