PROF. AGA&SIZ'S 



sides, all along the upper surface of the body. So 

 also Glaucus on both of the sides, (Plate XLII, fig. 

 A.) So it is also in Doris, where, however, the 

 mass of gills is placed only at the posterior extrem- 

 ity of the body, and has long tentacles at the an- 

 terior extremity. 



But, without entering into more details, youlmay 

 have already remarked that whatever differences 

 exist between these animals in the inequalities of 

 the two sides, we can reduce their symmetry to the 

 regular arrangement of parts on the two sides of 

 the body, more or less developed on one aide than 

 the other. And passing from these Snail-like Mot- 

 3usca to the Cephalopoda to the Cuttle Fishes we 

 shall have again (Plate XXXVI fig. A) all the parts 

 analagous to the symmetrical Gasteropoda, the 

 eyes and the gills are here again in pairs on the 

 two sides of the animaL 



But how will Cephalopoda and Gasteropoda 

 compare with the Acephala, is the great question. 

 The fleshy mass which is in the centre in Acepha- 

 la, is below the mass of organs in Gasteropoda. 

 We have the liver, we have the alimentary canal, 

 and we have the heart all shown. Those main 

 organs are above the fleshy mass, and hanging over 

 the fleshy mass, we have only the gills and the 

 mouth. 



Let us for a moment suppose that the mantle 

 was not so long, and would not hang in such large 

 folds on the two sides of the body, but be shorter. 

 And let us at the same time suppose that this 

 fleshy central part was not so contracted, (Plate 

 XLIV, fig. B) but stretched down, and you see 

 at once what analogy we have. You may change 

 at once such a bivalve shall into a univalve (Plate 

 XLIII, fig. A) with a single shell. Suppose the two 



[PLATE XLIIf. MARGARITA.] 



valves were united, and you will have what we 

 observe in Patella, where there is a shell spreading 

 on the back of the Mollusk, without any spiral on 

 the summit; and among bivalves there are several 

 in which the two valves are immovable ; the 'di- 

 vision is well marked in youth, but they unite to- 



gether in old age. This is observed in the 

 of Naiades, among those which constitute the 

 genug Alasmodonta. And that the cover be shield- 

 like or divided into two valves, does not indicate a 

 great difference. 



[PLATE XLYL NAUTILUS.] 



We have already noticed the little value of such 

 differences when speaking df the Crustacea, in 

 which we had among the Entomostraca, some 

 whose bodies were covered with Sat shields, and 

 others in which the bodies were enclosed between 

 two moving valves, as in Cypris. Suppose this 

 Patella was articulated in the middle, and the man- 

 tle was drawn down, there would be the first ap- 

 proach to the Scallop or the Oyster. Suppose that 

 the foot was reduced to one central fieshy mass, 

 and the analogy would then be almost complete ; 

 only the difference between the eyes and tentacles 

 would remain. 



That this is no vague supposition Jo admit of 

 such a division, is showa by some shells, in which 

 there is a notch on one side, in the longitudinal di- 

 ameter of the shell, for instance in Parmophorus, 

 and in Emarginula, there is realty a deep fissure. 

 So that we pass almost gradually into the type of 

 two connected valves, and into those which have 

 moveable parts. Now for the eyes and for the 

 other parts which are modified in this structure. 

 The eyes are here (Plates XLII, XLIII,) placed 

 around the mouth. The mantle in many of the 

 Mollusk univalves, extends all along the shell, as 

 you will observe in Phasianella, in Buccinum. &c, 

 But there are no eyes except in the head. Last 

 winter, however, it was my good fortune to meet 

 with a little Margarita in Boston Harbor, in which 

 we have (Plate XLIII, fig. A) tentacles all along 

 the body ; and at the base of each tentacle, are 

 dark spots similar to the eye which is observed in 

 the anterior part of the animal. On examination, 

 I noticed that the mantle is constructed as it is in 

 the Scallop. (Plate XXXVI, fig. B). We have, 



