794. CLASS XIII. 
three branchiz, two or a single one, composed of numerous leaves, 
arranged parallel like the teeth of a comb. 
Very rarely in place of branchiz a vascular network in the walls 
of respiratory cavity. Sexes separate, external organs of copula- 
tion distinct. 
Pectinibranchiates. In all two feelers and two eyes are present ; 
these last are often pediculate. The mouth has the form of a pro- 
boscis, of which the structure has been described at length by 
CuviER in the whelk (Buccinum). It consists of two tubes, which 
push one into the other and are connected together by the upper 
margin, so that when the innermost tube is unrolled and elongated, 
the outermost becomes shorter. This instrument is moved by many 
muscles'. The hinder part of the body contains the liver and the 
sexual organs. The sexes are distinct. In the male the penis is 
situated on the right side, behind the head, and in some, as in the 
whelk, is very large. It is folded round and concealed in the respi- 
ratory cavity, but is not retracted within the body, except in the 
genus Paludina, where it is protruded and retracted through an 
aperture in the right tentacle, which had been observed already by 
Lister, but was afterwards incorrectly denied by Draparnaup. 
Through the penis runs a tortuous canal, which on copulation, 
when that organ is erected, probably loses its tortuosities. Ac- 
cording to BastER and Buarnvitte, the shells indicate a difference 
of sex, and those of the female are wider, particularly in the last 
wreath’. 
The females secrete a kind of common envelope for the eggs, 
which they deposit at the same time with the eggs. On our shores 
round clumps of yellow vesicles may be frequently observed, which 
resemble bunches of grapes, and are the masses of whelk’s eggs in 
question®. According to Cuvier, this envelope is secreted by a 
1 Ann. du Mus. Xt. 1808, Mémoires s. l. Moll, No. 17, pp. 6, 7, figs. 8—1o. 
2 See Baster Natuurk. Uitsp. 1. bl. 39, 40; Buainv. Journal de Physique, XCIv. 
p- 92; Mucxen’s Archiv f. d. Physiol. Vil. s. 571—573, 1822. BasTER says that in 
Buccinum the shell of the male is somewhat smaller, that it has a greater number of 
wreaths, but which are thinner than those of the female. 
3 See figures of them in Baster Natuurk. Uitsp. 1. Tab. v. figs. 2, 3 of Buccinum 
undatum; Tab. vi. figs. I—3, of a species of Murex. Comp. ibid. bl. 38—45; see 
also Lunp Recherches sur les Enveloppes d’wufs des Gastéropodes pectinibranches, Ann. 
des Sc. nat., 2e Série, 1. Zoologie, pp. 84—112. By ARISTOTELES these masses of eggs 
are called jwedcchpae (translated favagines); he did not, however, suppose that shelled 
