776 
CLASS XIII. 
it was probably the reticulated muscular fibres that he took for vessels 
in these dermal lamelle [Clio Mediterranea GRGENB., tentacles very short, 
no eye-points; Cl. flavescens GEGENB. G. thinks that the 3,000 suckers in 
each tentacle described by ESCHRICHT are merely forms of epithelium. | 
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Orver II.  Gasteropoda. 
Molluses with head distinct, in most tentaculate, the inferior 
surface of abdomen flattened or grooved, or produced into a com- 
pressed lamina. Some hermaphrodite, others distinct in sex ; many 
aquatic, some terrestrial. 
fin, 
Gasteropods. In most the heart lies on the left side of the body. 
In those, however, that have left-handed shells (see above, p. 684) 
the heart lies on the right side. In many the sexes are distinct ; 
others are bisexual, so that mutual impregnation of two or some- 
times more individuals occurs. 
The inferior surface of the belly forms an elongate flattened disc, 
which is very muscular, as is commonly known in slugs and snails ; 
these animals, met with everywhere, give an idea of the typus of 
this order. But in others, this ventral disc, usually named foot, is 
compressed laterally, and serves for swimming. We separate these 
molluses from the rest as a distinct group. 
Family Hi. A. Heteropoda. Foot compressed, resembling a 
furnished with a dise or suctorial acetabulum. Branchiz pec- 
tinate or pinnate. Sexes distinct. 
Heteropods. These molluscs all live in the sea, and usually swim 
with the fin-shaped foot upwards, and the back downwards. 
Forskin, to whom we owe the first description of this family of 
animals, gave them the name of Pterobranchea. The existence of 
distinct sexes was discovered by LaurtiLtarp and Mitne Epwarps. 
[The part named ‘ Foot’ is highly developed in this family, and in 
some attains a high potentiality (Gucrns.). It does not exactly 
correspond to the foot of Gasteropods, but to one part of it alone. 
Huxutry (On the Morphology of the Cephalous Mollusca, Phil. 
Trans. 1853, Pt. 1. pp. 29—65). A foot has four parts, the pro- 
podium, mesopodium, and metapodium, found in Heteropods, and a 
fourth, the epipodiwm, not found in them. The fin-shaped foot of 
heteropods is the propodiwm. Besides the works just referred to, 
comp. also, on this division, Leuckart Zoologische Untersuchungen, 
Drittes Heft, Giessen, 1854, Der Baw der Heteropoden, pp. 1—68.] 
