776 CLASS xiir. 



it was probably the reticulated muscular fibres that he took for vessels 

 in these dermal lamellae [Clio Mediterranea GEGENB., tentacles very short, 

 no eye-points; Cl. flavescens GEGENB. G. thinks that the 3,000 suckers in 

 each tentacle described by ESCHEICHT are merely forms of epithelium.] 



ORDER II. Gasteropoda. 



Molluscs 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. 



Gasteropoda. 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 

 molluscs from the rest as a distinct group. 



Family III. A. ffeteropoda. Foot compressed, resembling a 

 fin, furnished with a disc or suctorial acetabulum. Branchiae 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. 

 FORSKAL, 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 LAURILLARD and MILNE EDWARDS. 

 [The part named ' Foot' is highly developed in this family, and in 

 some attains a high potentiality (GEGENB.). It does not exactly 

 correspond to the foot of Gasteropods, but to one part of it alone. 

 HCTXLEY (On the Morphology of the Cephalous Mollusca, Phil. 

 Trans. 1853, Pt. i. pp. 29 65). A foot has four parts, the pro- 

 podium, mesopodium, and metapoditim, found in Heteropods, and a 

 fourth, the epipodium, 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 Bau der Heteropoden, pp. 1 68.] 



