32 



HARDWICKE'S SCIENCE-GOSSIP. 



[Feb. 1, 1869. 



Land, while he himself was born on board a Dutch 

 brig in the Straits of Gibraltar. 



Dr. Gray interposed the lanthinidrc between the 

 Apple-snails and the Heteropods ; others affiliate 

 them to the Neritee ; while Mr. Woodward con- 

 signs them to the Haliotidce. They appear (by 

 their anatomy) to constitute a distinct family, whose 

 nearest affinities are with Solarium] and Scalaria, 

 the lingual dentition offering a close resemblance, 

 though the proboscis is very much longer in these 

 than in Ianthina ; oddly enough, as indicative of 

 the probability of the relationship here alluded to, 

 Scalaria secretes a purple fluid closely resembling, 

 if not absolutely identical with, that produced by 

 Ianthina. 



Fig. 1". Ianthina communis, nat. size. 



The foot is the organ upon which I purpose to 

 dilate as far as the limits of " Gossip " will permit, 

 and as it is a certainty that amongst its many 

 readers there are some who are not fully conver- 

 sant with the ideal archetype of the mollusca, I 

 think I may venture upon an elementary definition of 

 the parts of the foot for their benefit. 



The molluscan foot consists of four distinct 

 portions, though some authorities omit the fourth, 

 erroneously regarding it as a part of the body. 



1. The Propodium, or fore-foot, which, in this our 

 pelagic example, forms a boat-like expansion which 

 helps to float the mass. 



2 The Mesopodium, or middle foot, which con- 

 stitutes the creeping surface in the common snail. 



3. The Metapodium, posterior or hinder foot, the 

 upper surface of which, when protruded, may be 

 called the heel, and is known as the opcrculigerous 

 lobe, from its bearing the operculum when present. 



4. The Ppipodium, — literally, on or uponthe foot, — 

 spoken of as a single part in an ideal foot only, but 

 consisting of two lobes or processes in reality. The 

 term epipodia, in the plural, freely rendered lateral 

 feet, would perhaps be a better term ; but adhering 

 to the current word in the singular, I may define it 

 as lying immediately above the three preceding con- 

 stituents, and between them and the meso-soma, or 

 floor of the body. In some genera it scarcely pre- 

 sents a distinguishable division, while in others its 

 boundary is marked by a collar or ridge ; in others 

 it displays an overhanging mantle-like fold, and in 



others is developed into swimming-organs, as in the 

 Pteropods. 



The Float of Ianthina is somewhat inaccurately 

 styled " an apparatus of locomotion," and is no less 

 incorrectly spoken of as " a nidamental contrivance," 

 and as " developed only in the breeding season, and 

 thrown off as soon as the ova are deposited." The 

 simple term float in its common acceptation conveys 

 the most correct expression of its nature and uses. 

 It is present even when Ianthe is in her babyhood, 

 and is not by any means "a modified operculum." 

 In the first place, an operculum, when present, is 

 invariably borne on the back or upper surface of the 

 heel, and not on its under side ; whereas our sea- 

 nymph's raft is attached to the under surface or sole 

 of the metapodium : in the embryonic stage she is 

 furnished with a rudimentary operculum on the 

 back of her heel, which becomes arrested in develop- 

 ment, and is ultimately thrown off, leaving no trace 

 in the adult. 



That the float is not of necessity the homologue 

 of the operculum, we have proof positive in the 

 Macgillivrayia, in which the two are co-existent ; 

 neither is the float a mere egg-raft, for it is possessed 

 by creatures that never attach their ova thereto, 

 but deposit them elsewhere. Nor does our heroine 

 propel herself by " a lateral expansion of the 

 mantle," but by means of her largely developed 

 epipodia before mentioned. 



The underside of the metapodium is composed of 

 transverse rugae, in the recesses of which are im- 

 bedded the secreting follicles which produce the 

 plastic fluid of which the vesicular float is con- 

 structed, presenting us perhaps with an analogue of 

 the byssus-making foot of the mussel tribe. 



The manufacturing process may be readily com- 

 prehended by glancing at the following drawings, 

 which are copied from originals taken from nature 

 by Dr. J. D. Me Donald, R.N., F.R.S., and placed by 

 him at my disposal. The species here represented is 

 Ianthina exigua (fig. 18). 



Fig. 16. Foot x 12. 



Fig. 19. x 12. 



Here we see the spatulate fore-foot of the crea- 

 ture floating at the surface of the water, expanded 

 to catch a globule of air (fig. 19). 



Here we have the propodium closing over a 



