THE MOLLTJSCAN FOOT. 795 



14. The Molluscan Foot. 



The foot of a Gastropod in its fullest development consists of a median sole 1 and 

 lateral appendages resembling metapleural folds. 



Huxley {Phil. Trans. 1853) divided the median portion of the foot into three portions, 

 propodium, mesopodium, metapodium, and called the lateral portion the epipodium. He 

 pointed out that of all Mollusca the Heteropod genus Atlanta possesses the best de- 

 veloped foot-proper, and has its parts best specialised and separated, the peculiar ciliated 

 sucker of this pelagic animal representing the mesopodium. 



Twenty years later Grenacher established a more fundamental subdivision of the 

 molluscan foot, retaining Huxley's epipodium, but viewing the entire median portion, 

 whether simple or differentiated into regions, as the protopodium*, which is not to be 

 confused with Huxley's propodium. 



Huxley's threefold subdivision of the protopodium still remains, though of subordinate 

 importance. An interesting example of it is afforded by the widely distributed species 

 Harpa, ventricosa, which I met with in the course of my travels. In this form the proto- 

 podium as a whole is capable of enormous extension so as to cover a relatively immense 

 superficial area, and the propodium is not only marked off from the rest of the foot by 

 a deep notch on each side, but Brock 3 has discovered that it possesses a special inner- 

 vation in the form of a very remarkable nervous reticulum. The mesopodium and meta- 

 podium of Harpa appear at a hasty glance not to be differentiated from one another as 

 there is no operculum, but closer inspection reveals a line of division between them, and 

 if the hinder or caudal end of the foot of the living animal be held firmly in the hand 

 the metapodium is cast off and remains in the hand while the animal falls to the ground. 

 This is an interesting example of muscular autotomy which was known and is referred to 

 in the Cambridge Natural History*, though the suggestion that it is effected by pressure 

 of the shell is erroneous (PI. LXXVI. fig. 3). 



The metapodium is cut off with a clean concave anterior surface, the posterior surface 

 of the mesopodium after the act of autotomy being convex. There is no effusion of blood. 



In a large species of Oliva which was common at Lifu, the protopodium is also 

 capable of great extension and will wrap itself round land-snails which are offered to 

 it as bait by the natives. 



The sides of the protopodium may be produced into great natatory folds as in 

 Aplysiidae. These lateral expansions of the protopodium are called parapodia (Pelseneer) 

 or pteropodia (von Jhering). Whether or not these are of the nature of true epipodia 

 is a question which does not concern us here. 



The epipodia are presented in their most typical development in certain proso- 

 branchiate genera, particularly in Haliotis, where they appear as a pair of deeply cleft 



1 The " foot- proper" of Huxley. 



2 Grenacher, H., " Zur Entwickelungsgeschiehte der Cephalopoden. Zugleieh eiu Beitrag zur Morphologie 

 der hoheren Mollusken." Zeitschr. wiss. Zool., Bd. 24, 1874, see p. 465. 



3 Brock, J., "Zur Neurologie der Prosobranchier. " Zeitschr. wiss. Zool., Bd. 48, 1889, pp. 67—83, Taf. 6 

 and 7. 



4 Cooke, A. H., "Molluscs." Gambr. Nat. Hist., Vol. nr. 1895, p. 45. 



w. vi. 104 



