108 COMPARATIVE ANATOMY chap. 



developed as a simple or fringed border, which cariies a few tentacles (usually four 

 on each side) of varying length (Fig. 3, p. 4). At the base of each tentacle there 

 is a lateral organ. Eyes are said to occur at the bases of the epipodial tentacles in 

 Euniargcrita and Scissurclla. 



The epipodiuni is, as a rule, wanting in Docoglossa, lint one is found beset with 

 papillic in the genus Helcion, and in PatincUa and Nacella it is fringed ; these 

 epipodia correspond in ])osition with those of other iJiotocardia. 



A well-developed epipodiuni rarely occurs among the Monotocardia, but lanthina 

 has a typical ejiiiiodial border, and the Lifiopidce and many Jiissoidce have an 

 epipodium with several (1-5) tentacles on each side. Many other Monotocardia 

 liave retained either the anterior or posterior portions of the epipodium. 



{a) Anterior vestiges of the epipodium are found in Vcnnetus in two anterior 

 pedal tentacles, and in Paludina and Arnpullaria in two nuchal lobes, which 

 must not be confounded with true cephalic tentacles. In Fahidina, the right 

 nuchal lobe, and in Arnpullaria the left, forming a longitudinal groove, becomes a 

 sort of siphon. Calyptrcca jiossesses on each side under the neck a semicircular 

 epipodial fold. 



(6) Posterior vestiges of the epipodium are found in Lacuna in the form of an 

 epipodial fold with a process on each side above the foot. Narica has, above tlie 

 metapodium on each side, a wing-like epipodial lobe. 



(c) Median and jjosterior vestiges of the ejdpodium are found in Choristcs, where 

 there is a median papilla on each side, and posteriorly a ]iair of tentacles below the 

 operculum. 



The epij)odium is always innervated from the pedal nerve cords or the homolo- 

 gous pedal ganglia, or from the pleural ganglia which separate off from the latter. 



The foot of Hipponyx undergoes a curious transformation. Hipponyx is a 

 Monotocardian genus, with a conical shell ; the animal attaches itself firmly" to 

 rocks or the shells of other Molluscs, which it excavates, either directly or by 

 means of a shell plate, which probably answers to the operculum. The median part 

 of the sole of the foot has lost its muscle layer, and its edge has united with the 

 edge of the mantle, leaving only an anterior aperture through which the head can 

 be protruded. On the lower side of the foot, the columellar muscle which descends 

 from the shell gives rise to a liorseshoe- shaped muscular area surrounding the 

 central non-muscular part. 



Without going into details as to the method of locomotion of the Prosobranchia, 

 it may be stated that most of them creep or attach themselves by means of the flat 

 sole of the foot. 



Heteropoda. — The Heteropoda are pelagic Prosobranchia [Monotocardia), -which 

 have exchanged the creeping for the swimming manner of life. The foot has in 

 them become peculiarly adapted to this new method of locomotion. The propodium 

 has become changed into a narrow vertical rowing fin (carinate foot), which when 

 the animal is in its swimming position is turned ujiward. 



The development of this vertical fin can be traced almost step by step within 

 this division, starting with Oxyyyrics, and proceeding through Atlanta and Carinaria 

 to Ttcrotrachca. In this series, the typical outer appearance of the Prosobranchiate 

 (its shell, visceral dome, mantle, and gills, which are still retained in Oxygyrus and 

 Atlanta), gradually disappears owing to development in another direction. 



Oxygyrus (Fig. 99, A) still has the characteristics of a Prosobranchiate. The 

 foot consists of (1) a propodium, the creeping sole of which has been somewhat 

 hollowed out or deepened ; anteriorly it possesses a fin-like outgrowth, which is used 

 as a propelling organ in swimming ; and (2) a distinct metapodium directed backwards 

 like a tail, and bearing an operculum. The derivation of such a foot from that of 

 certain Prosobranchia, which have distinct propodia and metajiodia, such as the 



