GASTROPODA. 357 



It very generally bears on its hind end a horny or calcareous 

 operculum for closing the shell-aperture when the animal is retracted. 

 The form and size of the foot present various modifications according 

 to the condition of life. ^ 



In the sedentary Gastropods it is reduced in Vermetus and Magilus, which 

 are fixed, to a small discoidal projection ; and in Thyca and Stilifer, which are 

 parasitic, to a small appendage. In the pelagic forms it is flattened laterally 

 us a fin (Heteropoda) 

 or may even be absent 



(Pkyllirhoe). In the ^^ O 



leaping forms (Strom- 

 bidae) it is also flat- 

 tened. 



The creeping surface 

 is sometimes divided 

 by a longitudinal fur- 

 row, and the two halves 

 may move alternately 

 (Cyclostoma). The two 

 anterior corners of the Fio. 27P. Helix pomatia. eyes at the extremities of the long 

 foot may be prolonged 

 as tentacles. 



The anterior part of the foot may project beneath the head, forming the 

 propodium (burrowing forms); in Natica the propodium projects back on to 

 the cephalic region. 



The epipodia (sometimes called parapodia) are fin-like, produced lateral 

 portions of the foot (in many Opisthobranchs) ; in Notarchus the epipodia 

 fuse over the dorsal surface. In the Rhipidoglossa the epipodia are present, 

 and carry papillae. The posterior part of the foot is often marked off from the 

 r^est as metapodium ; in such cases when a propodium is present as well, the 

 middle part is called mesopodium. 



The pedal glands secrete a mucous substance, which lubricates the 

 surface on which the animal moves, or hardens on contact with air or 

 water into a thread by which the animal suspends itself (Limax, 

 Litiopa, Cerithidea, etc.), or, as in lanthina, becomes entangled with 

 bubbles of air and forms a float to which, in the female, the eggs are 

 attached. The secreting cells are distributed as unicellular glands in 

 the epithelium of the foot, and are often aggregated in special invagi- 

 nations of the skin. Such are .the supra-pedal gland which opens at 

 the front edge of the foot (Pulmo?iata, etc.); the labial glands 

 which open into a furrow at the anterior end of the foot (creeping 

 Streptoneura and Opisthobranchs) ; and the median pedal gland 

 which opens on the ventral surface, and is comparable to the byssus 

 gland of Lamellibranchs (Cyclostoma, Cypraea, etc.) ; it was formerly 

 mistaken for an "aquiferous pore." Finally, in some forms there 

 are glands at the hind end of the foot, either dorsal or ventral. 



