192 
FOOT AND PERIPODIAL GROOVE. 
THE PEDAL OR VENTRAL REGION. 
'I'he Podium or foot, vliich arises daring development as a promi- 
nence between tlie month ami the anus, is the most per.sistent and 
characteristic mollnscan organ, and is formed by an excessive develop- 
ment and specialization of the cellular and nnstriated somatic muscula- 
ture of the ventral surface of tlie body, in adaptation to a creeping- 
mode of life, receiving its innervation from special nervous enlargements 
called the jiedal ganglia. The -whole surface contains iunumerahle 
unicellular mucous glands, and, in addition, there are important and 
extensive aggregations of similar cells situate in certain definite parts, 
M'hich render i)rohable a mori)holog-ical connection between the glands 
of the Pelecypoda and those of the (histroi)oda. 
In (lastroi)ods the foot, though- varying greatly in size and shape, 
usually ])resents a long and broad 
])lantar surface or sole, which 
occupies the whole ventral surface 
of the body, but tho.se species with 
narro-w plantar area are ai)parently 
endowed with more rai)id loco- 
motive j)owers than those -with a 
more exi)anded surface. In the 
aquatic species the surface of the 
foot is richly covered -^vith vibra- 
tile cilia, but in those of terrestrial 
hal)it the ciliated area is compara- 
tively restricted. 
'I'he Peripodial (Jroove or pedal 
fuiTow is a more or less distinctly 
marked groove, separating the body 
region from the sole and is more especially found in the nude species ; 
it runs parallel with the edge of the sole of the foot along its whole 
length round the body, and is occupied by a more or less con.spicuons 
row of narrow, oblong, horizontal 
tubercles, upon which the tuhercles 
of the .sides of the body rest un- 
comforniably. 'I'he groove in some 
genera rises slightly towards the 
tail, where there is often a Caudal 
mucous gland containing a greater development and concentratifui of 
Fig. 376. — Ventral aspect of the fool of 
Helix asf>cysa^ showing its character and 
general form, as seen crawling up a slip of 
glass (from an instantaneous photograph). 
Fig. 377. — Peripodial Groove and Foot 
Fringe of Arion ater x 2. 
fr. foot fringe ; p.f. peripodial groove or 
pedal furrow. 
