GLANDULAR SYSTEM — VENTRAL GLAND. 
315 
The gland is composed of large and oval nucleated secretory cells, 
with pale finely grannlar contents, which lie in the snrroniiding tissue 
and open on its walls, and is usually a vertically compressed and 
conspicuously folded canal 
strongly ciliated on its 
ventral surface and often 
(piite as long as the foot 
itself, occupying the median 
line of the b(jdy above the 
sole, and partially embedded 
amongst the i)edal muscles, 
but sometimes lying ([iiite free within the body cavity, it serves as a 
reservoir and duct for the mucus issuing from the innumerable 
intercellular passages leading to it. 
The e.xternal aperture of the gland is between the mouth and the 
anterior end of the foot, the mucus being driven out by the action of 
the cilia, by muscular comiu’essiou 
of the cavity, by the undulatory 
action of the foot during loc(jm(j- 
tion, and by the general movements 
of the whole body, and forming the 
glistening iridescent tracks which 
so distinctly mark the path tra- 
velled by the mollusk (see p. 313, 
f. G()‘2, and which are not (jnly 
percei)tible on land, l)ut in the stiller pools these slime-tracks are left 
on the surface of the water by the larger Lhnna'cv, and may at times 
be observed of considerable length. 
The numerous mucous tracks of Llmnajn dagnalis observed by Mr. 
Henry Crowther upon the surface of Tag Lijck, near Elland, Yorkshire, 
on a warm still day in August, 1393, were several yards in length, 
crossing and recrossing each other many times. 
Slime-tracks do not, however, actually possess the gias.sy smoothness 
of surface they apparently present to a cursory observer, but are really 
composed of a multitude of transverse wavy ridges, about a millimetre 
apart with intervening depressions, this appearance being evidently a 
retlecti(ni and effect of the undulatory movements of the foot-sole. 
The Ventral Gland present in Cgclostoma and other Streptoneura, 
as well as in the Limnwkhv, is a ramified cavity within the foot, 
Fig. GDC. — Portion of pedal gland of 
Testae t'lla hnliotidca Drap. , highly magnified 
(after Lacaze-Duthiers), showing its convo- 
luted character and the investing tissue. 
s c 
Fig. 605. — Transverse section through the pedal 
gland of Liniax maxiiuus L., highly magnified. 
hi. lumen or cavity of gland ; s.c, nerve cells ; g.c. 
ganglion cells ; /. lacunae. 
