CEPHALIC TENTACLES AND LABIAL PALPS. 
189 
to its containing the male organ of reproduction, tlie tentacle being- 
perforate at the apex for its protrusion, but when not located within 
the tentacle, this organ may be placed as a non-invaginable projecting 
Fig. 366. — Head of Vhnpara vivipara d 
to show the malformed right tentacle, due to 
containing the male organ of reproduction 
(after Simroth). 
Fig. 367.— Valvata piscinalis showing the 
large non-invaginahle male organ on the right 
side of the neck (after Moquin-Tandon). 
process of the body wall, behind it, upon the right side of the neck 
and there simulate a third tentacle, as in the Yalmtidw. 
The two tentacles of the Basommatophora are also contractile only 
and are elongately subulate or flatly triangular processes, according to 
the genus, the right tentacle being 
often distinctly broader basally 
than the left ; the whole surface is 
covered with fine, transparent and 
very sensitive vibratile cilia, which 
have a very vigorous rhythmical 
movement, producing perceptiljle 
currents in the surrounding water, 
as is evidenced by the attraction or 
repulsion of small floating particles, 
according as they have become 
involved in the apiiroaching or departing current. This action is 
always most striking when the animal is in 
motion with its tentacles extended, becoming- 
weak and languid if the tentacles are con- 
tracted or the animal he injured or sickly. 
In Ancyliis fluviat'dis cilia of the ten- 
tacle continue in motion for nearly a hour 
after the excision of the tentacle from the 
head, the g-yrations it performs recalling the rotation of the emhry 
which is also caused by cilial action. 
The Labial Palps or lobes in (Gastropods are, when distinctly 
developed, exceedingly mobile processes around the mouth, capable 
of a considerable amount of extension and contraction, and delicately 
sensitive to tactile impressions, receiving- their innervation from the 
cerebral ganglia, and ])reserved in a moist and sensitive condition by 
Fig. 368. — Head of Planorhis juarghiatiis^ 
showing the contractile and slender subulate 
tentacles, X 10. 
Fig. 360. — Head of Liimura 
stagnalis^ showing the character- 
istic thin, triangular and contrac- 
tile tentacles. 
0 , 
