188 
CEPHALIC TENTACLES. 
Both pairs are exceedingly Ilexible and can, at the will of the 
animal, be withdrawn for safet)" within the body cavity by the con- 
traction of the special retractor ninscle, hxed to the apex of each 
tentacle, a mode of retraction termed .Vcrendiolic by Prof. Ray 
Lankester; their eversion and also elongation being accomplished by 
blood pressure and the successive or wave-like contraction of the 
annular or transverse muscular hbres of the organ, the apex or bulbous 
end being the last part i)rotruded ; this method of eversion or unfolding 
by the sides has been distinguisheil by the term Pleurecbolic, in con- 
Fig. 3G2. Fig. 3(>3. 
Fig. 3(U. 
Rctraciile tentaclc.'% of a Styloiniuatophore, showin;? llie retractor muscle and illustrating the 
various stages of the mode of retraction and eversion, termed Acrembolism and Pleurecbolism. 
'The figures viewed from left to right show the tentacle in process of retraction and from right to 
left .as undergoing extension or protrusion. 
Fig. 3(>1 .shows the tentacle completely extended ; Fig. 301 completely invaginated ; Figs. 302 
and 303 illustrating tlie intermediate stages. 
tradistinction to the terms Acrecholic and Pleurenibolic, which 
distinguish the i)rocesses by which the introvertiltle or retractile pro- 
bosces of some ^Streptoneures are ])rotruded and retracted. 
The Streptoneura of our fauna have only two rvell-developed 
tentacles, wliicli may be comparatively thick and rounded at the e.x- 
tremity, as in Cijclodoma elegdns;, or extremely long and delicately 
tajiering to a tine point, as in Buthhihi tentafiddt((, which receives 
its specific name fnmi the length and delicacy of its tentacles, but 
l)oth forms are contractile only, 
and cannot be retracted or drawn 
within the body cavity for protec- 
tion, as can the hollow retractile 
tentacles of the Stylommatophora. 
Although, in the terrestrial species 
of Strei)toneura, an organ of olfac- 
tiem may be located near the apex of each tentacle, as in the Stylom- 
matophora, yet the eyes are placed on sluu't i)edicels behind, which 
are more (jr less intimately fused or combined with the more develoi)ed 
tactile tentacles. 
The right tentacle of the male in some gnmps, as in the Vhlpdrw, 
is noticeably stouter than the left, this sexual dimorphism being due 
Fig. 3lV). — Dor.'tal a.'^pect of the cephalic 
region of Cyclostonia elcgans^ showing the con- 
tractile tentacles x 1 (after Motiuin-Tandon). 
