pallial outgrowths and degeneration. 
‘ 20 ;^. 
When thiLS developed, the interior surface of the mantle having- 
become permanently external, assumes special sculpture and markings 
in harmony with those of the body. In the Limaces it becomes 
more or less regularly concentrically fniTowed, hut in Avion these 
concentric wrinkles are not perceptible, the whole surfiice being some- 
what uniformly granular, 
although during strong 
Fig. 101 . — Arion horteiisis F^r., Horsforth, near COlltractlOn ail appcaraiice 
Leeds, illustrating the complete infolding of the shell n . • .j. • j • 
by the mantle and its consequent atrophy and loss. Oi COllCGlltriC StriRtlOll IS 
apparent at the anterior end. In Amalia the mantle, though simply 
rugose, displays a smooth, somewhat rhomboidal furrow, which is 
deepest on the right side ; it is sub-angulate in fi-oiit, rounded on the 
left side and distinctly angulate on the right, the point joining the anal 
furrow anterior to the respiratory orihce ; its use may possibly be to 
drain some secretion into the mantle cleft. 
In the Pelecypoda of our fauna we have no examples of such high 
pallial specialization as we find in the Arionidw and Limacidw, 
although such a form is actually known ; the genus Chlamydoconcha 
having the mantle lobes so greatly developed that they have com- 
pletely enclosed the shell, the valves of which have lost their con- 
necting ligament and adductors and are now separately imbedded 
within the pallial lobes. 
The Reduction in Size of the mantle and shell may be in correlation 
with the detorsion of the visceral sac or from adaptation to special or 
Fig. 402. — Tcstacella haliotidea Drap., Oxford, collected by Prof. E. B. Poulton, 
Illustrating the reduction of the mantle and shell resulting from the detorsion of the body and 
adaptation to a special mode of life, and incidentally showing the strongly marked lateral grooves. 
peculiar habits of life, and we have a striking exemplification of this 
feature in the genus Testacella, a group of mollusks especially adapted 
to a subterranean existence, in which the visceral sac has become 
untwisted and the respiratory organs returned to the rear of the 
animal, the mantle being reduced to the smallest dimensions and 
placed at the extreme hinder end of the body, its shell forming a shield 
or protection at the rear while the creature is traversing the worm- 
burrows in search of the worms upon which it feeds. 
