PALLIAL OUTGROWTHS. 
201 
Fig. 397. — Planorbis corneus {L,.), showing 
the exsertile Auriform Lobe (after Pelseneer). 
rJ. respiratory or auriform lobe. 
Tlie Extension or outgrowth of the mantle margins may, as in the 
Euthynenra, form the Auriform Lobe, known as the respiratory 
and also as the fecal lobe, a tegumentary appendage near the respira- 
toiy orifice, which may bear the 
anus or termination of the rectum. 
In Planorhis this lobe is largely 
develoi)ed and exsertile with a rich 
vascularization, and a similar but 
much smaller and slightly twisted 
prominence has been observed to 
exist in certain species of the Helicida'. The “balancier” of Vitr'ma 
is a somewhat spatulate outgrowth above the respiratory orifice, 
kept in almost incessant motion by the animal. The Columellar 
Lobule, a somewhat triangular pallial process of the hinder margin, 
is also developed in some species and is especially noticeable in those 
forms in which the umbilicus is closed by a shelly deposit. 
In the Streptoneura of our fauna the instances of pallial outgTOwths 
are few and insignificant. In Yimpara, the right mantle margin 
bears a number of slender but hollow processes of various sizes, 
which give rise to the spiral rows of minute hairs which encircle the 
young .shell, and Valvata possesses a well developed pallial tentacle 
on the right side which has been thought to be the vestigial 
representative of the vanished, but primitively left ctenidium. 
In the Pelecypoda similar processes evidently exi,st on the mantle 
of Sphcurium corneum, whose shell when young bears numerous 
small and projecting hair-like processes. 
The Enclosure of the shell within the mantle, owing to the exten- 
sion and fusion of the pallial margins, is known to exist in exotic 
species belonging to each of the groups of mollusca represented in our 
fauna, yet it is only in the Euthyneures that we have species which 
enable us, though disconnectedly, to show some of the stages in this 
particular line of pallial specialization, which leads us from the typical 
Helix, with the mantle margins extending to or slightly overlapping 
the aperture of the shell, to Avion, with the shell covered in and 
practically lost by the overfolding of the mantle. 
That Avion and other naked species are derived from forms with 
distinctly developed shells is shown not only by their retention of the 
vestiges of a shell, but by passing through a stage of development in 
which a distinct spiral shell, containing the intestinal sac, is present. 
