CEPHALIC RETRACTORS — DICHORHIZA AND MONORHIZA. 343 
features which are more or less specifically or generically charac- 
teristic, the retractor of the right ommatoi)hore, in many species, 
passing between and separating the penis and the vagina, while the 
retractor of the right lower tentacle passes beneath the penis, which 
is thus confined within the loop the retractors form. 
Although the cephalic retractors are less specialized in the Strep- 
tonenra than in the Stylommatophora, yet the coluniellar retractor 
in the former group exhibits a special modification, due to the pre- 
sence of the operculum upon the dorsal surface of the foot, the latter, 
however, in coiTelation with the acquirement of paired invaginable 
tentacula, exhibit greater complexity anteriorly, the tentacular 
muscles functioning for the independent or collective withdrawal of 
these organs within the body; more delicate muscular strands may 
also be given ofi’ to other organs to facilitate their retraction. 
The cephalic muscles vary in complexity and may for the purposes 
of .study be divided into three groups : Dichorhiza, Monorhiza, and 
Trichorhiza, based upon the number of their points of attachment to 
the columellar muscle or to tlie integument in the nude .species. 
The Dichorhiza fii i)arts ; a root) or two-muscled 
species exhibit the simplest form of cephalic musculature posse.ssed 
by our Gastropoda, and would appear 
to be especially characteristic of the 
Streptoneura and the Basommatophora, 
being quite deficient of any trace of 
tentacular retractors in correlation with 
their possession of n on-retractile ten- 
tacles; the pharyngeal retractors, which 
alone are present, are two distinctly 
separated muscular strands, arising from 
the great columellar muscle and affixed 
to the buccal bulb. In the Basommato- 
phora the penial retractors may also 
emanate from the columellar muscle 
near the same spot. 
The Monorhiza (/xdi'o?, single ; p'i(a, a root) are those forms in 
which the muscles for the retraction of the head, tentacles and 
adjacent organs are combined together medially to form a common 
stem at their point of origin from the columellar muscle. This group 
embraces tlie Helices, certain Ihjalinkv, and the genera Amalia, 
Fig. 636. — Cephalic retractors of 
Liptincca (MiiU.), X 3, illus- 
trating the section Dichorhiza, and 
incidentally .'showing the origin and 
character of the penial retractors. 
c.m. columellar muscle ; p.r. 
pharyngeal retractors ; r. penial re- 
tractor; p.s. penis sheath ; v.d, vas 
deferens. 
