844 
CEPHALIC RETRACTORS — TRICHORHIZA. 
Advioliimur, Lhud.v, Siicciiiea, etc., and in respect to this portion of 
tlieir organization these genera are less highly specialized than Ar/oii, 
Tc:<f<a-eUn, etc, which ai)pertain to the groiq) 'I’richorhiza. 
The tentacular retractors are uppermost 
and originate mo.st posteriorly upon the 
eohunellar innsele, and in some species 
midway in their course give off mnscular 
strands to the anterior region of the foot. 
Before reaching the base of the tentacles 
each muscle divides, the larger branch enter- 
ing the ni)per tentacles or ommatophores, 
while the smaller one enters the lower or 
anterior tentacles, dividing again in most 
species before doing so and sending a branch 
to the labial lobe. In Sncc/um the labial 
retractors are absent, but the lower tenta- 
cnlar retractor at the right side gives off a 
short stout muscle, which is attached to 
and acts as a sid)sidiary retractor to the penis-sheath. 
The pharyngeal or hnccal retractor is the median muscle and passes 
through the nerve-ring, becoming attached at one or more points to 
the buccal bulb; it may, as in Sacclnea, be cleft to its base or, as is 
more usual, form a single strand which divides or merely dilates at its 
attachment to the buccal bulb. 
The Trichorhiza (rptx'), in three parts ; a root) or species 
with three distinct pro.ximal muscular 
attachments embrace Testace/ht, Arlon, 
and certain The muscle 
strands in tho.se .s})ecies pos.sessing well 
developed shells, as Ilijd/lnld, arise from 
the great eohunellar retractor, the 
}»haryngeal muscle always having the 
mo.st posterior origin upon it, differing 
in this respect from the group Monorhiza, 
in which it usually has a more anterior 
origin. In the Arionidw, etc., the 
proximal terminations of the pharyngeal 
and tentacular retractors are affixed separately to the integument 
in the rear of the resi)iratory cavity, the pharyngeal retractor is 
Fig. 638. — Cephalic retractors of 
Arion suh/uscus Drap. X 3, illustrat- 
ing the section Trichorhiza. 
/. pharyngeal retractor ; t. tenta- 
cular retractors. 
I ■ 
! ^ 
j - 
' A 
Fig. 637. — Cephalic retractors 
of Limax maxbnus L., X 2, 
showing their common origin 
from the vestigial columellar 
muscle, and illustrating the 
section Monorhiza. 
/. pharyngeal retractor ; i, 
tentacular retractors. 
