COLUMELLAR MUSCLE. 
342 
or wanting in .slnggisli mollnsks, and most prononnced in tlie 
more active forms. 
Tlie general cliaracter of tlie muscnlatnro of tlie molliisca is, in a 
great measure, dependent upon the development of a protecting shell, 
which furnishes a firm attachment to the great retractor muscle.s of 
the body, hy whose action the animal is withdrawn within its .shelter, 
the columellar muscle of the Gastropods apparently representing the 
more nnmerons retractors of the Pelecypoda. 
The Columellar i\luscLE in the testaceons Gastropods is the 
principal retractor of the body, its presence being typically correlated 
with the possession of an external shell into which the body may be 
wholly or jjartially withdrawn for protection ; it also constitntes the 
sole connection between the animal ami its shell, the muscle being 
organically tixeil to tbe .shell-axis or columella and running forward 
beneath the resi)iratory cavity, the constituent glistening Avhite fibres 
dividing and becoming distally distributed among and interlacing with 
the mnscnlatnre of the foot, chiefly in the longitudinal median region ; 
it also gives off from its npi)er surface subsidiary yet powerful 
anteriorly-directed muscular strands which are known as the cephalic 
retractors. 
d'he Cei’iialio Retractors (K£</jaA/;, the head) form a very impor- 
tant part of the musculature of the body, the constituent parts being 
distinguished as Pharyngeal orTentacnlar 
retractors, according tn the function they 
discharge. 
'riie Pharyngeal or Buccal retractors 
are always median and may be formed of 
paired and slightly divergent muscular 
slips or of a more powerful single 
muscle, but they always pass through 
the nerve ring to become fixed to the 
buccal bulb by divided or by simply 
expanded extremities. 
The Tentacular retractors are always 
l)aired, corresponding muscles being pre- 
sent for the right and the left sides of 
the body, which may be united posteriorly with the great colnmellar 
muscle or, as in certain of the nude species, may arise inde^ien- 
dently from the integument. They also exhibit special minor 
Fig. G35. — Cephalic retractors of 
.-i ma/ia (l)rap.) X 2, showing 
the airangement of the pharyngeal 
and tentacular retractor muscles. 
/. pharyngeal retractors ; /. tenta- 
cular retractors. 
