ALIMENTARY SYSTEM — BUCCAL BULB. 
247 
The Stomodeum (o-rojaa, month ; oSatos, belonging to a way) or 
anterior region of the alimentary tract, which comprises the month 
with its related parts and also the oesophagus, is always placed at the 
anterior end of the body, and in some forms originates as a simple 
epiblastic invagination of the body-wall of the embryo, which meets 
with and opens into the mesenteron or median part of the alimentary 
canal ; sometimes, however, it is formed by the persistence of the 
blastopore, or orifice of embryonal invagination formed during the 
gastrula stage of development. 
In the Gastropoda the oral aperture is surrounded by variously 
shaped lips which open into the buccal cavity or pharynx, whose walls, 
especially ventrally and laterally, are formed by thick longitudinal 
and annular muscular layers constituting the buccal bulb, which in 
the more primitive inollusk was placed behind the cerebro-pedal nerve 
collar, but in the more specialized forms is now in front of it. 
The buccal bulb is protruded or retracted by means of special 
muscles, which, according to their function, are distinguished as 
Protractors or Retractors respectively. 
In the Streptoueura, Garnault has studied their an’angement in 
Cyclostoma elegans, and has described the protrusion of the buccal 
mass as mainly due to the contraction 
of the powerful lateral protractors, 
which arise from the smaller segments 
of the mandibles and are attached 
to the external integument near the 
oral aperture : these are supplemented 
by longitudinal muscles within the 
walls of the buccal bulb. 
The retraction is chiefly effected 
by two long paired muscles affixed to 
the larger jaws, which pass somewhat 
divergently backwards through the oesophageal ring, and mingle with 
and become lost amongst the pedal strands of the columellar muscle. 
In the Euthyneura the protractors of the buccal mass are several 
small muscular bands attached to the buccal bulb and to the anterior 
walls of the head region. The retraction is chiefly performed by a 
branch or branches usually arising from the columellar muscle, which 
are attached to the ventral and lateral surfaces of the bulb and exhibit 
much interesting variation in the different species. 
Fig. i99. — Buccal bulb of Cyclostoma 
elegans fMull.), X 10, showing its chief 
retractor and protractor muscles (after 
Garnault). 
a.p. anterior protractors; b.b. buccal 
bulb ; l.p. lateral protractor muscles ; 
£P. oesophagus ; p.r, pharyngeal or 
buccal retractors ; 7’.s. radula sac. 
