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a strong sphincter muscle and a thin sheet of longitudinal fibres which arise from 

 the equator of the pharynx and end among the circular fibres. The free edge 

 of the membrane forms a circular lip. The contraction of the sphincter trans- 

 forms the peristomial tube into a cone, thus practically closing the mouth and 

 throwing the otherwise smooth or slightly wrinkled surface into folds. 



The pharynx is armed with a pair of powerful chitinous jaws to which a 

 number of strong muscles are attached. The mandibles and muscles together 

 enclose the pharyngeal cavity and form a large oval mass . the pharynx or 

 buccal bulb, which is enclosed in the muscular case (described in connection 

 with the foot) formed by the bases of the arms. A pair of salivary glands and 

 the infrabuccal ganglion are attached to the dorsal end of the pharynx and the 

 oesophagus is attached near but a little above them. 



The space between the pharynx and its case is an extensive but incapa- 

 cious blood sinus which is divided into two cavities, the outer, and the inner 

 buccal sinus, by a thin- walled conical muscular tube which is attached to the 

 equator of the pharynx and extends backward to the dorsal end of the pharyn- 

 geal case. This tube or tubular muscle is the retractor of the pharynx. Its 

 uniform contraction draws back the pharynx into the sheath while the contrac- 

 tion of one or more segments of the muscle tilts the pharynx in the correspond- 

 ing direction. The outer buccal sinus contains three small pharyngeal muscles. 

 One muscle, the levator mandibuh, which seems to be a specialized portion of 

 the pharyngeal retractor, arises just outside it from the upper portion of the 

 pharyngeal case, and passes forward (ventrad) in the median line to its insertion 

 on the upper mandible. The other two muscles are a pair of thin bands which 

 arise near together from the upper side of the dorsal end of the pharyngeal 

 case and, passing downward around the pharynx, are inserted upon its lower 

 surface. These muscles rotate the pharynx on its long axis. The pharynx is 

 easily movable so that it can be thrown forward more than half its own length, 

 apparently being squeezed out by the contraction of the muscles of the phar- 

 yngeal case, and can be retracted or rotated by the muscles described above. 

 The inner buccal sinus contains the infrabuccal and suprabuccal ganglia, the 

 pharyngeal arteries , the duct of the median salivary gland , and the oesophagus. 

 It is prolonged backward as a canal thru the oesophageal nerve ring. The 

 buccal sinuses are lined by the vascular endothelium. 



The mandibles, or jaws (Text figure 11) resemble a parrot's beak inverted. 

 The upper mandible is longer, straighter, and more compressed than the lower 

 which is strongly convex and closes outside the upper jaw. Each mandible is 

 formed of two boat-shaped lamellae which unite at an acute angle and make the 



