A. D. Ppiacock 
105 
origins far back in the epicranium. They are inserted externally at the 
broadest part of the pumping-pharynx externally to the medial dilators. 
These appear to be the muscles which Harrison calls “the retractors of the 
buccal cavity running from the cornua backwards, upwards and outwards.' 1 
(4) Ventral retractors —one pair. They retract the pumping-pharynx after 
feeding. Each originates in the floor of the head in the same transverse 
plane as the eyes and is closely associated with the origin of the ventro¬ 
lateral dilator of the second pharynx. It runs upward and inward with 
this dilator for some distance, next runs close under the second pharynx 
where the girth of the pharynx is greatest, and from thence to its insertion 
at the posterior portion of the pumping-pharynx where its constriction begins. 
This retractor is not mentioned by Harrison. 
Pumping-pharyngeal tube (ph.t., PI. VI, fig. 1, Text-fig. III). The 
paired elements of the tube run forward from the beginning of the floor of the 
pumping-pharynx nearly to the teeth. Each is a simple half-tube (PI. VI, 
fig. 5), crescentic in cross-section, the rims being sharp edged whereby the 
half-tubes are capable of overlapping one another (Text-fig. II) and anteriorly, 
perhaps, may be applied to the arch of the buccal funnel. How the two elements 
are brought together to form a tube I do not know. Harrison suggests that 
it is done by a “pair of elastic folds, which lie at the base of the tube, and 
which pass obliquely upwards and backwards.” 
Pharynx (ph., PI. VI, fig. 1). This dilatation of the food canal is curiously 
shaped and musculatured. Distended with blood, it is bulbous, but at rest 
its floor, walls and roof have each a broad deep furrow. In cross-section the 
lumen has the shape of a four-rayed star-fish with the two dorsal arms larger 
than the ventral. The wall is composed of three layers (1) the inner lining of 
thin chitin, (2) the middle epithelial and chitinogenous layer and (3) the 
outer muscular layer. The latter appears to include an inner layer of longi¬ 
tudinal fibres, and, in certain areas, special, large, outer, circular sphincter 
muscles. 
The muscles consist of sphincters and dilators disposed as follows; 
dorsally and in succession backwards—the anterior dorsal dilator, the anterior 
sphincter, the medial dorsal dilator, the medial sphincter, the post-dorsal 
dilator, the post-sphincter; laterally and in succession backwards—the 
lateral dilator between the anterior and medial sphincter, the post-lateral 
dilator immediately after the medial sphincter and between it and the pos¬ 
terior sphincter. 
1. Sphincters —three (sph., PL VI, fig. 1 and Text-fig. Ill), (a) The anterior 
sphincter is broad and encircles the pharynx just behind the insertion of the 
anterior dilator, (b) The medial sphincter encircles the pharynx about the 
beginning of its posterior third, (c) The posterior sphincter, the smallest, 
encircles the pharynx where it tapers to the oesophagus. 
2. Dilators —five pairs (a-e) as follows (dil., PI. VI, fig. I and Text-fig. III). 
(a) Antero-dorsal. Each has its origin in the anterior part of the epicranium 
