672 
.1. DAVIDSON. 
On removing the embryos from the dorsal surface the 
whitish-yellow coils of the digestive canal are seen lying 
beneath. 
The alimentary canal, as seen in side view (fig. 35), leads 
from the mouth into a well-defined pharynx (p/i.), which 
structure passes upwards through the head, and leads into 
the narrow, tubular oesophagus (oss.). The oesophagus 
passes in the median line over the thoracic ganglion (t. 
and enters into the stomach (st.) or sac-like dilation of the 
mid-gut. The stomach narrows towards its posterior end, 
and leads into the coiled intestine (i.), from which the rectum 
(r.) passes in the median line over the vagina (v.) to the 
anus (an.). 
The salivary glands lie obliquely in the thorax, above the 
oesophagus, and consist of a small anterior gland (s. a.) and 
a large posterior gland («. p.), situated on each side of the 
median line. The salivary duct from each side passes beneath 
the thoracic ganglia, and both unite in the mid-ventral line to 
form a median salivary duct (s. d.) which leads to the salivary 
pump (.s.), situated at the posterior end of the buccal cavity. 
The supra-oesophageal ganglia occupy the greater portion of 
the head. E.xtending beneath the oesophagus is an elongate 
ganglionic mass, which comprises the sub-oesophageal 
ganglion, with its two broad commissures, and the fused 
thoracic and abdominal ganglia. 
The respiratory organs consist of fine tracheal tubes, which 
ramify over the body in two definite systems, a dorsal and a 
ventral tracheal system, the main trachege from which lead on 
each side of the body to the nine pairs of spiracles. 
Four conspicuous bands of longitudinal muscles extend 
along’ the floor of the body, two on each side of the median 
line, and four similar bands extend beneath the dorsal 
segment. 
A. The Digestive System (PI. 38, fig. 5; PI. 41, fig. 35). 
The mouth or oral opening is situated on the ventral surface 
of the head, being bounded anteriorly by the labrum (Ihr.), 
