92 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
The circular muscles do not form a continuous sheet, but consist of 
small bundles which lie partially embedded in the hypodermis. Some 
of these muscles cross the mid-ventral line external to the nerve, thus 
causing a partial separation of cord and hypodermis, Between the 
muscle bundles, however, the neurilemma of the nerve cord is in contact 
with the hypodermis. The brain also lies deeper than the hypodermis, 
from which it is suspended by a narrow membrane lying in the median 
plane. 
a. Brain, 
The form of the brain is roughly that of a trapezoid (Plate 1, Fig. 1, 
ceb.), the anterior pair of eyes marking approximately the extremities of 
the longer one of the parallel sides, while the posterior pair marks the 
limits of the shorter one. The anterior angles of the trapezoid are 
drawn out toward the palps, thus making the anterior margin of the 
brain slightly concave. The dorsal aspect of the brain is broadly cor- 
date, the re-entrant angle being at the anterior side. Fourteen pairs of 
nerves arise from the brain by distinct roots. As they are arranged 
symmetrically, it will not be necessary to describe both sides. Beginning 
anteriorly at the median line, and numbering and describing the nerves 
of one side in order, there is first near the median line a group of three 
nerves (I, II, IIT), which arise near together. 
The first nerve (I) passes forward, then downward, and finally back- 
ward along the dorsal wall of the proboscis; the second (Il) goes 
directly forward to the antenna; the third (III) runs forward along the 
dorsal wall of the head. 
At the anterior lateral angle of the brain there is another group of 
three nerves (IV, V, VI). The fourth nerve (IV) divides into two 
branches, one going to the ventro-median wall of the palp, the other to 
the dorso-median wall of the same organ. The fifth nerve (V) extends 
ventrally to the proboscis; the sixth (VI) is the large sensory trunk 
of the palp; and the seventh (VII) arises from the brain laterally, 
between the group just described and the anterior eye of the same side; 
it passes forward along the lateral wall of the palp. 
The eighth, ninth, and tenth nerves are the three roots of the circum- 
œsophageal commissure. They unite in the commissural ganglion, which 
lies a short distance ventral to the anterior eye. The eighth (VIII) is 
a small nerve arising near the seventh, passing out parallel with it, and 
then turning down into the ganglion. The ninth nerve (IX) arises 
laterally from the brain, passes out directly beneath the eye, and then 
bends down to the commissural ganglion. The tenth (X) arises from the 
