284 WILLIAM BLAXLAND BENHAM. 
16). The proximal portion of the lateral heart is narrow, but 
the vessel soon swells into a globular form; the vessel presents 
a series of such dilatations. The moniliform appearance is 
due to circular muscles placed at certain distances along the 
vessel (fig. 18). These five pairs are contractile: thatis why 
they may be called “ lateral hearts,” whilst the other two pairs 
of commissural vessels are non-contractile. 
A few very small vessels leave these “ hearts” and go to the 
posterior septum, and another larger one has a similar course 
from near the ventral end of the “ hearts ;”’ but in these somites 
there are no vessels from the dorsal trunk to the alimentary 
tract. 
In somites v and rv the commissural vessels (com.) are not 
moniliform and are much narrower than the “ hearts ;” like 
them they leave the dorsal trunk at the undivided portion. 
Shortly after leaving this trunk, each gives off a small septal 
branch (com. spt.) to the posterior septum of its somite ; whilst 
in addition they give off a vessel to the cesophagus (com. al.). 
In somite 111 the dorsal trunk has become very narrow and 
remains single; just in front of the first septum it gives off a 
branch to the wall of the pharynx on each side (ph. v.), which 
after giving off a branch to the septum (sp¢.) breaks up amongst 
the muscles of the pharyngeal wall inito a network (ph. nw.) 
with which the lateral trunks (L.) are connected. 
About half-way along the pharynx, that is, in somite 1, a 
second pair of branches is given off to the pharyngeal wall 
(ph. v’.), which also helps in the formation of the network just 
mentioned. 
A third pair of branches (47.) to the pharynx occurs by the 
division of the now very delicate dorsal trunk into two vessels, 
just behind the cerebral ganglia. Each of these passes down- 
wards, close behind the circumpharyngeal nerve commissure, 
either to enter the ventral trunk directly, or to break up into 
the network from which both ventral and lateral trunks take 
origin. The existence of the three pairs of branches from the 
dorsal trunk in front of the first septum seems to confirm the 
idea, that this region is formed of three somites. 
