No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 57 
here, and do not call for special notice. A portion of a cross- 
section through the anterior region of the collar of a worm 
at this stage is drawn in Fig. 75. In the upper part of the 
figure is seen the nervous system in cross-section. It has 
increased in size, and the number of its nuclei has doubled. 
A large lumen is present throughout the length of the chord. 
Beneath the nerve cord is a flat space bounded at the sides 
and below by the anterior median protrusion of the third body- 
cavities. Beneath this lies the flattened notochord, cut just 
in front of its opening into the oesophagus. It has a large 
lumen, wide from side to side. Ventral to the notochord are 
two chitin-like rods that are formed on each side in the folds 
of entoderm. These are the supporting-rods of the neck, united 
anteriorly into a single rod in the neck and base of the pro- 
boscis. A large space lying between the rods in the figure 
is probably an artefact. The splanchnic layer of the meso- 
derm on each side is differentiated along the rods into a thick 
muscle-layer. Anterior to this region a pair of forward lateral 
extensions of the collar body-cavity follows the course of the rod, 
and forms a layer of muscles, as is shown at 4.c.?, in Fig. 77. 
It is very difficult to follow the course of the blood-vessels in 
the young worms, owing to the elasticity of the walls ; so that 
at times a large vessel may be found in section distended with 
blood, and at other times the same vessel cannot be found, 
owing to the absence of blood, and the collapse of the walls. 
In one worm of this stage a very large accumulation of blood 
was found in the blood-vessel between the notochord and pro- 
boscis vesicle, so that the blood-vessel seemed suspended from 
the dorsal well of the notochord into the cavity of the proboscis 
vesicle. From the blood-vessel the blood is continuous with 
the irregular spaces between the inturned cells of the proboscis- 
cavity. At the base of the proboscis vesicle the vessel is 
nearly obliterated, and inasmuch as the dorsal vessel a few 
sections back is empty of blood, no connection between the 
two could be seen. A median dorsal and a median ventral 
blood-vessel can always be found in the body proper, formed 
by the apposition of the walls of the second and third pairs of 
body-cavities. 
