388 E. W. MACBRIDE. 
tween the ectodermic and notochordal portions a wedge of tissue 
projects—really the modified tip of the ventral pocket of the 
proboscis celom (Pr. cw. v.) ; Kohler, however, mistook it for 
a blood-vessel, and described the skeletal rod as completely 
divided into two by vascular tissue. In fig. 2 the great ventral 
vessel is seen as wellas thedorsal. Fig. 3 is a transverse section 
through the base of the proboscis, showing the mutual relations 
of heart, notochord, proboscis gland, and proboscis celom. 
Fig. 4is a transverse section through the collar region, the 
main point to notice being that the dorsal vessel has on each 
side of it a space half filled with muscular tissue—the peri- 
hemal cavity (PH.). This, as can be seen in Fig. 1, is a for- 
ward diverticulum of the trunk ceelom. 
Fig. 5 is a transverse section through the branchial region 
of the trunk. The figure-of-8 form of the alimentary canal is 
well seen, the upper part being the branchial tube, the lower 
the cesophagus. Where the one passes into the other there is 
on each side a ridge of thickened epithelium, and in life these 
two ridges are closely apposed, entirely separating the two 
tubes. The tongue-bar is seen on each side of the branchial 
tube in longitudinal section, and is seen to contain a portion 
of the trunk celom. On each side of the dorsal surface is seen 
a slight projection—the genital wing—in the proximal part of 
which the gonad opens on each side. Sections of the dorsal 
and ventral vessels are seen, and also of the dorsal and ventral 
nerve-cords. The whole ectoderm is ciliated, and has a well- 
developed plexus of nerve-fibrils amongst the bases of its cells. 
The dorsal and ventral nerve-cords are mere local thickenings 
of this plexus: the latter is confined to the trunk region; the 
former, however, passes into the posterior end of the central 
nervous system through the posterior ectodermic pit. Just 
behind the collar a ring of nervous matter puts the dorsal and 
ventral nerve-cords in communication with each other. The 
nerve-plexus is well developed in the proboscis region, and is 
especially thick round the narrow neck by means of which 
the proboscis joins the collar, constituting a kind of anterior 
nerve-ring. 
