154 JULIA WORTHINGTON. 
circular canal (fig. 5). This canal lies in the horizontal 
plane, its ends directed cephalad. A longitudinal section of 
the brain running through this canal and the infundibulum 
makes the two together look like the head of an anchor. 
There is a bulb-like dilatation of each end of the semi-circular 
canal, these dilatations lying in the two hillocks seen on the 
ventral surface of the ’tween brain just caudad of the infun- 
dibular stalk. This same relation of parts is found in 
Myxine (Retzius, 1895), but Johnston (1902) makes no mention 
of it in describing the infundibulum of Petromyzon. 
The Mid Brain.—The mid brain consists, dorsally, of 
two equal and symmetrical lobes, divided by a median longi- 
tudinal cleft that becomes shallower as it proceeds caudad. 
Anteriorly the cleft widens out, the mid brain lobes separate 
from each other on the dorsal surface, and admit between 
them the hind part of the habenular ganglia (fig. 1). The 
lobes are about the same in length as those of the fore brain, 
but are narrower behind (fig. 1). Ventrally they taper to a 
comparatively small point that can be seen in section (fig. 6), 
but is obscured superficially by the medulla and ’tween brain. 
Structurally the mid brain is somewhat similar to the fore 
brain, and merges into it gradually, so that in examining a 
series of cross-sections it is difficult to tell definitely just 
where the mid brain begins and the fore brain ends. Like 
the fore brain it may be divided into two parts—an outer shell, 
the tectum opticum, and a central mass. The relative 
size of the two parts, however, is more nearly equal (fig. 9), 
nor are they as sharply marked out one from the other. The 
tectum has but two layers, as against the four in the outer 
division of the fore brain, an outer glia layer, and an inner 
layer of cells. These layers are in every respect similar to 
the corresponding layers of the fore brain. The central mass 
consists of cells densely grouped together, except where 
separated by the fibre-tracts passing through this part of the 
brain. 
The mid brain, though not of the same importance in 
Bdellostoma as in higher forms possessing better developed 
