100 
GEORGE E. NICHOLES. 
thelium of the sub-commissural organ {s. c. o.), and an optocoel 
(op.), which appears as a short bluntly ending and dorsally 
directed canal much more reduced than the corresponding 
canal in Bdellostoma, both open postero-ventrally into the 
sinus mesocoelicus (s. m.). From this, two canals, isthmic 
(i. c.) and ventricular (v. c.), lead backwards, but whereas in 
Bdellostoma it is the isthmic canal which is much the 
larger, in Myxine the ventricnlar canal is large and the 
isthmic canal has become a very narrow channel (compare fics. 
25 and 27). 
The latter opens widely into the upper portion of the 
sinus mesocoelicus on the level of the posterior end of the 
sub-cornmissural canal, but diminishes rapidly (funnel- wise) 
and extends through the greater part of the thickness of the 
postero-ventral portion of the optic lobes as a very fine canal, 
nearly oval in transverse section and comparatively remote 
from the backwardly-bulging sinus mesocoelicus. 
The sinus mesocoelicus at its lower end is somewhat 
constricted and passes into the ventricular canal, which curves 
upwards and backwards to meet the isthmic canal as it makes 
its exit from the mid-brain. The junction of the two canals 
marks the beginning of the fourth ventricle, which, as in 
Bdellostoma, comes, at this point, more nearly to the 
dorsal surface of the brain than elsewhere. In Myxine, 
indeed, it is separated from the vascular tissue which every- 
where envelops the brain by a thin layer of epithelial tissue 
only. 
Posteriorly the roof of the hind brain thickens consider- 
ably, and again as in Bdellostoma, a series of small 
irregular chambers are to be made out beginning from a point 
immediately behind the posterior end of the isthmic canal. 
In some specimens they actually begin on either side of the 
isthmic canal, at its point of junction with the ventricular 
canal, as a pair of small chambers. Thence they extend in 
the middle line backwards to a point near the middle of the 
length of the medulla oblongata. Beneath this series of small 
chambers the fourth ventricle passes backwards, changing 
