BRAIN AND CRANIAL NERVES OF BDELLOSTOMA DOMBEYI. 155 
eyes, and consequently larger optic nerves, is still an impor- 
tant section of the brain, and its fibre-tracts are more con- 
spicuous than those of any other section except the medulla. 
The most striking of these is the dorsal decussation, whose 
fibres connect not only the roofs of the two lobes, but their 
sides and floorsas well. Unlike Petromyzon (Johnston, 1902) 
the decussation does not extend through the whole mid brain 
roof, but is confined to its caudal part. Numerous small 
bundles of fine fibres leave the cell layer of the tectum and 
proceed towards the central mass, but they are so very fine 
that it is impossible to trace them in ordinary hematoxylin 
sections, or to separate from among them the fibres of the 
optic nerve. Conspicuous in the base of the mid brain, 
filling entirely its basal cone (fig. 6), are the bundles of 
Meynert, already described, on their way to the medulla; 
and, dorsal to their decussation, the ansulate commissure, 
comparatively small in Bdellostoma. ‘This commissure is 
caudad of the bundles themselves, and lies so close to them 
that a small bundle of fibres of the right bundle is deflected 
by it and pierces down through the commissure instead of 
passing in front of it, rejoining its fellows later on. Leaving 
the central mass on each side is the tractus tecto- 
bulbaris et spinalis, running to the medulla and spinal 
cord. These, with the tractus habenulo-tectalis, already 
described, are the principal tracts found in the mid brain of 
Bdellostoma. 
The iter enters the dorsal part of the mid brain as a 
straight open tube, whose lumen is about ‘09 mm. in 
diameter. But before the habenular ganglia are reached 
the tendency to close asserts itself, the ependyma cells crowd 
in, the tube narrows, and the lumen becomes very smal]. In 
some specimens the lumen, in the fore part of the mid brain, 
is, in places, entirely closed. 
The Cerebellum.—The cerebellum is the smallest divi- 
sion of the brain of Bdellostoma, though very much larger 
and more conspicuous than the same section in the brain of 
Petromyzon. lt consists, in Bdellostoma, of two small lobes, 
