ANATOMY OF THE NERVE CENTRES. 447 
is a precisely similar shifting of the origin of the optic tract in 
Arthropods and in Vertebrates from the thalamus to the 
mesencephalon as development advances, or when the less 
differentiated are compared with the more highly differentiated 
forms. 
The Metacerebron consists of two pairs of reticular nodules ; 
an internal pair, which are connected with the great ceso- 
phageal connectives (crura of the cerebron), and which are 
united with each other by a transverse commissure; and an 
external pair, from which the nerves of the great antenne 
arise. 
Thus each of the great divisions of the brain in the Crayfish 
consists of a pair of central ganglia and a pair of sensory 
ganglia. The ganglia of the antennules belong to the thala- 
mon, those of the great compound eyes should be regarded as 
the sensory ganglia of the mesocerebron, and those of the 
antennee are undoubtedly the sensory ganglia of the meta- 
cerebron. 
In Insects the metacerebron is but feebly developed ; it con- 
sists of a commissure and of the ganglia of the crura, which, 
like the metacerebron of the Crustacea, give off the pharyngeal 
and stomo-gastric nerves. 
The Crura of the Cerebron, as in Insects, are connected not 
only with the metacerebron, but with the mesocerebron, thala- 
mon and optic peduncles, and probably with every part of the 
supra-cesophageal nerve mass. In the Crayfish these form a 
thick layer on the ventral surface of the whole supra-ceso- 
phageal nerve centre, and many of the fibres decussate across 
the median plane (Fig. 55). 
Ventricles.—In the Crayfish there are three distinct groups 
of nerve cells, which penetrate into the interior of the brain 
from its dorsal surface ; one in the thalamon, a second between 
the thalamon and the mesencephalon, which lies above the 
commissure of the mesencephalon, and a third behind and 
between the lobes of the metencephalon. These cavities in 
the reticular substance are filled with nerve cells; but in Insects 
they are frequently developed into distinct ventricular cavities. 
30 
