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 osso- 

 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 antennas 

 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 

 antennae 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-oesophageal 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 



