64 Miscellaneous, 



The Brain of the Crustacea. — Like the Insects and the Myriapods, 

 the Crustacea possess a brain formed of three segments — protocere- 

 bron, deutocerebron, and tritoccrcbron. 



The protocerebron of the Crustacea is constructed upon the same 

 plan as that of the Insects ; we find in it an optic ganglion formed 

 of the same portions, as well as pedunculate bodies, a central body, 

 and a pons of the protoccrtbral lobes. In the same way their 

 deutocerebron is in every respect similar to that of the Insects and 

 the Myriapods. 



In the Insects and the Myriapods the third cephalic zonite is 

 devoid of appendages and only bears the labrum ; in the Crustacea, 

 on the contrary, the same zonite bears, in addition to the labrum, 

 the second pair of antennae. This difference entails a slight modifi- 

 cation in the structure of the tritocerebron. While in the Insects 

 and Myriapods the tritocerebron is represented only by a pair of 

 oesophageal ganglia, in the Crustacea the same cerebral segment is 

 formed by a pair of oesophageal ganglia, and, in addition, by a pair 

 of antennary lobes intercalated between the latter and the deuto- 

 cerebron. 



The oesophageal ganglia in the Crustacea, as in the Insects and 

 Myriapods, are united with one another behind the oesophagus by 

 the transverse commissure of the oesophageal ring, and each of them 

 gives rise, by means of a common trunk, to the nerve of the labrum 

 and to a root of the visceral nervous system. 



The antennary lobes, the commissural fibres of which pass with 

 those of the oesophageal ganglia behind the oesophagus, give rise to 

 the nerves of the second antennae, to a pair of tegumentary nerves, 

 and to the motor nerves of the eye-stalk. 



The visceral nervous system of the higher Crustacea differs from 

 that of the Insects in a single point, which is, however, of but slight 

 importance. In the Crustacea the unpaired and the lateral ganglia, 

 instead of being separated as in the Insects, are all fused into a 

 median mass applied to the wall of the stomach, and known by the 

 name of the stomotogastric ganglion. This mass is united to the 

 brain bj- roots, which are strictly homologous with those which we 

 find in the Insects. Like the frontal ganglion of the Insects, the 

 stomatogastric ganglion of the Crustacea is united to the oesophageal 

 ganglia by a pair of roots, which are generally double, and, like the 

 lateral ganglia of the same animals, it is connected with the deuto- 

 cerebron. 



We therefore conclude, from what has been stated above, that, 

 from the point of view of cerebral structure, there exists the closest 

 relationship between the Crustacea, Insecta, Myriapoda, and 

 Peripatus. 



Liniulxs and the Arachnids, of which in other respects many 

 zoologists recognize the affinities, constitute, as regards the organiza- 

 tion of their brain, a most homogeneoixs group, but one which 

 recedes considerably from the rest of the Arthropods. 



In Limulus and the Arachnids the brain is composed of two 



