NERVOUS SYSTEM. INVERTEBRATA. 21 



the connectives join the suboesophageal ganglion an 

 elongated mass formed by the longitudinal concentra- 

 tion and fusion of eight pairs of ganglia ; it innervates 

 the mandibles, maxillae, and five anterior pairs of thoracic 

 appendage*. The last three thoracic and the six abdominal 

 ganglia are segmental in position ; each gives off three pairs 

 of nerves, distributed respectively to the appendages and 

 lateral muscles of the same segment and to the flexor muscles 

 of the one behind. The terminal ganglion supplies the 

 sixth abdominal segment and the telson. All the ventral- 

 chain ganglia show a high degree of transverse concen- 

 tration, combined (except in the case of the subcesophageal 

 ganglion) with well-marked longitudinal separation. The 

 connectives are bound up in a common neurilemma-sheath. 

 Bellonci, Ann. Mus. Civ. Stor. Nat. Geneva, vol. xii. 

 1878, p. 518. 



D. 14. Two specimens of the nervous system of a Lobster 

 (Homarus vulgaris). The cerebral ganglion is lodged 

 immediately below the bases of the eye-stalks ; it is 

 roughly quadrilateral in shape, with a pair of conspicuous 

 rounded eminences (globuli) upon its lateral margins. 

 Each of its upper angles is connected by a nervous tract 

 to a rod-shaped optic ganglion that lies within the eye- 

 stalk ; from its lower angles arise the antennary and in- 

 tegumentary nerves ; the circumoesophageal connectives 

 are given off from the middle of the ventral border. The 

 cerebral ganglion, as in other Decapods, has considerable 

 structural complexity (fig. 12). Three regions can be 

 traced in it, of which the anterior two correspond probably 

 to the proto- and deuto-cerebrum of the Insect brain, while 

 the third is a part of the tritocerebrum peculiar to Crus- 

 tacea. The protocerebrum is the optic centre ; it consists 

 of the optic ganglia, and of a quadrilateral mass (proto- 

 cerebral lobes) that forms the upper part of the cerebral 

 ganglion. The protocerebral lobes are separated by a 

 slight median furrow; they are traversed by numerous 

 commissural fibres, and have in the middle of their sub- 

 stance a transverse bar of dense neuropile that probably 

 represents the corpus centrale of the Insect. 



