NERVOUS SYSTEM. INVEHTEBRATA. 



The mid-brain is the centre of origin for the nerves to 

 the eyes and tentacles. Its postero-lateral parts are pro- 

 longed backwards to form a pair of indistinct lobes, from 

 which fibres extend to the nuchal region they apparently 

 represent the third area of the Polychsete brain (hind- 

 brain). From each side of the mid-brain a second root is 

 given off to the circumoesophageal connective; it unites 



Fig. 5. 



C.T, 

 I 



6.P. f ' C i T 6.C. 



^ \ '* * - 



\ 



N.R. 

 P.S. 



Transverse section through a Ventral-chain Ganglion of Marphysa 

 sanguined. X 75. 



C.M. Central medulla. C.T. Connective tissue. G.C. Ganglion-cells. 

 G.F. Giant fibre. L.M. Longitudinal muscles. N.R. Nerve- 

 root. P.G. Pigment masses. 



with the first (derived from the fore-brain) immediately 

 outside the ganglion. The ganglia of the ventral chain 

 lie close together, one in each segment, and are united by 

 definite though short fibrous connectives. A pair of para- 

 podial nerves rises from each ganglion. Upon the ventral 

 surface of the cord runs a single large " giant fibre/' 

 Jourdan, Ann. Sci. Nat., ser. 7, t. ii. 1887, p. 250. 



D. 6. Two specimens of the nervous system of a Sea Mouse 

 (Aphrodite aculeata). The cerebral ganglion is of large 

 size and complicated structure ; it lies in the prostomium 

 and consists of a central mass of neuropile, separable into 

 two main centres the fore- and mid- brain. Between the 

 two, on their posterior surface, lies a third, very definite mass 

 of neuropile, from which a pair of stalk-like processes project 

 upwards towards the dorsal integument. Each stalk 



