248 



LECTUEE XII. 



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LECTURE XII. 



ANNULATA. 



Hitherto the highest condition of the nervous system which we have 

 observed, has been that of detached unganglionic fila- 

 ments diverging from a single sub-oesophageal gang- 

 lion or from a simple oesophageal ring, continued un- 

 connectedly along the abdomen, or diverging in rays 

 down equidistant tracts of the common parietes of 

 the body. If we have met with ganglionic masses 

 in connection with coloured ocelli, these have been, 

 as, e. g., in the Acalephge, either so situated as to 

 give no indication of a head, or so multiplied as to 

 lose all significance as a common cerebral centre of 

 sensation. 



In the class Annulata, the nervous system has 

 reached a higher type and more constant plan of 

 arrangement. It always commences by a symme- 

 trical bilobed ganglion, meriting, both by its- situation 

 above the mouth and by the parts which it supplies, 

 the name of brain, which it has commonly received. 

 The sub-oesophageal ganglion is, however, the ana- 

 logue, if not the homologue, of the medulla oblongata, 

 and should be included in the encephalic division of 

 the nervous system in both the Articulate and Mol- 

 luscous animals. 



In the medicinal leech there are sent off from the 

 super-oesophageal ganglion {fig. 107, a) ten distinct 

 optic nerves (b b), besides many smaller filaments to 

 the integument and other parts of the head ; each 

 optic nerve or filament terminates by expanding 

 upon the base of a black eye-speck or ocellus, ten of which you will 

 easily distinguish by the aid of a moderate magnifying power, dotting 

 at equal distances the upper margin of the expanded suctorial lip. 



The principal nervous productions of the brain of the leech are 

 what may be termed its crura, which diverge as they descend to 

 embrace the oesophagus, and are called the oesophageal chords ; they 

 then converge and reunite to join the large cordiform sub-oesophageal 

 ganglion (c). From this ganglion the muscles of the three serrated 



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Leech. 



