ANELLATA. 



141 



The branchiae of the Anellata errantia * are usually in the form of 

 shorter tufts than the cephalic ones of the Tubicola ; and they are 

 attached to the upper part of the sides of a greater or less number of 

 segments. In some species, as the Nereis lamelligera^ the branchia is 

 formed by a flattened vesicle, including the ramifications of the 

 branchial vessels, and attached to the base of the upper tubular foot. 

 We shall afterwards see the homologue of this respiratory plate 

 taking an important share in the locomotive functions in the higher 

 organised forms of Articulata. 



LECTUEE XII. 



ANELLATA. 



Hitherto the highest condition of the nervous system which we 

 have observed has been that of detached ungang- 

 lionic filaments diverging from a single sub-oeso- 

 phageal ganglion or from a simple oesophageal ring, 

 continued unconnectedly 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 in the Acalephae, either so situated 

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

 as to lose all significance as a common cerebral 

 centre of sensation. 



In the class Anellata the nervous system has 

 reached a higher type and more constant plan of 

 arrangement. It always commences by a symme- 

 trical bilobed ganglion, which, both by its situation 

 above the mouth, and by the parts which it sup- 

 plies, merits the name of brain, which it has com- 

 monly received. 



In the medicinal leech there are sent off from this 

 ganglionic centre {fg. 74. a) ten distinct optic 

 nerves (bb\ 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. 



"^ Preps. Nos. 987, 988, 989. 



