MORPHOLOGY OF ANIMALS 



thus forming a centralized nervous system; other nerve 

 cells remaining scattered throughout the epidermis serve 

 to connect the nerve strand with the muscles. 



(3) The ganglionic type. In ctenophores, a sense organ 

 from Which nerves radiate, is found at the apical pole 

 (Fig. 2, C, so)^ and in a great many of the higher animals 

 the earliest formed and most generally present portion of 

 the nervous system is a group of nerve cells, or ganglion, 

 which appears at the apical pole of the gastrula, and be- 

 comes in the adult the cerebral ganglion^ or brain, lying on 

 the dorsal side of the oesophagus (Figs. 3, 1 1, 12, A) . Nerve 

 trunks are always given off from this ganglion, and very 

 generally two of them run down on each side of the 

 oesophagus to its ventral side, thus forming a circum-oeso- 

 phageal nerve ring (Fig. 3, D). In different phyla longi- 

 tudinal nerve trunks may be given off from different parts 

 of this ring; in the case of annelids and arthropods this 

 ring connects on the ventral side of the oesophagus with 

 the ''ventral nerve chain ^'' which consists of a bilateral 

 pair of ganglia in each somite connected with those in 

 front and behind by nerve cords. The first one in the chain 

 is the suh'oe so phageal ganglion^ connected with the cere- 

 bral ganglion by the circumoesophageal connectives (Fig. 

 23, A). In the moUusks the nervous system consists of a 

 pair of supra- and sub-oesophageal ganglia {^cerebral and 

 pedal) which with their connectives form an oesophageal 

 ring. To these is usually added a pair of plural and parietal 

 ganglia forming a loop which extends back into the body, 

 while ventral trunks {pedal cords) may be present in the 

 foot (Fig. 13, A). 



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