490 Mr. Garner on the Nervous St/stem of Molluscous Animals. 



branchio-visceral. The foot has become too insignificant to require appro- 

 priate ganglia. Though there are in this animal, and in some species of Doris, 

 no eyes, the author thinks he has found the rudiment of them in two minute 

 black spots which he has noticed, one on each side the brain ; and he infers 

 that so unusual a circumstance must arise from the pignientum nigrum exist- 

 ing on the brain before the external eye is developed. These spots mark the 

 superior ganglia to be the cerebral, and M'^e find the tentacles supplied from 

 them. Externally the nerves are derived which supply the mantle, branchiae, 

 and viscera. In Doris and EoUda (fig. 5.) the same conformation exists. 

 According to Cuvier the four ganglia are quite separate in Tritonia ; and it 

 would appear, from the observations of the same anatomist, that the nervous 

 system of the genera Phi/llidia, Otichidium, Tethys, Testacella and Pleuro- 

 branc/ius is more or less upon the same plan. The |)haryngeal ganglia are 

 often small, but exist as usual. 



The nervous system of the Aplysia, which is not figured in tiie plates 

 accompanying this paper because it is so minutely described by Cuvier, is 

 particularly interesting. The cerebral or sentient ganglia, giving origin as 

 usual to filaments forming the pharyngeal ganglia, are conjoined, as in other 

 naked Gasteropoda, into one situated above the oesophagus. The two late- 

 ral ganglia give off internal filaments to the foot, and external ones to the 

 mantle. It will be seen, as we ascend, that there are separate ganglia for 

 the foot, mantle and branchiae. In the Aplt/sia there is, besides, another gan- 

 glion, — the one supplying the branchiae and visceral organs at the posterior 

 part of the body. That each of the lateral ganglia is in reality composed of 

 two, appears from its supplying the two parts above mentioned, which, in most 

 of the Gasteropoda and in the Cephalopoda, have separate ganglia for each : 

 besides, the fellow ganglia are connected together by two separate filaments, 

 and between tliem the aorta passes, which in many of the higher Gasteropoda 

 and in the Cephalopoda distinguishes by its course that part of the ring which 

 supplies the foot from that which supplies the mantle and viscera. Lastly, 

 each lateral ganglion is connected to the sentient lobe by three nerves, beipg 

 those which it receives from the pedal and from the branchial ganglia, and 

 from that of the mantle. 



In Bullcea (fig. 9.) we find the pedal ganglia (B.) distinct from the two sup- 



