NERVOUS SYSTEM. 215 



have the greatest portion of the ganglionic matter of the ceso- 

 phageal nervous ring accumulated in a cerebral position above 

 the entrance of the alimentary canal, as seen in the annexed 

 figure of the harpa elongata (Fig. 92,) from New Guinea, 

 where the mantle (0,) is opened to show the branchiae 

 (dy } ) and the syphon (c } ) on the left, and the mucous 

 follicles the colon (n.) and the male organ (f.) on the 

 right side of the respiratory cavity. On opening the an- 

 terior part of the trunk the retracted proboscis (g) with 

 its muscles (h.) are seen extending backwards over the 

 brain (i.) which rests on the inferior turn of the oesophagus 

 where the two salivary glands (k. k.) are also placed. 

 From this cerebral mass (i.) large nerves are seen ex- 

 tending forwards to the head (#,) the tentacula, with the 

 eyes (s. s,) at their base, and to the broad fin-like ante- 

 rior fold (a,) of this long tapering inoperculate foot. Other 

 nervous chords extend downwards to the ventral surface 

 of the abdomen, and backwards to the sympathetics which 

 supply the abdominal viscera. This gradual concentration 

 of the ganglionic matter of the great cesophageal nervous ring 

 of the gasteropods into a cephalic position and form, on 

 the median plain above the alimentary canal, is a pre- 

 liminary to its enclosure in a distinct cranial covering, 

 which takes place in the cephalopods. 



The nervous system of the pteropods presents the same 

 general plan and the same varieties of form in its ce- 

 phalic masses as seen in the gasteropods, especially 

 in the naked and swimming species. In the clio bo- 

 realis (Fig. 93, B.) one of the small naked swimming 

 pteropods, there is a double nervous collar around the 

 ossophagus, as in the aplysia and many other gasteropods. 

 Two small ganglia approximated to each other to form 

 a bilobate brain are placed above the oesophagus im- 

 mediately behind the lips (93, B ,) and indicate by their 

 diminutive size the imperfect development of the organs 

 of the senses in this animal which scarcely presents a 

 trace either of eyes or tentacula. Behind these central 

 ganglia are two larger lateral ganglia connected together 

 by a transverse band below the oesophagus, and which 

 supply the principal nerves to the muscular closed mantle 

 enveloping the trunk. Two nervous bands proceed from 



