204 



STREPTONEURA AND EUTHYNEURA 



left cerebral ganglia. The osphradial nerve springs from one of 

 the ganglia on the visceral loop, the osphradium itself being situ- 

 ated (in Limnaea) immediately above the pulmonary orifice and 

 adjacent to the anus (Fig. 102). This massing of the ganglia 

 is still better illustrated by the accompanying figure of Physa 

 (Fig. 103), in which the animal is represented as if transparent, 

 so that the ganglia and nerves are seen through the tissues. 

 Of the streptoneurous Gasteropoda, the nervous system of 



Fig. 102.— I. Nervous system of 

 Limnaea stagnalis L. The 

 oesophagus has been cut and 

 pulled forwards through the 

 nerve-collar, so as to expose 

 the lower surface of the buccal 

 mass (dissected byF.B. Stead). 

 B.M, 'buccal mass. 

 B.G, buccal, C.G, cerebral, 



Os.G, osphradial, Pe.G, 



pedal ganglia. 

 Pl.G, pleural ganglia. 

 Op.N, optic, Os.N, osphradial, 



Te.N, tentacular nerve. 

 Ot, otocyst ; V.L, visceral loop. 

 R, rectum, dotted in to show 



its position relative to the 



osphradium. 

 II. Right side of the head of Lim- 

 naea stagnalis. The over- 

 hanging flap of the man tie has 

 been cut in the middle line, 

 and the right half twisted 

 back, so as to expose the pul- 

 monary orifice, etc. The points 

 A A on the mantle edge were 

 continuous before the mantle 

 was cut ; the line BA is part 

 of the free edge of the mantle. 



An, anus ; F, female genera- 

 tive orifice ; J, portion of jaw; 

 M, male generative orifice un- 

 der right tentacle ; Os, osphra- 

 dium ; P.O, pulmonary orifice. 



.-Os.G 



I 



Fissurella and Haliotis shows distinct points of similarity to that 

 of the Amphineura. The pedal nerves are united by transverse 

 commissures throughout their entire length, while a double com- 1 

 missure unites the cerebral ganglia to the mass from which the ■ 

 pedal nerves proceed. In the great majority of the Streptoneura 

 the ganglia (except the visceral)" are more concentrated and the 

 commissures are consequently much shorter. The accompanying 

 figure of Cyclostoma^ in which the animal is represented as in 





