204 



STREPTONEURA AND EUTHYNEURA 



of the ganglia on the visceral loop, the osphradinni itself 

 being situated (in Limnaea) immediately ahove the ])nlmoiiary 

 orifice and adjacent to tlie anus (Fig. 102). Tliis massing 

 of the ganglia is still Letter illustrated hj the accompanying 

 figure of Pliysa (Fig. 103), in wliicli the animal is re])reseiited as 

 if transparent, so that the ganglia and nerves are seen through 

 the tissues. 



Of the streptoneurous Gasteropoda, tlie nervous system of 



_ Fro. 102. — T. Nervous .<5ysteni of 



Limnaea stacjnalis L. The 

 oesophagus has been cut and 

 pulled forwards through the 

 nerve-collar, so as to expose 

 the lower surface of tlie buccal 

 mass (dissected by F. 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 

 osphradiuni. 

 TF. Right side of the head of Lhn- 

 iiaea stagiudix. The over- 

 hanging thip of the mantle 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 under 

 light tentacle ; Os, os{>hradium ; 

 P.O, pulmonary orifice. 



_.Os.C- 



FissurcIIa 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- 

 missure unites the cerebral ganglia to the mass from which the 

 pedal nerves proceed. In tlie great majority of the Streptoneura 

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

 commissures are consequently much shorter. The accompanying 

 figure of Cyelostoma, in which the animal is represented as in that 

 of Fliysa just described, illustrates this grouping of the ganglia, 



