258 MARTIN P. WOODWARD. 



along the course of the posterior connective joining the cere- 

 bral to the pleuro-pedal cords. This condition is practically 

 the same as that seen in Pie urotom aria if we imagine a 

 crowding together of the ganglionic cells at the point of 

 origin of the visceral nerve, and is the natural outcome of 

 that tendency towards a shortening of the nerve-tracts and 

 concentration of the nerve-cells into ganglia which is so 

 characteristic of the Gastropoda. 



From the condition seen in Pal u din a it would be very 

 easy to derive, by a shortening of the cerebro-pleural con- 

 nective, the condition of all other Tteuioglossa, with the 

 possible exception of the Cycle ph or id eb and Ampullaria, 

 which are probably special and independent derivatives of 

 more specialised Diotocardia. 



From the above consideration I conclude that Pleuro- 

 tomaria in its nervous system, as in some other points in 

 its anatomy, is the most primitive of existing Diotocardia, 

 and presents a condition from which that of the majority of 

 the TsGnioglossa may be derived, — possibly also that of the 

 other Diotocardia, the form in the latter being attained by a 

 shortening of the pleuro-pedal connective, thus causing the 

 pleural centres to be approximate to the pedal ganglia; thus 

 the condition seen in Haliotis, Trochus, Fissurella, and 

 Patella would be a derived and not a primitive one. 



While it is fairly easy to derive the Monotocardian type of 

 nervous system, radula, gill, and reproductive system from 

 the corresponding organs of existing Diotocardia, yet in the 

 conformation of the kidneys we meet one of the greatest 

 stumblingblocks in our attempt to derive the former group 

 from the latter. 



All the Diotocardia with the exception of the aberrant 

 Neritinoid group possess two kidneys, and in the majority 

 these two organs differ markedly in structure and function.^ 



1 In Fissurella and Patella, both of which, however, are specialised 

 forms, the two kidneys, though differing in size and relationship, are both 

 excretory in function; but the left kidney, as in other Diotocardia, derives its 

 blood-supply from tlie auricles. 



