384 Miscellaneous. 



BuflBciently below to lodge the viscera, and the mantle fonns nothing 

 more than a little disk or buckler ; in the TestacfUa and the BuUetw 

 the foot follows the neck in its excessive development, and the 

 mantle remains rudimentary at the end of the body ; in the Aplysi(r 

 the foot and the neck become much developed upward, but the foot 

 still increases sufficiently in its lower part to cover with its two lobes 

 the back and even the mantle, with which it has been erroneously 

 confounded. 



The criterion which I propose enables us to determine the homo- 

 logous parts. 



Four groups of nervous ganglia characterize the MoUusca in 

 general and the Gasteropoda in particular: these are, first, the 

 stomnto-jostric, the cerehroid, and the jjedal ganglia. The fourth 

 group, intermediate between the latter two, always placed a little 

 behind and below the pedal centre, is nmymmetrical — that is to say, 

 formed by an uneven number of ganglia, generally five. It charac- 

 terizes the Gasleropod group, and, except the head, the neck, the 

 foot, and the viscera, it innervates all the organs. The name which 

 would best designate its relations would be that of Iranchio-cardio- 

 paUio-genital ; but I shall simply call it the median or inferior centre. 

 It varies much : sometimes it forms a very small ring, sometimes an 

 extremely long curve which seems to modify and change all the 

 relations. Thus in the Limnfn^, the PJanorhes, and the Ancyli, 

 although its ganglia are a little disjointed, it is very close to the 

 other centres. Again, in the Helices, the TestaceUcn, the Limaces, &c. 

 its five ganglia lie upon the pedal centre, and are united to it in 

 such a manner by a common conjunctive tissue that they have been 

 described as the posterior pedal ganglia. 



In the Ajylysicf, the Bullea, all the Pectinihranchia, the Ualiotides, 

 and the Cydostonvita, the commissure which unites the inferior 

 ganglia is long and tA\-isted, and the homologous parts are difficult 

 to recognize. Notwithstanding this, the general connexions remain 

 constantly fixed. 



With regard to the mantle, the following facts leave no doubt. 

 By numerous dissections of the most different types, I believe I am 

 able to establish that this part of the body is exclusively innervated 

 by the inferior centre, and that henceforward we may define it thus : — 

 Any fold or ci' famous part of the hody of the Gasteropod rectiviny 

 nerves from the inferior or vnsyrnmetrical centre is either the mantle 

 or a dependence of the mantle. The forms of the pallial fold may 

 varj- infinitely, their connexions never. How, therefore, can we, m 

 Apiysia, regard. the two large lobes which ascend at the back and at 

 each side upon its back as being dependences of the mantle, when 

 their nerves all come from the pedal ganglia ? These lobes are the 

 foot itself, and they serve for swimming. 



The dorsal shield of the Limaces is the mantle very slightly deve- 

 loped ; it receives all its nerves from the inferior centre ; and the 

 part which is drawn out along the lower part of the body and con- 

 tains the viscera, is the foot, for its nerves are derived from the an- 

 terior centre. -Again, in the Testacella>, it is the upper part of the 



