324 MOLLUSCOUS ANIMALS. 



ed tendencies to violence or to motory efforts. The 4tli exhibits a 

 lower modification of the nutritive system tending tovrards the vege- 

 tative or absorbent mode of obtaining food, whilst the fifth represents the 

 lowest or embryonic condition of life. If we assume these character- 

 istics of the greater divisions of the animal kingdom, to be repeated 

 under each of them, always in consistency with the special type of 

 each, and to be again repeated under each subdivision as far as divi- 

 sions are required, we shall have a scheme of classification expressing 

 a general system prevailing throughout nature, and which would pro- 

 duce at once the differences, the affinities, and the analogies which are 

 actually observable. This general idea has in fact been attained by 

 observing how in all parts of the animal kingdom, the best arrange- 

 ments proposed, those which put the objects together in the most 

 intelligible and satisfactory order, almost constantly present the same 

 number of divisions of any given rank, analogies between correspond- 

 ing divisions in different groups continually striking the mind, and 

 when once the general idea had been obtained, its power in suggesting 

 improvements and removing difficulties, proving so remarkable as most 

 strongly to confirm the truth of the principle and encourage its ex- 

 tended application. In what follows I shall explain its application to 

 the Molluscous sub-kingdom confining myself at present to the con- 

 sideration of its classes and sub-classes not without hope of illustrating 

 on a future occasion the orders, families, and sub-families. 



The following important groups laying claim to the rank of classes 

 have been pointed out amongst the Mollusca, proper attention to which 

 may probably lead us to a right conclusion as to those which it is pro- 

 per to admit t 1st, Cephalopoda, the Nautilus, cuttle-fish and Ammo- 

 nite tribe ; 2nd, Pteropoda, so called from their wing-like organs of 

 motion, reduced by some under Gasteropoda, by others placed in a 

 lower position, but in my view properly occupying the position usually 

 assigned to them ; 3rd, Heteropoda of Cuvier a small anomalous 

 group, now generally, and I think justly, regarded as an order of the 

 following class ; 4th, Gasteropoda, the crawling Mollusca generally, 

 with a few swimmers evidently resembling them in structure con- 

 stituting the most numerous and the most typical division of the sub- 

 kingdom ; 5th, Lamellibranchiata, sometimes called Conchifera, Mol- 

 lusks generally covered by a pair of shells ; 6th, Brachiopoda of 

 Cuvier, often and perhaps better called Palliobranchiata to mark the 

 distinction in the mode of aeration from the preceding group ; 7th»- 



