CLASSIFICATION. 307 



the animals classed as Molluscoida, possess rude appliances 

 for the transfer of force : the peri- visceral sac, or closed 

 cavity between the intestine and the walls of the body, 

 serves as a reservoir of absorbed nutriment, from which the 

 surrounding tissues take up the materials they need. The 

 more highly-organized animals, belonging to whichever sub- 

 kingdom, all of them possess definitely- constructed channels 

 for the transfer of force ; and in all of them, the function of 

 expenditure is divided between a directive apparatus and 

 an executive apparatus — a nervous system and a muscular 

 system. But these higher sub-kingdoms are clearly separated 

 from each other by differences in the relative positions of 

 their component sets of organs. Prof. Huxley defines the 

 type of the Vertebrata, as one in which the ganglionic nervous 

 system lies on the dorsal side of the alimentary canal, while 

 the central vascular system lies on its ventral side ; and one 

 which is yet further characterized by the possession of a 

 second, and more conspicuous, nervous system, placed on the 

 dorsal side of the vertebral axis — an extra endowment which 

 is perhaps the most essentially distinctive. The types of the 

 Annulosa and Mollusca, are together marked off from the 

 vertebrate type, by the singleness of the nervous system, and 

 by its occupation of the ventral side of the body: the 

 habitual attitudes of annulose and molluscous creatures, is 

 such that the neural centres are below the alimentary canal 

 and the hsenial centres above. And while by these traits the 

 annulose and molluscous types are separated from the verte- 

 brate, they are separated from each other by this, that in 

 the one the body is " composed of successive segments, 

 usually provided with limbs," but the other, the body is not 

 segmented, " and no true articulated limbs are ever de- 

 veloped." 



The sub-kingdoms being thus distinguished from one an- 

 other, by the presence or absence of parts devoted to funda- 

 mental functions, or else by differences in the distributions of 

 such parts ; we find, on descending to the classes, that these 



