THE ANATOMY 



OF 



UnTVERTEBEATED animals, 



INTRODUCTION. 



I. — THE GENERAL PRINCIPLES OF BIOLOGY. 



The biological sciences are those which deal with the 

 phenomena manifested by living matter; and though it is 

 customary and convenient to group apart such of these phe- 

 nomena as are termed mental, and such of them as are ex- 

 hibited by men in society, under the heads of Psychology 

 and Sociology, yet it must be allowed that no natural boun- 

 dary separates the subject-matter of the latter sciences from 

 that of Biology. Psychology is inseparably linked with 

 Physiology ; and the phases of social life exhibited by ani- 

 mals other than man, which sometimes curiously foreshadow 

 human policy, fall strictly within the province of the biolo- 

 gist. 



On the other hand, the hiological sciences are sharply 

 marked off from the abiological, or those which treat of the 

 phenomena manifested by not-living matter, in so far as the 

 properties of living matter distinguish it absolutely from all 

 other kinds of things, and as the present state of knowledge 

 furnishes us with no link between the living and the not- 

 living. 



These distinctive properties of living matter are — 



1. Its chemical composition — containing, as it invariably 

 does, one or more forms of a complex compound of carbon, 

 hydrogen, oxygen, and nitrogen, the so-called protein (which 

 has never yet been obtained except as a product of living 

 bodies) united with a large proportion of water, and forming 



