16 PHYSIOLOGICAL CLASSIFICATION 



common to all protozoa is also that which is 

 common to all metazoan cells, except their in- 

 dependence. The positive distinction between 

 protozoa and metazoa is thus justified alike on 

 morphological and on physiological grounds. 



It remains to be seen whether there are not 

 some other properties of animals that could be 

 utilised temporarily for the purpose of obtaining 

 a comprehensive survey, beyond those which 

 are afforded by their bodily composition. A 

 grouping of physiologically comparable forms 

 may indeed be very instructive, and it has 

 this additional advantage that it enables us to 

 visualise, in a single perspective, a multitude 

 of facts which would otherwise lie beyond our 

 range for the time being. It will thus be seen 

 that physiology can render great service in 

 marshalling together scattered morphological 

 units. In support of the soundness of these 

 assertions I may quote the following statement 

 from Prof. A. A. W. Hubrecht's recent mono- 

 graphic essay on the " Early Ontogenetic 

 Phenomena in Mammals." 1 Referring to the 

 relation between trophoblast 2 and amnion of 

 which he is the exponent, he says : — 



" A. Willey ('98) has speculated upon similar 

 relations between arthropod embryos and their 

 larval envelope, also designated as amnion, in 



1 Quart. Journ. Micro. Sc, vol. liii., 1908, p. 76. 



2 The trophoblast is the outer absorbent germ-layer attached to 

 the embryos of many viviparous animals. 



