206 Comparative Physiology. 



between Phytozoa, tlie lowest group of animals, and Proto- 

 phyta, the lowest of plants. They approach each other so 

 closely as to be distinguished often by supposed differences only. 

 They correspond in their nutritive functions, the lowest in each 

 absorbing by their whole surface. The lichens absorb by one 

 surface only, and in this resemble Polypifera, which have the 

 absorbent power restricted to the sides of the digestive cavity, 

 the external surface being excluded by its hardness. Traces of 

 the higher animals have been found among the lower, as if 

 Nature, at the commencement of her work, had given us a 

 sketch of the different forms she intended to adopt in the higher 

 parts of the scale. So, in like manner, among the lower plants, 

 in Protophyta, traces of Endogens and Exogens may be found. 

 The fungi and Radiata correspond in the tendency exhibited 

 in the higher divisions of fungi to the arrangement of parts 

 around a common centre, so characteristic of the Kadiata (star 

 fish). The ferns have the spiral mode of developement very 

 evident, in the arrangement of the leaves both in themselves 

 and around the stem *, to which the Gastropoda (typical Mol- 

 lusca) approach more than other animals ; they also form their 

 shells by additions to the edges, as ferns do their stems by the 

 addition of the petioles of the leaf. The Articulata (insects) 

 resemble Endogens, in their having the hardest portions or 

 organs of support external ; the additions to their tissue being 

 formed from within, and the trachea being distributed, like the 

 tubes in Endogens, through the whole system. Finally, Ex- 

 ogens may be considered analogous to Vertebrata, in the 

 internal situation of their hard parts, the formation of new 

 tissue from without, and the confinement of the internal respi- 

 ratory apparatus to a particular situation in the fabric. The 

 process of decortication in some plants reminds us of the exuvi- 

 ation among serpents. The following table will place in an 

 obvious aspect the position of the principal groups of the animal 

 and vegetable kingdoms : — 



ANIMALS. VEGETABLES. 



MOLLUSCA. ^V" "^ ACROGENS, 



VERTEBRATA. J® \ i " " ^ EXOGENS. 



PHYTOZOA. # PROTOPHYTA. J 



A J 



ENDOGENS. 



* Some authors say the spiral is the true mode of developement in plants ; 

 and that the radiated or whorled is an arrestment only of that spiral ; even 

 the opposite leaves they reckon unnatural. 



