in Organized Beings. 103 



root modified to imbibe fiuid, and the nutriment rapidly con- 

 veyed by the vessels of the stem to distant parts of the plant, 

 where it undergoes certain processes of elaboration, which ren- 

 der it fit to be applied to the purposes of nutrition. 



The compound Polypifera may probably be i-egarded as pre- 

 senting us with the first appearance of a distinct circulating 

 system in animals. The motion of water through the pores 

 and canals of the sponges, can scarcely, I think, be regarded 

 in this light, since the proper function of absorption does not 

 commence until the fluid comes in contact with the soft tissue 

 lining these passages ; which have been justly compared, by Dr 

 Grant, to the ramified roots of a plant turned inwards. It is 

 in the Echinoderma, however, the bulk and solidity of whose 

 tissues prevent that immediate absorption, either from the 

 stomach or the external surface which prevails in less de- 

 veloped animals, that we first perceive a complete vascular sys- 

 tem ; and this is employed like that of plants in receiving di- 

 rectly, from the absorbent surface, the fluid aliment, and in 

 conveying it to the distant parts of the organism. We are 

 then to regard the stomach and alimentary canal of animals as 

 organs to which no analogy exists in plants ; in the latter, the 

 nutriment is directly received from the surrounding medium, 

 in a fluid form, no solid material being capable of being intro- 

 duced into their system until first dissolved. Their food, there- 

 fore, consists of water, holding various matters in solution ; and, 

 I think, is capable of being proved, that water and carbon in 

 some forms constitute all that is essential to the growth of 

 plants. They are, therefore, entirely dependent on the inor- 

 ganic elements around them ; and as these are constantly with- 

 in their reach, vegetables have no need either of organs of lo- 

 comotion, or of an internal cavity to store up or prepare their 

 food. Animals, on the other hand, being chiefly dependent 

 upon matter previously organized, which can only be procured 

 under certain circumstances, require peculiar means of obtain- 

 ing it, and a particular apparatus for preparing it to be intro- 

 duced into the system. We cannot regard any substance to 

 have been so introduced, until it shall have been absorbed ; and 

 the only difference between the skin and the mucous membrane 

 in this respect being, that the latter absorbs with the greatest 



