54 Part second. 



and with all their innocent and flower-like beauty, lead not only active 

 but predatory lives. For the most part it may be affirmed with certainty 

 that they are the descendants of freely swimming animals which have 

 prefered security to independence and monotony to danger. 



Roughly speaking, the differences between plants and animal are as 

 follows. Few, if any , plants are hnown to digest solid food ; almost all 

 animals do so. Few, if any, animals have the peculiar green colour which 

 enables a plant in the sunlight to feed on air; most plants possess it. 

 The great majority of plants derive their strength from a supporting 

 honeycomb of cellulose (the substance of which cotton is composed) or 

 allied woody matters containing no. nitrogen; these have been met with 

 in the animal kingdom only among the Tunicates (see p. 84). Most animals 

 possess supporting structures of the nature either of horn or gristle, con- 

 taining much nitrogen; these substances have not been found in plants. 

 Plants rarely possess power of movement , animals are rarely without 

 it. Last, but not least, an animal shows signs of relationship to other 

 animals, and a plant to other plants. With the exception of the occur- 

 rence of a kind of cellulose in those most undoubted animals , the Sea- 

 squirts, there is not one of these characters in which any of the sedentary 

 creatures above referred do approach the plants. 



It is worth adding a word to say that the old conception of the 

 Vegetable as a half-way house between Animal and : Mineral 

 is very delusive. The structure of the final living matter in plants and 

 animals appears almost identical , and all the differences which can be 

 enumerated between them sink to nothing beside the gulf that separates 

 both from non-living matter. 



SPONGES (PORIFERA). 



Although in the earlier part of the 17th century it was debated 

 whether sponges were plants or animals, close investigation soon rendered 

 undoubted their animal nature. It was early remarked that sponge 

 when burnt gave off a smell of burning hair or horn, and exact analysis 

 showed it to be nearly allied to these substances* This in itself gave 

 reason to suppose that the chemistry of their life was animal rather than 

 vegetable. Though a living sponge is fixed and apparently motionless, it 

 was found that the holes in its surface are capable of opening -and shut- 

 ting , and that from the larger of them, when open , there is usually a 

 strong stream of water issuing. This is compensated for by small entering 

 streams through other holes far more numerous but generally invisible 

 without magnification. Further it was found that the young sponge (varying 

 from microscopic size to that of a pins's head) swims freely about by 

 means of little waving hairs ( flagella ) over its surface. Finally it was 

 shown that sponges live on solid food. While thus possessing all those 

 characters that are more frequent among animals than plants (see p. 53) 

 they never contain any traces of the cottony and woody substances espe- 

 cially characteristic of the vegetable kingdom. 



The we,ter entering by the small pores passes through a system of 

 branching and fine canals , and is collected again by a similar system 



