54 Part second. 



and Avith all their innocent and flowei*-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 Avhich have 

 preferred security to independence and monotony to danger. 



Roughly speaking", the differences between plants and animals are as 

 follows. Few, if any, plants are known 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 jjlants derive their strength from a support- 

 ing 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, containing much nitrogen; these substances have not been found 

 in plants. — Plants rarely possess power of movement, animals are rarely 

 Avithout it. — Last, but not least, an animal shoAvs signs of relationship 

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

 occurrence of a kind of cellulose in those most undoubted animals , the 

 Sea-squirts, there is not one of these characters in Avhich any of the se- 

 dentary creatures above referred to approach the plants. 



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

 « Vegetable » as a half-Avay liousc betA\^een « 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 tlie gulf that separates both from 

 non-living matter. 



*& 



SPONGES (PORIFERA). 



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

 Avhether sponges were plants or animals, close iuA^estigation soon rendered 

 undoubted their animal nature. It was early remarked that « sponge » 

 Avhen 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 Avas found that the holes in its surface are capable of opening- and 

 shutting, and that from the larger of them, Avhen 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 A\'itliout magnification. Further it Avas found tliat the young sponge 

 (varying from microscopic size to that of a pin's liead) swims freely about 

 by means of little Avaving 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 Avoody substances 

 especially characteristic of the vegetable kingdom. 



The water entering by the small pores passes through a system of 

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



