2 A PRIMER OF BIOLOGY 



of lower grade than horses or oak trees, such, for 

 example, as corals and seaweeds, we may well find 

 it difficult, without special training in zoology and 

 botany, to decide to which of the two great classes 

 above mentioned these organisms respectively belong. 

 Even so great a naturalist as Linnaeus included 

 representatives of both in the same category. A 

 seaweed like that shown in Fig. 1 looks, at first sight, 

 not at all unlike the coral by its side. Both are, when 

 alive, reddish-white in colour ; both live in the sea ; 



FIG. 2. A, Flustra, a Polyzoon ; B, Padina, 

 an Alga. (^ natural size.) 



both are attached to rocks ; both are stony in texture 

 and, in the main, composed of calcium carbonate ; 

 yet the living substance of the one is vegetable, of 

 the other, animal. Albums of pressed seaweeds 

 frequently contain specimens of marine animals, 

 which in many cases bear a strong superficial resem- 

 blance to genuine seaweeds. 



In studying the very lowest types of life we en- 

 counter even greater difficulties, and the trained 

 zoologist or botanist may well hesitate before pro- 

 nouncing a definite opinion on their essential nature. 

 The remarkable Slime^ Fungi, for example, which 



