80 WHAT IS A PLANT ? 



of Urtica^ the stinging-nettle ; in the beautiful purple hairs grow- 

 ing in the centre of Tradescaritia^ the spider-wort, a discovery due 

 to Robert Brown fifty years ago, and in many other cases. 



3. — Certain Alg^e give rise to bodies known as zoospores^ 

 from their likeness to some Infusoria ; these are embryonic forms 

 of future adult Algae, and being set free, move rapidly by the aid 

 of their cilia, already named. Good examples of these are seen 

 in the motile forms of Pi'otococcus^ Vaiicheria^ our old friend, 

 Pero?iospora, and others. 



4. — Many e?itire plaiits spend their whole life in a state 

 of brisk activity. For example, the Desviids and Diatojns^ 

 microscopic organs of wonderful variety, beauty, and delicacy 

 found in fresh-water pools, show this power, and the closest 

 attention of such observers as Huxley and Dallinger has so far 

 failed to detect either the mechanism of the movement, or the 

 distinction (if any) between it and the motion of Infusorians. 

 Volvox globator is a kind of hollow sphere, formed of numerous 

 green bodies, each owning two vibrating cilia. By means of these 

 it rolls over and over, hither and thither, paddling with great 

 rapidity, as if it were a kind of " Queen's messenger " in the plant 

 realm. 



On the other hand, many animals spend all their lives in a 

 fixed condition. Such are the Sponges^ the Corals^ and many of 

 the Polyps (therefore once thought to be plants) ; many Rotifera, 

 as Fioscularia, Melicerta the building rotifer, and others, termed 

 " tube-dwellers " ; Flustra^ and most of the Polyzoa ; some Crus- 

 tacea, such as Balanns (acorn-shell), Lepas (Barnacle) ; finally, 

 some MoLLUSCA, such as the Oyster^ Mussel^ and Ship-worm^ and 

 the Ascidioida, or sea-squirts, our supposed ancestors. Locomotive 

 power, therefore, as a distinctive character of animals, being 

 brought up before the Bar of Science, must have the verdict 

 given against it, let the counsel for the defence say what he may. 



VI. — Function of Digestion. The presence of a digestive 

 cavity formed, in Cuvier's judgment, the most important charac- 

 ter of animals. He made this therefore his primary distinctio?i. 

 He held that they must needs possess it, because, being more or 

 less constantly in motion, they needed some internal compartment, 

 or digestive cavity, whence their food might be taken by the 



