6 AN INTRODUCTION TO ZOOLOGY. 



paralysed by tbe cold of our climate. The above are 

 instances of irritability, that is to say, movement that 

 occurs in response to some stimulus. Instances of 

 spontaneous movement are seen in the spores of cer- 

 tain Algee and Ferns [zoospores, antlitrozoids, figs. 2, 3), 

 which by means of cilia (see p. 137) swim about bodily 

 from place to place with extreme rapidity : and more 

 remarkable even than these, are some of the smaller 

 algse, which are round balls consisting of many little 

 cells each provided with cilia, by the harmonious 

 action of which the whole swims about in the water. 

 So animal-like are these last-named forms [Volvox, 

 . Pandorina, see pp. 13, 14), that in some books they 

 will even be found classed with animals ^ (figs. 3, 4). 



Again, although as a rule the gaseous interchange 

 of plant life is as stated above, yet Fungi constantly 

 give off carbonic acid gas; so do the buds of flowers, 

 and the germinating seeds of plants : yet these could 

 not possibly be confused with animals. 



The fact is, that although the majority of animals 

 and plants are sharply marked off from one another, 

 yet there are also certain broad characters which they 

 share in common ; and the two groups may be likened 

 to two great branches that have spread in opposite 

 directions from one hidden root. We must not be 

 surprised to find a few intermediate twigs, given off 

 from the parent stock half-way between them, scarcely 



' For their place when classed in the vegetable kingdom, 

 see Prantl and Vines' " Text-book of Botany." 



