20 ELEMENTS OF BIOLOGY. 



which at one period of their life exhibit an aggregate of 

 phenomena such as to justify us in speaking of them as 

 animals, whilst at another they appear to be as distinctly 

 vegetable." 



In the case of the higher animals and plants there is no 

 difficulty; the former being at once distinguished by the 

 possession of a nervous system, of motor power which can 

 be voluntarily exercised, and of an internal cavity fitted for 

 the reception and digestion of solid food. The higher 

 plants, on the other hand, possess no nervous system or 

 organs of sense, are incapable of independent locomotion, 

 and are not provided with an internal digestive cavity, their 

 food being wholly fluid or gaseous. These distinctions, 

 however, do not hold good as regards the lower and less 

 highly organised members of the two kingdoms, many ani- 

 mals having no nervous system or internal digestive cavity, 

 whilst many plants possess the power of locomotion; so 

 that we are compelled to institute a closer comparison in 

 the case of these lower forms of life. 



a. Form. As regards external configuration, of all char- 

 acters the most obvious, it must be admitted that no abso- 

 lute distinction can be laid down between plants and ani- 

 mals. Many of our ordinary zoophytes, such as the Hydroid 

 Polypes, the sea-shrubs and corals as, indeed, the name zoo- 

 phyte implies are so similar in external appearance to plants 

 that they were long described as such. Amongst the Mol- 

 luscoida, the common sea-mat (Flustra) is invariably re- 

 garded by sea-side visitors as a sea-weed. Many of the 

 Protozoa are equally like some of the lower plants (Proto- 

 phyta); and even at the present day there are not wanting 

 those who look upon the sponges as belonging to the vege- 

 table kingdom. On the other hand, the embryonic forms, 

 or "zoospores," of certain undoubted plants (such as the 

 Protococcus nivalis, Vaucheria, &c.), are provided with 

 ciliated processes with which they swim about, thus coming 

 so closely to resemble some of the Infusorian animalcules as 



