8 MANUAL OF ZOOLOGY. 



two kingdoms, many animals having no nervous system or in- 

 ternal 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 charac- 

 ters the most obvious, it must be admitted that no absolute 

 distinction can be laid down between plants and animals. 

 Many of our ordinary zoophytes, such as the Hydroid Polypes, 

 the sea-shrubs and corals as, indeed, the name zoophyte 

 implies are so similar in external appearance to plants that 

 they were long described as such. Amongst the Molluscoida, 

 the common sea-mat (Flustra) is invariably regarded by sea- 

 side visitors as a seaweed. Many of the Protozoa are equally 

 like some of the lower plants (Protophyta) ; and even at the 

 present day there are not wanting those who look upon the 

 sponges as belonging to the vegetable kingdom. On the other 

 hand, the embryonic forms, or " zoospores," of certain un- 

 doubted 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 to have been referred to that divi- 

 sion of the Protozoa. 



b. Internal Structure. Here, again, no line of demarcation 

 can be drawn between the animal and vegetable kingdoms. 

 In this respect all plants and animals are fundamentally 

 similar, being alike composed of molecular, cellular, and fib- 

 rous tissues. 



c. Chemical Composition. Plants, speaking generally, exhibit 

 a preponderance of ternary compounds of carbon, hydrogen, 

 and oxygen such as starch, cellulose, and sugar whilst 

 nitrogenised compounds enter more largely into the compo- 

 sition of animals. Still both kingdoms contain identical or 

 representative compounds, though there may be a difference 

 in the proportion of these to one another. Moreover, the 

 most characteristic of all vegetable compounds, viz. cellulose, 

 has been detected in the outer covering of the sea -squirts, 

 or Ascidian Molluscs; and the so-called "glycogen," which is 

 secreted by the liver of the Mammalia, is closely allied to, if 

 not absolutely identical with, the hydrated starch of plants. As 

 a general rule, however, it may be stated that the presence in 

 any organism of an external envelope of cellulose raises a strong 

 presumption of its vegetable nature. In the face, however, 

 of the facts above stated, the presence of cellulose cannot be 

 looked upon as absolutely conclusive. Another highly charac- 

 teristic vegetable compound is chlorophyll, the green colouring- 



