14 MANUAL OF ZOOLOGY. 



either amongst vegetables or amongst animals. Thus, Dr 

 Ernst Hgeckel has proposed to form an intermediate kingdom, 

 which he calls the Regnum Protistiaim, for the reception of all 

 doubtful organisms. Even such a cautious observer as Dr 

 Rolleston, whilst questioning the propriety of this step, is 

 forced to conclude that "there are organisms 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 animals 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 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 sea-weed. 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," (fig. i, a and b) 

 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 to have been referred to 

 that division 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, 



