Sub-Kingdom I.— PBOTOZOA. 



The organisms which, following von Siebold, we here include under 

 the name " Protozoa," form a division of the animal kingdom whose 

 members, in part at least, can only be distinguished with difficulty, 

 if indeed at aU, from the lowest plants. ^ 



The most essential characteristic of these Protozoa lies in their 

 minuteness and in the simplicity of their structure, two characters 

 which to a certain extent involve each other. For the most part 

 invisible to the naked eye, and but rarely attaining the diameter of a 

 milhmetre or more, they possess a body devoid of anything deserving 

 the name of organs, and often discharge their vital functions as simple 

 minute masses of animal substance, without any differentiation what- 

 ever. 



As early as 1845 von Siebold endeavoured to compare the structure 

 of the Protozoa with that of the ordinary cell, or, in other words, to 

 define the Protozoa as unicellular animals ; ^ and this assumption has 

 been fully justified in regard to the vast majority of these organisms. 

 It is, however, true that we have in the course of time discovered some 

 Protozoa in which as yet a nucleus has been sought for in vain (the so- 

 caUed " Monera"), and others which might perhaps be more correctly 

 termed cell-aggregates, as evidenced by the possession of numerous 

 nuclei, were it not that in all these cases the component cells are so 



' It is not my intention to enter further into the relations between plants and animals. 

 This much, however, must be noted, that in the simplest animal and vegetable organisms we 

 find neither in structure nor in function any of those fundamental differences which usually 

 separate the higher representatives of the two kingdoms. Prom this it does not, how- 

 ever, in any way follow that we are bound to separate off these simple organisms from the 

 others, and make a special intermediate kingdom of them, as Hogg and Haeckel have 

 lately done (Protoctista, Hogg — Protista, Haeckel). By the creation of this heterogeneous 

 intermediate kingdom, which includes forms so widely separated as the Infusoria and 

 Fungi, the difficulty of determining the boundaries is not diminished, but doubled ; instead 

 of there being only one uncertain boundary line, there are two. [With respect to this 

 point, it may be urged that a scientific classification is not merely an arrangement for 

 practical convenience, but rather a method of expressing our views as to the relations of 

 organisms. If, in conformity with the evolution theory, we hold that plants and animals 

 sprang from a common stem, then " Protista " must have once existed, and if so, they 

 ought to find a place in our scheme. The question whether it is easier to draw one 

 boundary line or two seems quite beside the mark. — W. E. H.] 



''-"Lehrbuoh d. vergl. Anat. d. wirbelloeen Thiere:" Leipzig, 1845; and Zeitschr. f. 

 wist. Zool., Bd. i., p. 270. 



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