DISTINCTIVE CHARACTERS OF THE PROTOZOA 3 



Siebold's generalization was a great step in advance, introducing 

 clear and orderly ideas into the place of the chaotic notions pre- 

 viously held, and setting definite limits to the group Protozoa by 

 excluding from it various types of organisms, such as Sponges, 

 Rotifers, etc., which had hitherto been classed as Protozoa, but 

 which were now referred definitely to the Metazoa. Nevertheless 

 Siebold's definition presents many difficulties, especially when con- 

 sidered from a wider standpoint than that of the zoologist. This 

 will be apparent if the two words of the definition given above, 

 ; ' unicellular animals," be considered critically. 



1. " Unicellular."- Accepting the standpoint of the cell-theory, it 

 has already been noted that many other organisms besides Protozoa 

 must be regarded as single cells. Moreover, it is found that many 

 organisms which must be classed as Protozoa appear constantly in 

 a multicellular condition ; such are the well-known genus Volvox 

 and its allies, besides examples of other orders. In all cases of this 

 kind, however, the constituent cells are morphologically equivalent, 

 and are to be regarded as complete individuals more or less inde- 

 pendent, showing as a rule no differentiation, or, if any, only into 

 reproductive and vegetative individuals ; and the multicellular 

 organism as a whole is to be regarded as a colony of unicellular 

 individuals primitively similar but secondarily differentiated, it may 

 be, in relation to special functions. Such multicellular Protozoa 

 present, in fact, a perfect analogy with the colonial forms seen in 

 many groups of animals higher in the scale, especially the Coelentera, 

 where also the members of a colony, primitively equivalent and 

 similar amongst themselves, may become differentiated secondarily 

 for the performance of distinct functions by a process of division 

 of labour among different individuals. It is not possible to con- 

 found the multicellular Protozoa with the Metazoa, in which the 

 organism is not only composed of many cells, but exhibits also 

 cell-differentiation based on mutual physiological dependence of 

 the cells on one another, leading to the formation of distinct tissues ; 

 that is to say, aggregations or combinations of numerous cells, all 

 specialized for the performance of a particular function, such as 

 contraction, secretion, and so forth. 



The essential feature of the Protozoa, as contrasted with the 

 higher animals or plants, is to be sought in the independence and 

 physiological completeness of the cell-individual. The Metazoa 

 are tissue-animals, in which the primitive individuality of the cell, 

 is subordinated to, or has a restraint imposed upon it by, the 

 corporate individuality of the cell -aggregate. In the Protozoa the 

 cells are complete individuals, morphologically and physiologically 

 of equal value. If, however, as feAv will doubt, the Metazoa have 

 been evolved from simple unicellular ancestors, similar to th<> 



