INVEBTEBRATA. 429 



PHYLUM PROTOZOA. 



The general characteristic of the Protozoa as distinguished 

 from all other animals is that in them the cell is the indi- 

 vidual or organism, while in others the individual is 

 composed of multitudes of cells which are not capable of 

 permanent existence apart from the compound organism to 

 which they belong. The Protozoa, however, are not always 

 simple cells each with a single nucleus. Many have two 

 nuclei, like Paramecium, or more than two, as in one stage 

 of the Foraminifera. Such forms may be considered as 

 equivalent to several cells potentially, not actually, divided. 

 Again, many Protozoa when they multiply by simple 

 division do not separate but remain connected by peduncles 

 or otherwise, forming compound structures, just as the 

 zooids of Obelia remain connected with one another. Such 

 forms are usually called colonial, which as explained in the 

 case of Obelia is not an appropriate term. We cannot call 

 the compound in this case a gemmarium, because the new 

 individuals are not formed by gemmation but by fission ; 

 we may therefore call it a fissorium. In such cases every 

 cell is equally capable of conjugating and reproducing the 

 compound, whereas in Metazoa these functions are pos- 

 sessed by special cells of the compound organism and have 

 been lost by the others. Again, in the compound Protozoa 

 all the cells are similar and each performs all the necessary 

 functions of life, and the cells are not arranged in layers. 

 In Metazoa the cells are arranged in layers which have 

 different functions, one layer being specialised for digestion. 

 In the Protozoa parts of the cell are differentiated to form 

 organs, in the Metazoa organs are formed of groups of 

 cells. 



The Protozoa are divided as follows : 



CLASS : EHIZOPODA. 



Protozoa without distinct cuticle, the amoeboid form 

 being predominant, the protoplasm capable of protrusion 

 in the form of pseudopodia, which are not constant in form 

 nor permanent. This class is sometimes called Gymnomyxa. 



