180 THE EYE IN EVOLUTION 



Protozoa 



PROTOZOA are the most primitive and simplest of animals, some of 

 which might with equal justification be considered as plants ; they are 

 essentially single-celled but sometimes form loose colonies by budding 

 or by cell-division, showing some degree of co-ordination but never 

 forming differentiated tissues. Of all animal types they are the most 

 numerous, being found in every continent, on land, in fresh water, in 

 the seas and impartially distributed as parasites within all animals 

 (including some of their own kind), among which not the least fre- 

 quented is Man ; their skeletons contribute largely to the oozes of the 

 seas and to the composition of the rocks of which the land-masses are 

 made. 



Within the phylum four methods of activity are evident — 

 amoeboid movement, flagellate and ciliary progression, and encystment 

 with spore -formation, characteristics under which the upwards of 

 15,000 species may be conveniently grouped into 4 classes (see p. 179). 



SABCODiNA (or rhizopoda), Organisms which progress by sending out 

 finger-Hke pseudopodia into which the protoplasm of the cell pours itself. This 

 class comprises such types as the fresh -water Amoeba, the parasitic Entamceha 

 or the marine Foraminifera with chalky shells and Radiolaria with siliceous 

 shells which after death enter largely into the formation of the oozes of the bed 

 of the ocean. 



FLAGELLATA (or mastigophora), Organisms which swim by the lashing 

 movements of one or a few whip-like flagella. The class comprises such types 

 as the common Euglena and colonial forms such as Volvox almost universal in 

 fresh-water ponds, the parasitic, disease-producing Trypanosomes and Tricho- 

 monads, Dinoflagellates including Noctiluca which gives luminescence to the seas,^ 

 and Cystoflagellates, important constituents of the plankton of lakes and the 

 oceans. 



ciliophora, organisms which progress by the coordinated movements of 

 many hair-like cilia. The class comprises the Ciliates (such common types as 

 the slipper-shaped Paramoscium, the bell-shaped Vorticella or the trumpet- 

 shaped Stentor) and the Suctorians which lose their cilia in adult life and in their 

 place develop tentacles used as suckers by which they capture and suck out the 

 bodies of their protozoan prey. 



sporozoa, encysted organisms without a locomotive mechanism ; they are 

 parasitic on almost every species of animal and are spore-forming in habit 

 (Coccidia, Hsemosporidia, Plasmodium, etc.). 



In view of the fact that the response to light in these primitive 

 forms is motorial, it is not surprising that receptors are not found in the 

 passive parasitic Sporozoa ; in the first three classes responses to light 

 are found among the freely-swimming active types, but as would be 

 expected in imicellular organisms, the receptor mechanisms are of the 

 most pr i itive nature. In the Sarcodina (Amceba) and some Ciliates 



1 p. 738. 



