12 THE PROTOZOA THE DAWN OF LIFE 



carbon dioxide, which it cannot work up into food products, as it 

 is destitute of chlorophyll, the green colouring matter present in 

 all green plants. The minute plant, on the other hand, under the 

 action of light and by the presence in its tissue of chlorophyll 

 bodies (Chloroplastids) can utilise the carbon dioxide for nutri- 

 tion, the carbon being used for the building up of such compounds 

 as starch and sugar, while the oxygen is liberated. Here, then, 

 is an association beneficial to both organisms, and in this sym- 

 biosis existing between the Radiolarian and the alga Zooxanthella 

 the latter benefits the former, contributing to its respiration by 

 the oxygen which it gives off, and to its nutrition by the sugar 

 and starches which it forms, while the Radiolarian supplies the 

 carbonic acid and other substances necessary to the alga. 



Diatoms are also found sometimes living symbiotically in the 

 protoplasm of certain species ; while an amphipod crustacean 

 (Hyperia) has been observed as parasitic on colonial forms. Such 

 colonial forms, which may attain considerable size, and float freely 

 in the sea, are produced by the original central capsule dividing 

 again and again, thus giving rise to a number of central capsules 

 which remain embedded in a firm, gelatinous substance. 



Mention must be made of the Proteomyxa, 1 a division of the 

 Protozoa chiefly of interest to the specialist, and containing a 

 number of somewhat obscure forms which cannot well be placed 

 under the Rhizopoda. Very little is known about the life-history 

 of these organisms. In many instances only one stage has as 

 yet been observed, but it seems probable, according to our pre- 

 sent knowledge, that most, if not all, at some stage, form cysts or 

 spores. The spores escaping from such cysts may have pseudo- 

 podia like Amoeba, when they are called Amcebulce, and they 

 may be furnished with one or two flagella. To this group belongs 

 the Protamceba of Professor Haeckel, an organism more primitive 

 than Amoeba, for it is without any definite nucleus or contracting 

 vacuole, and is found, like Amoeba, living in both fresh and sea water. 



A number of the Proteomyxa are parasitic upon fresh-water 

 algae, at least at one stage of their life-history, and one species is 

 known to occur in the muscles of the domestic pig, but apparently 

 without causing any harm to its host. The only species that is 

 at present recognised as of any economic importance, and in some 



1 Greek, protos, first ; myxa, mucus. 





