SYMBIOSIS 297 



non-motile vegetative cells are usually found in the peripheral 

 layers of the polyp, the larval stages of the host commonly being 

 devoid of the alga. Most of the algal symbionts are known to have 

 a motile stage and hence are capable of an independent existence. 

 The function and relations of these symbiotic algae in the coral 

 polyps has been discussed at great length by Yonge (1932), and on 

 the whole there would appear to be evidence for a symbiotic 

 relationship, the alga obtaining food from the animal, and the 

 animal oxygen and also perhaps nitrogen from the alga. The prob- 

 lem of the relationships between algae and animals is by no means 

 completely worked out, and it is not impossible that in some cases 

 we really have an animal that is parasitizing the alga. This is 

 probably especially true in the case of the worm-like creature 

 Convoluta Roscojfensis and its algal associate Carteria, because the 

 animal apparently cannot live unless infected with the alga, whilst 

 under certain conditions it also devours the green cells. 



Other examples of symbiosis are provided by Anahaena Cyca- 

 dearum which lives in the root tubercles of species of Cycas, and 

 Anahaena Azollae, which is found in the leaves of the water fern, 

 Azolla filiculoides, though the species oi Nostoc that are to be found 

 in the thallus of the Liverworts Blasia and Anthoceros are probably 

 no more than space parasites obtaining shelter. 



Epiphytism is extremely common among the algae, whilst there 

 are also a number of epizoic forms. One may also find endophytic 

 species, such as Schmitziella mirahilis in Cladophora pellucida, and 

 endozoic species, such as Rhodochorton endozoicum in the sheaths of 

 hydroids. The origin of the symbiotic habit among the algae is 

 probably to be explained as cases of epiphytism in which the 

 relationship between host and epiphyte became more intimate: 

 similarly the relatively few cases of parasitism probably arose either 

 directly from an epiphytic habit or else passed through the 

 symbiotic phase. Examples of total or partial parasites are Notheia 

 anomala in the Phaeophyceae, Choreocolax Polysiphoneae in the 

 Rhodophyceae and Chlorochytrium Lemnae in the Chlorophyceae. 



