64 



LECTUEES ON BIOLOGY 



Algae have entered to their mutual advantage. Confined within 

 the cells of the polyp the Algae do not only find shelter for an 

 undisturbed growth, but share at the same time in the carbon 

 dioxide which the polyp exhales. In return the Algae supply the 

 polyp with the necessary oxygen, and also with a part of the 

 starch produced by them. 



In some of the marine polyps which, like the Hydra, give 

 shelter to Algae, mutual adaptation has made such progress that, 

 for instance, the sea-anemones have lost the power of taking their 



-~ec 



c-- 



FIG. 18. SYMBIOSIS OF SEA-ANEMONE AND UNICELLULAR ALGJE. 



A, Section of Sea-anemone : o, Mouth aperture ; t, tentacles ; g, stomach 

 cavity ; ec, ectoderm ; en, endoderm ; g, supporting lamella. B, Transverse 

 section of the body-wall (magnified) : c, Unicellular algse within the endoderm. 

 C, Unicellular algse (greatly magnified). (After O. Hertwig.) 



food independently, but are fed entirely by their ' lodgers/ 

 Experiments have shown that without Algae they are invariably 

 doomed to die from starvation. We may even regard Algae and 

 the polyp as one individual, for these minute plants even invade 

 the growing ovum of the polyp and are thus ' inherited ' by 

 the descendants from the mother. 



There is, however, a large class of plants the species of which 

 possess no chlorophyll at all. These are the fungi. As a result 

 of this defect fungi have lost the power of extracting their food 

 from the inorganic world and are, like animals, restricted to 



