OMNIVOROUS ECHINODERMS, &c. 271 



bodies are present in the green parts, especially the leaves, of 

 all the plants which make up the conspicuous part of vegetation, 

 though in some lowly forms there may be chlorophyll-bodies of 

 very different shape. That, however, is an unimportant detail, 

 the essential fact being that leaf-green in some way enables the 

 living substance which it permeates to build up the simple 

 chemical compounds already enumerated into complex organic 

 substance. In an ordinary land-plant the requisite carbonic acid 

 gas (CO 2 ) is supplied by the air, while the water and mineral 

 matters are taken up from the soil. In a word, green plants 

 bridge over, as it were, the gap between the non-living mineral 

 kingdom and living organisms. The upbuilding process asso- 

 ciated with the presence of chlorophyll is dependent upon light, 

 and this pigment, in some way not clearly understood, enables 

 the energy of the sun's rays to be used for the purpose. 



The nutrition of green plants has been mentioned here because 

 there are some animals in which chlorophyll is present, and which 

 therefore are partly, or in some cases it would seem entirely, able to 

 subsist upon carbonic acid gas, water, and mineral salts. A very 

 interesting example is afforded by a small marine flat-worm be- 

 longing to the group of Planarian Worms (Turbellaria) (see vol. i, 

 pp. 445-447). This creature (Convoluta Roscoffensis) is found 

 in large numbers floating in the shore-pools at Roscoff, on the 

 French coast, and is coloured green by the presence of numerous 

 chlorophyll-bodies, which enable it to live entirely after the manner 

 of a green plant. The Convoluta belongs to a carnivorous group, 

 and is no doubt descended from forms which depended upon 

 animal food. A gradual change of habit has, however, taken 

 place, and this is associated with corresponding modification of 

 structure. It is, however, doubtful whether the chlorophyll-bodies 

 are really an actual part of their possessors. Some authorities 

 regard them as resulting from the specialization of microscopic 

 green plants (algae), the ancestors of which acquired the curious 

 habit of living inside an animal. 



Well-known instances of the presence of chlorophyll are found 

 in the phylum of ZOOPHYTES (Ccelenterata). A case in point 

 is that of the Green Freshwater Polype (Hydra viridis) com- 

 monly found adhering to water- weeds in ponds and streams. 

 Hydra has already been described in some detail (vol. i, pp. 

 465-473), and it need only be remarked here that the body is 



