228 THE CONDITIONS OF LIFE IN THE SEA. [PART III 



accompanied by the progressive decrease of the digestive surface 

 of the autozooids. With this reduction we find that the animals 

 have lost the power of capturing and digesting living food. Among 

 the true corals (Madreporaria) the same association may be seen ; 

 that is, green cells, with contained chlorophyll and starch granules, 

 are contained in the tissues of the soft parts of the corals, and 

 these green cells are most abundant in those parts of the coral 

 which are exposed to the light, and are in continual contact with 

 the current of w^ater which passes through the cavities of the 

 animal. The green cells are included Zoochlorellae, and they 

 take up carbonic acid from the sea water in which they live, and 

 by virtue of the power of photo-synthesis which they possess, 

 elaborate starch, which is used by the coral in the same way as in 

 the case of the green Convoluta. 



Saprozoic animals. Such associations between animals and 

 plants — symbiosis, or parasitism, whatever name we give them — 

 do not really invalidate the distinction between the characteristic 

 holophytic mode of nutrition of the plants, and the characteristic 

 holozoic mode of nutrition of animals. We can still clearly dis- 

 tinguish between the two members of the association, and their 

 distinct modes of nutrition. But the existence of a saprozoic mode 

 of nutrition among animals deprives the method of feeding, con- 

 sidered as a means of distinguishing between the two kingdoms 

 of life, of its value. A saprozoic animal resembles precisely a 

 saprophytic plant, for both find their nutritive material in complex 

 chemical substances, such as peptones or extractives, or carbo- 

 hydrates and organic acids. Still further we find that there are 

 apparent instances of photo-chemical reactions in the metabolic 

 processes of some animals. 



Many of the protozoa are saprozoic in their mode of feeding. 

 If we add some sugar to a little water containing bacteria we 

 will find that very soon there will arise an abundant infusorial 

 grow^th. Very probably the infusoria feed upon the added carbo- 

 hydrate. All internal parasites are also saprozoic. A tape-worm 

 possesses neither mouth nor alimentary canal, and it lives 

 throughout its life (except perhaps for a very short period, when it 

 may inhabit water or other media) attached by means of suckers 

 or hooks to the wall of some internal cavity of an animal body. 



