226 THK CONDITIONS OF LIFE IX THE SEA [PART III 



A still further complication is encountered in the cases of 

 those animals which contain green or yellow cells in their tissues. 

 Here we have to deal with associations of two organisms — the 

 animal and the contained green cells, which belong to one of two 

 groups of algae — the Xanthellae and Zoochlorellae. These partner- 

 ships arise from the " infection " of the animals by the cells of the 

 plant, and by the further multiplication of the latter within the 

 tissues of the animal ; and we speak of such associations as 

 " symbioses," or sometimes as cases of parasitism. The most 

 familiar example of these associations is the well-known green 

 Hj^dra, the tissues of which contain green corpuscles, which have 

 long been regarded as " commensal algae." Many other cases are to 

 be found among the Protozoa, Sponges, Corals, Mollusca, Alcyonaria, 

 Hydrozoa, Medusae and Turbellaria ; and even among the much 

 higher groups of the Mollusca, Polyzoa and Echinoderms. Some- 

 times the association is an obligatory one, that is, neither of the 

 partners can exist without the other, or at least, one of them must 

 be associated with the other in order to live. Sometimes, more 

 often indeed, the association is a facultative one, that is, the 

 animal can exist apart from the alga, and vice versa. 



Now the point of interest for us in connection with these 

 associations is that two modes of nutrition may proceed simul- 

 taneously in the same (compound) organism. Indeed the question 

 of how the thing feeds itself is often an extremely complicated 

 one. The most exhaustive investigation of the life-history and 

 metabolism of such a symbiotic organism is that made by Keeble 

 and Gamble^ in the case of the Turbellarian worm Gonvoluta 

 roscoffensis. This animal begins life as a colourless creature, and 

 within three days after birth becomes infected with the cells of 

 a species of alga which lives saprophytically in the water in 

 which the worm also lives. This alga belongs to the group 

 Chlamydomonadeae. It multiplies in the tissues of the worm 

 and soon the latter becomes green, the colour being due, of 

 course, to the contained algal cells. The plant may exist outside 

 the tissues of the animal, when it possesses the form typical of its 

 group, but when the infection has taken place the cells undergo 



1 "The Green cells of Convoluta roscoffensis," Quai't. Joiirn. Micro. Science, 

 vol. LI. p. 167, 1907. 



