48 THE INVERTEBRATA 



chlamydomonad of the genus Carteria which lives as a zoochlorella in 

 the tissues of the turbellarian worm Convoluta roscoffensis (Figs. 35, 

 36). The photosynthetic partner in these symbioses benefits by a supply 

 of carbon dioxide and the nitrogenous excreta of its host ; the latter 

 has waste matters removed, is supplied with oxygen, and sometimes 

 draws on the supply of carbohydrates manufactured by the guest, 

 though it is rarely, as Convoluta, unable to dispense with this nutri- 

 ment, and often, as the reef corals (p. 193), makes no use of it. If kept 

 in the dark it is apt to devour the guest. A photosynthetic organism 

 is specific to a particular host species. In some cases the two partners 

 are capable of living apart; in others, they are mutually dependent. 





a. 



epd. 

 ■pyr. 



Fig. 35- Fig. 36. 



Fig. 35. A section through a portion of the superficial tissues of Convoluta 

 roscoffensis, showing symbionts belonging to a species of Carteria (Chlamy- 

 domonadidae, Volvocina). From Keeble. ci. cilia of epidermis; epd. epi- 

 dermis; gr.c. "green cells" (symbionts); nu. nucleus of symbiont; pyr. 

 pyrenoid. 



Fig. 36. A free individual of the species of Carteria which is symbiotic in 

 the resting stage with Convoluta roscoffensis. From Keeble. chl. chloroplast ; 

 e. eye-spot; nu. nucleus ; ^jyr. pyrenoid. 



The plant organism usually enters the host by being ingested but not 

 digested. It may be passed on from one generation to the next in 

 asexual reproduction or even, as with the green Hydra, in the ovum, 

 but is often lost in the gametes of its host, so that the zygote must be 

 reinfected. Protozoan hosts in symbiosis are usually members of the 

 Radiolaria (Figs. 32 A, 37, 69 A) or Foraminifera, but various ciliates, 

 Noctiluca, etc., also harbour holophytic symbionts. Zooxanthellae are 

 commonest in marine hosts, zoochlorellae in fresh water. 



The amoeboid faculty possessed by some members of the group 

 may be limited to ingestion, but is often exhibited also in locomotion. 

 Certain forms with such locomotion lose their flagella for shorter or 

 longer periods : some may have done so altogether. When species with 



