PHYTOMASTIGINA 43 



yellow or brown {Zooxanthellae) ; most green symbionts (Zoochlorellae) 

 are algae belonging to the Protococcaceae. An exception to this is the 

 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 a symbiosis benefits by a supply of 

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

 waste matters removed, is suppHed with oxygen, and sometimes 

 draws on the supply of carbohydrates manufactured by the guest, 

 though it rarely relies upon this alone. 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. 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, 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 

 c\\i2ites,Noctiluca, etc., also harbour holophytic symbionts. Zooxan- 

 thellae 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 amoeboid movement become 

 colourless they are only to be separated 

 from the Sarcodina by certain features (of ^^^ 

 their nuclei, cysts, swarm spores, etc.) 

 which prove them to be related to various 

 mastigophora. 



Of the orders of the Phytomastigina, that 

 which contains the most highly organized 

 members is the large and protean group ^ig. 37. Lithocircus annu- 

 Dinoflagellatay characterized by the posses- laris. After Lankester. cps. 

 sion of two flagella, one longitudinally central capsule ;«m. nucleus; 

 directed and the other transverse, usually ^^jj-^poreplate; 3;.^. yellow 

 in a groove around the body but in a few 



cases twisted about the base of the longitudinal flagellum. Three of 

 the remaining orders differ from the rest in the possession, in the 

 anterior part of the body, of a pit ("gullet") or groove, from which 

 the flagella usually arise. One of these, the Cryptomonadina, has 

 simple contractile vacuoles and its carbohydrate reserves are of starch : 

 it is held by some authorities to be related to the ancestors of the 

 dinoflagellates. The second, the Euglenoidma, has a more complex 



