SYMBIOSIS 215 



products of its symbionts. In C. paradoxa the algae are 

 yellow {Zooxanthellce), but their function is the same ; this 

 species does seek food on its own account, but it cannot 

 live without the help of the algae. Zoochlorellae occur in 

 many Rhizopods and Ciliates, in fresh-water Sponges, in 

 Chlorohydra, and in some Rotifers ; Zooxanthellae occur 

 in many Rhizopods and most Radiolarians, and in very 

 many Ccelenterates, especially Anthozoa. In many cases 

 the host can probably survive without the symbionts, 

 but they undoubtedly help it, especially in times of starva- 

 tion. Sometimes, in unfavourable circumstances, the host 

 will kill the goose that lays the golden eggs by digesting 

 the algae. In return for their services to the host, the 

 algae make use of the carbon dioxide and nitrogenous waste 

 products of the host's metabolism. 



Class Trematoda. Flukes, etc. 



The Trematodes are leaf-like^ or sometimes cylindrical 

 external or internal parasites. With their parasitic life may 

 be associated the absence of cilia on the surface of the adults, 

 the thick " cuticle'' the presence of attaching suckers {occasion- 

 ally with hooks), and the rarity of sense organs. After 

 embryonic life the ectoderm degenerates, ceases to be distinctly 

 cellular, and sinks inwards. It is likely that they have 

 arisen from free Turbellarian-like ancestors, and they resemble 

 the Turbellarians in being unsegmented, in having anterior 

 ganglia, from which nerves pass backward and forward, in 

 the rudimentary nature of the body cavity, in the ramifying 

 system of fine excretory canals, in the hermaphrodite and 

 usually complex reproductive system. The excretory and 

 nervous systems are, however, more complex than those 0^ 

 Turbellaria. The alimentary canal is usually forked, 

 often much branched, and always ends blindly. In many 

 cases the animals are self -impregnating, but cross- 

 fertilisation also occurs. The development of the external 

 parasites is usually direct, of the internal parasites usually 

 indirect, involving alternation of generations. They occur 

 on or in all sorts of Vertebrates, but those which have an 

 indirect development, and require two hosts to complete their 

 life-cycle, often pass part of their life in some Invertebrate. 



