iv] MODES OF NUTRITION 103 



pursuing this mode of life: (1) animals which live 

 attached to the outer surfaces of their host the skin, 

 the gills, the branchial cavities, and the cavities of 

 the nose or mouth. Examples of this class of parasites 

 are the Copepods (fish-lice), which are attached to 

 the external surfaces of many species of fishes ; some 

 leeches ; and some Trematodes (flukes). (2) Internal 

 parasites which live in the alimentary canals of their 

 hosts, either holding on by means of suckers or hooks, 

 or lying quite free in these cavities (tapeworms, 

 internal trematodes, and thread-worms); or which 

 live in the blood-stream or lymph channels, or in the 

 peritoneal or serous cavities; or embedded in the 

 substance of the muscles or other tissues (trematodes, 

 thread-worms, larvae of tapeworms, trypanosomes, 

 etc.). The external parasites usually pass through a 

 life-history which includes a free-swimming stage in 

 which the organism lives among the plankton as an 

 independent animal, and then at a certain phase, 

 coming in contact accidentally with its host, it attaches 

 itself thereto and undergoes metamorphosis to its 

 sexually mature form. Some of the internal parasites 

 also pass through a free-swimming stage in the course 

 of which they are taken into the alimentary canal, 

 or into some other part of the body of an animal 

 which acts as their larval host. They live in the 

 tissues of this host and do not undergo development 

 beyond a certain stage, and finally degenerate unless 



