v PHYLUM PLATYHELMINTHES i>7'. 



6. DISTRIBUTION, MODE OF OCCURRENCE, AND MUTUAL 



RELATIONSHIPS. 



Of all the great groups of the animal kingdom above the 

 Protozoa the Platyhelminthes are the widest in their distribution. 

 Members of the phylum occur on land, in fresh water down to the 

 bottom of some of the deepest lakes, on the sea-shore, in the deep 

 sea, and on the surface of the ocean ; and parasitic Flat- worms live, 

 in one phase or another, in animals of nearly every class of the 

 Metazoa. 



As regards their mode of life, they present almost every possible 

 gradation between free-living forms which procure their food con- 

 sisting of minute animals and plants by their own exertions, and 

 forms that are only capable of living in a special part of the 

 interior of a certain other animal, and are quite incapable of pro- 

 curing food for themselves, living by the passive absorption of the 

 juices of their host or of its digested food. The Turbellaria are 

 for the most part free-living, and their food consists of small 

 Crustacea or the larvse of larger forms, Insect larvae, Water-mites, 

 Rotifers, small Worms, and the like ; or sometimes of Diatoms and 

 minute Algse of various kinds. Some, however, live a life of true 

 parasitism. Such are certain Rhabdocceles which are parasitic in 

 the alimentary canal of various Holothurians and Gephyreans (vide 

 Sections IX and X). In these there is correlated with the inactive 

 mode of life a tendency to degradation of structure, a degradatior 

 which is characteristic of parasites in general : the pharynx ib 

 reduced in size as compared with that of non-parasitic allied 

 forms, not being required for the capture and swallowing of living 

 prey ; and the eyes, useless to an animal living in complete dark- 

 ness, are absent. Some of the Turbellaria, though not parasitic 

 in the strict sense, live in a state of commensalism with another, 

 larger, animal : that is to say, are more or less constantly associated 

 with it, living on its surface or in one of its cavities that open 

 freely on the exterior, and often sharing its food. An example of 

 this mode of life is the Triclad Bdelloura, which lives on the surface 

 of the King-Crab (Limulus). 



While a free existence is the rule in the Turbellaria, true 

 parasitism is the rule in the Trematodes, and is universal in the 

 Cestodes. The majority of the Monogenetic Trematodes are 

 external parasites, living on a part of the outer surface of a 

 larger animal, and feeding on mucus and other secretions of the 

 integument. Many are parasites on the gills of Fishes. A few, 

 however, inhabit the interior of various organs, and are true 

 internal parasites : one, for example (Polystomnm), lives in the 

 urinary bladder of the Frog ; another (Aspidogaster) lives in the 

 pericardial cavity of a Fresh-water Mussel. At least one family of 

 Trematodes (the Temnocephalea) are not parasites at all in the 



