284 ZOOLOGY sect. 



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 larvae of larger forms, Insect larva?, Water-mites. 

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

 minute Algae 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 in- 

 active mode of life a tendency to degradation of structure, a degrada- 

 tion which is characteristic of parasites in general : the pharnyx 

 is 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 ex- 

 ternal 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 

 strict sense of the term, living on the surface of the " host " 

 animal, depositing their eggs there, and being carried about by it, 

 but subsisting on minute living animals captured in the water. 



The Digenetic Trematodes are all internal parasites, and in the 

 adult condition inhabit, in nearly all cases, the alimentary canal, 

 liver, or lungs of some vertebrate animal, swallowing the 

 digested food or various secretions of their host. But, as mentioned 

 before in the account given of their development, they are internal 

 parasites, not only in the adult condition, but throughout the 

 greater part of their life. After a short period of freedom as 



