264 ZOOLOGY SECT. 



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

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

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

 parasitism. Such are certain Rhabdocoeles which are parasitic in 

 the alimentary canal of various Holothurians and Gephyreans 

 (vide infra, Sect. IX.). In these there is, correlated with the inactive 

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

 which is characteristic of parasites in general ; the pharynx 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 some part of the outer surface of some 

 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 (Polystomum), 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 

 the Monogenetica (the Temnocephalece) 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, or 

 the liver, or the 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 

 ciliated larva?, they again enter into a state of parasitism as sporo- 

 cysts or redia? in a second host, and, after a second free interval as 

 cercaria?, may enter the body of a third to become encysted. The 

 second host is, very generally, a Mollusc, and the cercaria may 

 become encysted in the same animal. 



The Cestodes are, of all the Platyhelminthes, those that are most 

 modified in accordance with the condition of internal parasitism 

 in which they remain throughout life. The adult Cestode is 



