PLATYHELMINTHES 173 



a hyperdevelopment adaptive to the hazards encountered in the worm's 

 life history. 



The classification of the tapeworms has been somewhat more artificial 

 than systematic in that it has not sufficiently taken into account mode 

 of development, a factor which should furnish the basis for their true 

 natural affinities. Their larvse may, with reference to method of develop- 

 ment, be placed in the five following forms: 1. Cysticercus (Fig. 107); 

 2. cxnurus (Fig. 114); 3. echinococcus (Fig. 117); 4. cysticercoid (Fig. 

 96); 5. plerocercoid (Fig. 112,-e). The first three are found in organs 

 or serous cavities of Herbivora and Omnivora, occasionally in Carnivora; 

 the fourth fives mostly in invertebrates, and the fifth in the musculature 

 of fishes. The more recent tendency in cestode nomenclature is to con- 

 fine the generic name Taenia to those tapeworms which have a cysticercus 

 stage in their life history. 



The cystic forms enumerated above, with the conditions which cer- 

 tain of them produce in their hosts, are taken up further on in the con- 

 sideration of the cestode larvse. 



The accompanjdng tabular arrangement of the principal tapeworms 

 considered in this work, with their adult and cystic hosts, is inserted 

 for convenient reference. 



