650 ^ ^- Cary. 



Looss lias pointed out that the three forms, sporocyst, redia 

 and cercaria are built upon the same fundamental plan and that 

 the essential differences between them is in the increasing- complexity 

 which is shown in the structure of the three members of the series 

 in the order named. This is most clearly shown in the development 

 of the dig-estive sj^stem, the water vascular system, and in the ner- 

 vous system; while the structure of the reproductive system sets 

 the cercaria sharply over against the parthenogenetic forms. 



To draw a comparison from the development of the cercaria, it 

 is as though the development of the body of the sporocyst had 

 stopped when the primordium of the gut had first appeared and the 

 water vascular system had developed to a stage when only the 

 main trunks with a single flame cell attached to each had been 

 differentiated ; while the redia represents a stage in which the deve- 

 lopment has gone on until the gut consists of the unpaired primordium, 

 the anterior sucker and the pharynx and oesophagus had been deve- 

 loped, and the water vascular system had become branched, while 

 the body has become elongate in outline. 



Except for the presence of mature sexual elements, the two 

 parthenogenetic generations represent larval forms in all their 

 structure. 



The power of reproduction which has been developed in these 

 larval forms may be interpreted as a special provision for an in- 

 crease of individuals which is necessitated in order to compensate 

 for the fact that there is only a very slight probability that any 

 cercaria will meet with the conditions necessary for its further 

 development. If only a single individual came from each fertilized 

 egg, there would be small chance that, with its complicated life 

 history with the consequent number of critical stages, enough embryos 

 would be able to transform to the adult condition to maintain the 

 life of the species. 



Summary. 



1. BiplocUscm temporafus Stafford differs in many characters, 

 particularly in the nurse generations and the cercaria, from the 

 European species AmpJnstomwn snhclavatmn with which it has been 

 generally confounded. 



2. For the formation of a complete cyst it is essential that the 

 cercaria become attached to some substratum by means of the ven- 



