428 FRESH-WATER BIOLOGY 



Certain important changes have occurred during this larval 

 growth period. These are most marked in the bladder-worm ces- 

 todes. The fully developed cysticercus shows a completely formed 

 scolex that corresponds in detail with the scolex of the adult ces- 

 tode save that it is reversed and lies turned into the internal cavity 

 of the bladder. When the bladder-worm reaches its final location 

 the head is everted and appears with the armament of suckers and 

 hooks that characterizes the adult. This scolex attaches itself in 

 the region appropriate for the adult and the bladder remnant is 

 lost by digestion while the neck continues to grow in length until 

 it has produced a full-sized adult worm. The formation of pro- 

 glottids and the growth within them of the reproductive organs 

 proceeds slowly as the worm lengthens, the oldest proglottids 

 being found regularly at the end furthest from the scolex. 



The life histories of North American cestodes are entirely un- 

 known and can only be inferred to be similar to related species 

 that have been studied in the Old World. The evidence fur- 

 nished by the latter indicates clearly that tapeworms are not 

 bound up with an aquatic existence in some stage as are the flukes. 

 Certain cestodes have aquatic larvae and others bladder-worm 

 stages in aquatic invertebrates, or vertebrates, but many of the 

 species parasitic in birds and mammals pass the larval period in 

 terrestrial hosts (insects, land snails, birds, mammals) and have 

 no relation to the aquatic fauna at any time. Such forms do not 

 belong rightly in such a synopsis as this; but the data available 

 are insufficient to mark out clearly which forms belong to the fresh- 

 water fauna during some phase of their existence and which are 

 entirely unconnected with it. 



Cestodes are found as parasites in all types of fresh-water verte- 

 brates. The adults occur most frequently in the alimentary canal 

 or pyloric ceca. Certain kinds, chiefly larval forms, are found in 

 the body cavity and the encysted stages, the bladder- worms, may 

 be encountered in almost any tissue, even in the brain; yet they 

 are most frequent in connective tissue and seem to find the liver a 

 preferential location. Usually only a few cestodes are found in 

 an individual host, but they may occur in such numbers that the 

 cavity of the alimentary canal is stuffed full and the wall of the body 



