23 Comparative Animal Physiology 



from mammals and from fresh-water and marine fishes are also isotonic with 

 or slightly hypotonic to host tissues. However, the concentration of the trema- 

 tode Fasciola hepatica was 40 per cent greater, as judged by freezing point, 

 than that of the bile of the host. -^^ Ascaris in hypotonic solutions decreased 

 rapidly in internal concentration; the worms swelled in dilute and shrank in 

 hypertonic solutions. Volume changes were initially rapid, but the curves 

 leveled at 6 to 10 hours and there was no tendency to return to original vol- 

 umes in 18 hours. The body wall is, therefore, permeable to water. Maximum 

 volume changes were about 50 per cent; the osmotically inactive volume of the 

 worm is probably large, but certainly the volume changes are less than if there 

 were no regulation. Failure of recovery in hypotonic media indicates that 

 there is Httle or no salt loss. It is likely that in a dilute medium the excretory 

 organs are getting rid of water. The acanthocephalan, N eoechinorhynchus 

 imydis, from the intestine of turtles is normally flat and wrinkled but it be- 



Nephrostome 



Nephridial canal 



Nephridiopore 



Fig. 8. Drawings of nephridia of Nereis dh'ersicolor (A), and of N. cultrifera (B). 

 Modified from Jiirgens.'"^ 



comes swollen and turgid in saHnities of less than 0.85 per cent sodium chlo- 

 ride. 235 



In Cysticercus tenuicollis, a cestode larva from the abdominal cavity of 

 sheep, volume changes are small (20 per cent swelling in a medium of one- 

 fourth the normal concentration), but as in Ascaris there is no tendency to 

 return to the original weight during approximately a day in dilute or hyper- 

 tonic media. --^ Freezing point measurements show, however, that Cysti- 

 cercus has considerable osmoregulating ability, maintaining itself hypertonic 

 in dilute media and hypotonic in concentrated media. Schistocephalns, a fish 

 tapeworm, has an osmotic concentration similar to that of the host (Af.i).= 

 0,44); the worms maintain constant weight in 0.75 per cent sodium chloride 

 solution but gain in more dilute and lose in more concentrated solutions. --** 

 More accurate data are needed regarding the salt and water exchange and the 

 osmotic concentration of various parasitic worms with and without excretory 

 organs. It is probable that many degrees of regulation will be discovered 

 among them. 



Osmotic Regulation by Storage of Water: Giinda iilvae. The flatworm 

 Gunda idvae lives in estuaries at the intertidal zone in England and may be 



