14 Comparative Anhnal Physiology 



then falls as salt is lost and, with it, water. As much as 37 per cent of the 

 body salt may be lost. After equilibrium is approached at a new low level, 

 often below the original weight, return to 100 per cent sea water causes loss 

 in weight because the normal sea water is now hypertonic. By ligating the 

 mouth it is shown that the body surface of Aplysia is permeable to both water 

 and salts. 



The echinoderms are an entirely marine group and are adapted osmotically 

 to the ocean. Indeed, in all except holothurians the ambulacral fluid is very 

 similar to sea water. The body fluid of Asterias freezes at>the same temperature 

 as the sea water and becomes dilute when the starfish is put into dilute sea 

 water. ^^ In 50 per cent sea water ^^° starfish increased in weight by about 

 25 per cent in 8 hours, but returned nearly to the original weight in 24 hours. 

 Starfish from the North Sea were unable to survive in 50 per cent sea water, 

 but animals from the brackish Kiel Canal survived in greater dilutions. -^^ 

 Echinoids have left the Baltic because of its low salinity. ^' Echinoids and 

 asteroids, because of their exoskeleton, are unable to swell as much as the 

 soft-walled animals such as Aplysia, hence volume changes are limited and the 

 animals must lose some salt to a hypotonic medium. ^~' 



Osmotic Adjustments by Cnidaria and by Ascidians. The osmotic concen- 

 tration of jellyfish has been measured by both freezing point and by vapor 

 pressure methods; jellyfish are composed of only 3 to 5 per cent solids ^^'^ and 

 have the same or very nearly the same osmotic concentration as the ocean. ^'•*' 

 46, 47, 73, 74 ^ftgj- 35 fiours in 75 per cent sea water the jelly disc of Cyanea 

 diff^ered in concentration from the medium by a negligible amount. ^^ Some 

 corals and ctenophores tolerate a 20 per cent reduction in salinity.^ ^^' ^^^ 



The blood of an ascidian (Ascidia mentula) is approximately isotonic with 

 the ocean, ^^ although the body fluid of Molgtila is slightly hypotonic and 

 its renal fluid hypertonic to sea water. ^^ 



In summary, some marine invertebrate eggs and some marine and parasitic 

 Protozoa and worms such as Phascolosonia are true osmometers when allow- 

 ance is made for osmotically inactive volume. They swell and shrink and show 

 no salt exchange over periods of many hours; they show neither volume nor 

 osmotic constancy. Numerous others, e.g., Nereis pelagica and N. czdtrifera, 

 Doris, Onchidium, the holothurian Candina, and Mytilus, likewise remain 

 swollen in dilute sea water. The amount of swelling is greater in some (Doris) 

 than in others (Mytilus); when the swelling is considerably less than the 

 amount predicted osmotically, salt may have been lost. Finally, many animals 

 such as Aplysia, and to a less extent Asterias, swell initially in a dilute medium 

 and then shrink as salt is lost; they regulate their volume while changing their 

 concentration. The regulation of volume and that of osmotic concentration 

 may, therefore, be separate functions. 



OSMOTIC REGULATION TO THE MEDIUM; 

 HOMOIOSMOTIC ANIMALS 



Simple Types of Osmotic Regulation: Nereis diversicolor. Osmoregulation 

 at a low level of cfl'ectivcncss is shown by Nereis diversicolor, as contrasted 

 to simple adjustment in dilute media shown by N. pelagica and N. cultrifera. 

 Due to its regulatory ability N. diversicolor is able to live in brackish water 

 (0.4 per cent salinity), whereas N. pelagica is restricted to the open sea. ^^^ 



