26 Comparative Animal Physiology 



Limulus, an arachnoid, shows some regulation of osmotic concentration. It 

 survives for a week or two in dilutions as low as one-fourth sea water but dies 

 in one-eighth sea water. One Indian species occurs in brackish water. ^^'^ 

 When Linndus was transferred from a medium of Ao=l-82 to one of 

 Ao=l-02, the blood concentration reached Ai=l-12 after 52 hours.^^ The 

 marine isopod Mesidotea also appears to maintain a hypertonic blood in dilute 

 sea water but to adjust to the external medium in concentrations greater than 

 its normal habitat. ^^ » 



Osmoregulation in Dilute Media : Shore Crabs. The shore crab, Carcimis, 

 is capable of considerable osmotic regulation and hence is found in more dilute 

 waters than any of the previously considered crabs. It can survive indefinitely 

 in dilute sea water of Ao=^0-60.'"'- C. maenas is found from the high-water 

 mark and tidal pools out to depths of a few fathoms, although it occurs in the 

 Elbe river at a freezing point of -0.682°.-^- Figure 12 shows that in ordinary 

 and in concentrated sea water Carcimis changes in internal concentration and 

 may be slightly hypertonic, but that in dilute sea water it maintains itself with 

 some constancy in a more concentrated condition. ^~ In brackish water 

 Carcinus remains hypertonic by as much as two times the external concentra- 

 tion. 



When Carcinus is placed in a dilute medium it swells hardly at all (Fig. 

 9). '■^" Yet approximately 12 hours is required for the blood to come to a 

 constant hypertonic concentration (Fig. 10, curve A). ^^^ This must mean 

 that the permeability to water is very low. Chloride is gradually lost from 

 Carcinus in a dilute medium. ^^^ In 50 per cent sea water the crab's weight 

 increased less than 0.5 per cent in 2^2 hours. ^^^ When, however, the open- 

 ings of the antennal glands (kidneys) were closed, the crabs in 50 per cent sea 

 water gained 2.2 per cent in weight in 1 hour and 5.5 per cent in weight in 3 

 hours. Apparently the gills are not impermeable to water, but the kidneys 

 excrete water as fast as it comes in. Nagel '^^'^ measured the urine output by 

 plugging the kidneys for 10-hour periods and then measuring the loss after 

 removing the plugs. He found urine output as follows: 



Urine outiput 



(ml./24 hrs./ Salinity of medium 

 50 gm. body wt.) (per cent) 



5.1 3.3 



6.3 1.62 



8.5 1.57 



Urine excretion increases with dilution of the medium, but not enough to 

 account for the increased water intake. There was greater weight increase in 

 66 per cent sea water with both antennal glands and mouth plugged (3.4 

 per cent in 4 hours) than with only the antennal glands closed (2.7 per cent 

 in 4 hours), hence water is eliminated by the mouth. •^'* Carcinus gives off 

 water by kidneys and gastrointestinal tract as fast as it takes it up in a hypo- 

 tonic medium. 



Carcinus has low permeability to water and also to salts, both incoming and 

 outgoing; it takes up less iodine from the medium than does Portiinas, Hyas, 

 or Cancer (Fig. 15). ^**^ Also, when iodine was injected into Hyas, after VA 

 hours 50 to 94 per cent of the amount injected had been lost to the outside 

 medium, while Carcinus lost only 5 to 16 per cent of the injected iodine in 



