OSMOTIC AND IONIC REGULATION 231 



to suggest that these three genera are living in a state of partial magnesium 

 anaesthesia, with depressed neuromuscular transmission. One might ob- 

 ject that other ions and hormones influence activity, and point to the gen- 

 eral anatomy of a spider crab as being unsuitable for swift movement. 

 Yet there seems to be some support for a causal relationship in Crusta- 

 cea between reduction of magnesium and increased activity, at least ac- 

 cording to studies on the neuromuscular system of the legs (Katz, 1936; 

 Waterman, 1941 ; Boardman and Collier, 1946). 



Sodium and chloride are the least variable ions in dififerent specimens 

 of a species, as one might expect, since they form such a large proportion 

 of the total ionic concentration, which is kept at the same level as that of 

 sea w'ater, within 1-2%. A reduction of magnesium as in Nephrops and 

 PJiascolosoma is accompanied by a rise in sodium to maintain osmotic 

 and cation balance. 



Sulphate in body fluids is usually less than the equilibrium value with 

 sea water, and falls below ZOfo in Loligo and Sepia. Reduction in this 

 anion is compensated by increase in chloride to maintain cation-anion bal- 

 ance and osmotic equilibrium. 



In body fluids potassium tends to be the most variable ion in specimens 

 of a species, but the mean values are fairly characteristic of species and 

 groups. In all so far examined except the Homaridae {Homarns and 

 Nephrops), potassium is higher. than the equilibrium value with sea 

 water, and highest of all in cephalopods, the mean values in the octopus 

 Eledone, the cuttlefish Sepia, and the squid Loligo being 152%, 205%, 

 and 219%, respectively. The meaning of the lower potassium in lobsters 

 is not clear, but it may have some physiological significance in relation to 

 the balance between the various ions, since these animals have very low 

 magnesium concentrations. In cephalopods, calcium and magnesium ions 

 remain near equilibrium levels with sea water, and perhaps the high values 

 of potassium contribute to the activity and pow-ers of active movement of 

 members of this class, since moderate increases in potassium ions are 

 known to have a stimulatory action on the neuromuscular system (e.g., 

 Wells, 1928; Ross and Pantin, 1940). 



The mechanism of ionic regulation in many groups is unknown, al- 

 though one may attribute it to the activity of the cells in contact with the 

 external medium in such groups as coelenterates and tunicates. In the 

 Arthropoda and MoUusca rather more is known, at least in the decapod 

 crustacean and cephalopod groups. 



In these animals an output of fluid through the excretory organs is 

 balanced by the uptake of a similar quantity of fluid elsewhere, so that the 

 weight remains constant. If the excretory apertures are closed, the w^eight 

 of the animal increases and estimates can be made of the amount of fluid 



