232 INVERTEBRATE PHYSIOLOGY 



produced. Analyses of the fluid from the antennal glands and molluscan 

 excretory tubules show that differential excretion of ions is taking place 

 (Table 2), tending to eliminate magnesium in the crustaceans and sul- 

 phate in all the animals, while conserving the other ions. In general, these 

 differences in the excretory fluids from pure ultrafiltrates of the blood are 

 tending to alter the levels of ions in the plasma, in relation to sea water, 

 towards those actually found. 



TABLE 2. ANTENNAL GLAND SECRETION AND RENAL SAC 



FLUID COMPARED WITH BLOOD PLASMA 



Concentrations as percentages of plasma 

 values (on water-content basis) 



Na K Ca Mg CI SO^ 

 Crustacea 



(Antennal gland secretion) 



Maia squinado 100 98 99 109 101 214 



Cancer pagurus 97 81 90 125 96 134 



Carcinus maenas* 95 78 94 390 98 224 



Palinurns vulgaris 98 65 86 137 101 98 



Homarus vulgaris 99 91 64 180 101 159 



Nephrops norvegicus 98 83 81 130 101 106 



Palaemon serratiis^ 82 86 95 670 106 380 



Cephalopoda 

 (Renal sac fluid) 



Eledone cirrosa 102 90 87 89 97 136 



Sepia officinalis 79 50 70 68 100 215 



*Webb (1940). 



t Parry (1954), on ml. basis. Palaemon shows hypo-osmotic regulation, but "urine" 

 is isosmotic with blood. 



Remainder of analyses from Robertson (1939, 1949, 1953). 



A deficiency of cations in the renal fluid of Sepia is apparent from 

 Table 2, since they are only 50-79% of those in the plasma. The missing 

 cation is ammonium, NH4'', which may be excreted in amounts as much 

 as 146 m.equiv., forming 24% of the total cation equivalents. The produc- 

 tion of ammonium in such amounts, presumably shortly after feeding and 

 protein breakdown, presents a problem in osmotic regulation which is 

 solved in Sepia by reduction of the other cations, so that the "urine" re- 

 mains isosmotic with the plasma. 



Loss of water and salts through excretory tubules must be made good by 

 absorption of a similar quantity of fluid through permeable portions of 

 the integument, usually the gills. Absorption of pure water would alter 

 the osmotic pressure of the blood, and ions must be absorbed with the 

 water in concentrations sufficient to make it isosmotic with the fluid it is 

 replacing. Consequently Robertson (1939) and Webb (1940) deduced 

 that sodium, potassium, calcium, and chloride must be taken up against 

 concentration gradients in many invertebrates, although magnesium and 



