Inorganic Ions qy 



SPECIAL FUNCTIONS OF CALCIUM 



The principal elements of living organisms are widely distributed in all 

 organs; certain elements may be concentrated in particular organs, as iodine in 

 the thyroid. Calcium becomes predominantly localized in skeletal structures 

 and in other organs which are of interest in comparative physiology. An 

 excellent discussion of the function and metabolism of calcium in inverte- 

 brates is given by Robertson. ^^-^ In many arthropods calcium as carbonate 

 and phosphate is deposited in and over the chitin, and no increase in size can 

 occur except at the time of molt. In crabs (e.g. Carcinus} much Ca3(P04)2 

 is stored in the hepatopancreas before a molt, while in crayfish calcium is stored 

 in gastrohths. Calcium may constitute 16 per cent of the skeleton of a normal 

 crab and only 1 per cent of the skeleton of a freshly molted one. ■^' The gastro- 

 liths grow rapidly before a molt, and may consist of concentric layers deposited 

 nocturnally. ^^^ Secretion of the gastroliths is normally regulated by an 

 inhibitory hormone from the sinus gland^" (Ch. 22). In insects the chitin 

 is normally not calcified. 



In earthworms the esophageal epithelium of the calciferous glands secretes 

 spherules of CaCOa. Four functions of the calciferous glands have been sug- 

 gested: (1) neutralizing the intestinal contents (this neutralization was shown 

 by Robertson ^^^ to be by other digestive secretions); (2) fixing metabolic 

 CO2; ^^ (3) providing buffer capacity for the blood when in an acid condi- 

 tion;'*''' ^"^ and, principally, (4) excreting excess calcium.'"'- ^-^ 



In molluscs the calcareous shell is an important source of base in the neutral- 

 ization of acids produced in anaerobiosis. In Venus kept out of water the total 

 CO2 binding capacity of mantle fluid increases and the calcium concentration 

 also increases. ^^ The shell is eroded by the mantle. Some nonvolatile acid 

 (possibly lactic acid HL) increases in the mantle fluid, and this plus the in- 

 creased CO2 equals the increase in calcium in the mantle fluid, thus forming 

 Ca(HC03)2 and CaLs: 2 CaCOa + 2 HL->Ca(HCO,)2 + CaLo. In the 

 fresh-water mussel Anodonta cygnea kept in water under oil, a similar anaero- 

 bic metabolism produces acid, and the pH of the blood remains about 7.B. 

 Here the carbonate increases more than does the calcium; hence there must 

 be some additional source of cations. '**'" 



CONCLUSIONS 



The maintenance of suitable ionic balance is an important step in freeing 

 an animal from strict dependence on its environment. In only a few marine 

 groups— particularly in the echinoderms, and to a lesser extent in the marine 

 coelenterates— are the various elements of body fluids in nearly the same 

 proportions as in the ocean. Ionic independence is necessary for life on land 

 or in fresh water. Nothing is known of salt balance in the body fluids of 

 parasites. In general, the farther removed a phylum or class is from the ocean, 

 the more divergent are the salt ratios from those of sea water. The bloods of 

 vertebrates and of insects are least like the ocean, and each is unlike the other 

 (Table 9). Toleration of changes in environmental salts has not been much 

 investigated, but some animals such as echinoderms readily alter their body 



