96 Comparative Animal Physiology 



aids the hardening of dental enamel, but in excess it causes abnormal bone 

 structure. Fluorine is found in some molluscan shells, as in the oyster and in 

 the mantle of the mollusc Archidoris. ^'^"^ 



Calcium as carbonate or phosphate is a common structural material of endo- 

 skeletons and exoskeletons. Because of the low solubility of CaCOa in alkaHne 

 water, much calcium is being continually stored up in masses of coral and of 

 molluscan shell. Magnesium is important in some skeletons— foraminiferans, 

 alcyonarians, echinoderms, and crustaceans. More magnesium is found in 

 skeletons from warm areas, as in crinoids. ^" Magnesium is an essential com- 

 ponent of chlorophyll. Strontium can be substituted experimentally for cal- 

 cium in bone; it has been reported as essential in a radiolarian skeleton. ^^ 

 Barium is sometimes accumulated and can be deposited in calcareous struc- 

 tures. SiHcon is the important material in the skeleton of most radiolarians 

 and diatoms, and in the spicules of siliceous sponges; it is a constituent of 

 radular teeth of certain molluscs. 



Heavy metals are essential in the prosthetic groups of numerous enzymes. 

 Iron is an important metal biologically in that it is an essential constituent of 

 heme. The respiratory function of heme compounds is discussed in Chapter 9. 

 The most widely distributed hemochromogen is cytochrome, a link in carbo- 

 hydrate oxidation. Iron seems essential for the growth of the autotrophic 

 flagellate Chilomonas -"- ^'^ only if thiamine is present. The radular teeth 

 of the marine mollusc. Patella, contain iron as Fe203 to the extent of 50 per 

 cent of the ash weight. Copper also is active in an oxygen-transporting pigment 

 (hemocyanin, Ch. 9) and is present in some respiratory enzymes; it is essential 

 for hemoglobin synthesis in vertebrates. ^'^ Copper is found in the red pig- 

 ment of feathers of South African turacos; traces of copper occur in oysters. ^^ 

 Manganese occurs in traces in many animals, particularly in molluscs. The 

 Mn content of oysters is high in gills and ovaries and is highest at the period 

 of sexual activity. ^^ Manganese is accumulated by some hymenopterans and 

 may be stored, as are some other metals, in mid-gut cells. -^" This element 

 is essential for the normal development of mammalian embryos, and lack of 

 manganese causes a bone disease of fowls. ^-*^ Deficiency studies indicate 

 that in the utilization of iron for hemoglobin there is some interaction between 

 Mn, Co, and Cu. ^-° Cobalt is more important for ruminant than for non- 

 ruminant mammals, and is a constituent of vitamin B12 (Ch. 5). 



Zinc has rarely been reported in animal analyses, although it is concentrated 

 by some bivalves, especially Pecten, and it doubtless is essential in traces. ^- 

 Its only proved function is as an essential constituent of the enzyme carbonic 

 anhydrase (Ch. 9). Vanadium is selectively concentrated by several genera 

 of ascidians (Ch. 9), by Pleurohranchus, and by a holothurian Stichopns. 

 97, 127 Nickel occurs in feathers; it has been reported in mammalian liver 

 and in two marine molluscs. ^'"^ Aluminum is widely distributed and may be 

 an essential constituent of the succinic oxidase system. ^-^ The study of trace 

 elements is made difficult by the necessity to prepare diets which are totally 

 free of the elements in question; it is quite probable that many elements are 

 required in specific reactions in amounts corresponding to relatively few 

 atoms per cell. ^^ 



