WATER, SALTS AND MINERALS 73 



are sometimes present in small amounts. Iodine is a normal constituent of 

 thyroxine in vertebrates, and in Amphioxus it is localized in the endostyle 

 and in mucous secretions of that gland. Further evidence is thus provided 

 of the homology of the protochordate endostyle and the vertebrate thyroid 

 gland. Both iodine and bromine occur in gorgonians and sponges, in 

 haloaromatic amino-acids which are normal constituents of skeletal 

 scleroprotein. Iodine is accumulated to some extent by lamellibranchs and 

 other marine animals. Organic secretions of many species besides corals 

 show high levels of iodine, possibly in combination with organic sub- 

 stances, namely tubes of Diopatra, Chaetopterus, Bispira (polychaetes), 

 byssus of Mytilus, test of Pyura (tunicate), etc. Fluorine occurs in traces in 

 the shells of some lamellibranchs, in remarkably large quantities in mantle 

 and other tissues of the nudibranch Archidoris (2% of cations), and in the 

 body wall of the brittle star Ophiocomina (2b, 15, 40, 53, 81, 115, 116, 

 136, 136a, 141). 



Some of the heavy metals are accumulated in appreciable amounts by 

 various marine invertebrates. Certain metals, particularly iron, are essential 

 elements in the prosthetic groups of many enzymes. Iron is found in 

 cytochrome, a widely distributed intracellular haemochromogen, and in 

 certain respiratory pigments (haemoglobin, chlorocruorin, haemerythrin, 

 see Chapter 4). The radular teeth of chitons (Chitonidae) and limpets 

 (Patellidae) contain large amounts of iron as Fe 2 3 (54% of ash in Patella 

 vulgata). The iron content of sea water is low (0-002-0-02 mg kg), and 

 Patella relies mainly on algal food for supplies of iron. The iron content 

 of certain gastropods, Lineus (nemertine) and Nephthys (polychaete) is 

 high (56). 



Copper is probably universally distributed among animals. It occurs in 

 the respiratory pigment of crustaceans, xiphosurans and molluscs (haemo- 

 cyanin, Chapter 4), and in some respiratory enzymes. Marine invertebrates 

 lacking haemocyanin generally have copper concentrations of 0-2-8 mg% 

 dry weight, but oysters show unusually large concentrations of copper, up 

 to 300 mg%. Manganese is another element which appears to be a normal 

 constituent of all animals, and occurs in traces. Amounts in Pecten and 

 Ostrea vary from 1-18 mg% dry weight. Gills and ripe ovaries contain the 

 largest amounts and there is an increase of manganese during the repro- 

 ductive period (36, 88). 



Certain other trace elements have an interesting biological distribution. 

 Vanadium is accumulated by some ascidians, Stichopus mobii (holothurian) 

 and Pleurobranchus (nudibranch). In ascidians, at least, the vanadium is 

 organically bound and is concentrated in blood corpuscles (Chapter 4). 

 Other trace elements reported for various marine animals are aluminium, 

 zinc, nickel, cobalt and titanium. Minor elements may be taken up directly 

 from solution. However, certain ions are adsorbed by hydrated oxides 

 of iron and manganese, and if the latter are collected by filter-feeders the 

 adsorbed elements become available to the animals (8, 38, 39, 74). 



Calcium is extensively utilized in skeletons of animals, and in calcareous 



M.A. — 3* 



