288 THE BIOLOGY OF MARINE ANIMALS 



haemoglobin, and contribute to the external coloration of the animal. A 

 protoporphyrin is found in the integument of starfish (Asterias), and is 

 regarded as a by-product of chlorophyll excretion. Porphyrins (uropor- 

 phryin and coproporphyrin) are deposited in the shells of testaceous 

 molluscs; in species without shells or with uncalcified shells {Duvaucelia 

 and Aplysia), uroporphyrin occurs in the integument (24, 25). 



MODES OF NITROGEN EXCRETION 



Ammonotelism 



Aquatic animals are usually ammonotelic, and produce ammonia as the 

 major end-product of protein catabolism. Small amounts of ammonia 

 may also arise from the breakdown of purines and pyrimidines. No species 

 is restricted entirely to one excretory product, however, and as Table 7.2 

 shows, ammonotelic animals excrete other nitrogenous compounds, 

 sometimes in traces, often in relatively large amounts. Actinians, for 

 example, are predominantly ammonotelic but they also excrete a little 

 urea, and small amounts of uric acid and tetramine (tetramethyl ammo- 

 nium chloride) are found in the tissues of some species, the latter substance 

 probably being formed by the methylation of ammonia (52a). The principal 

 nitrogenous excretory product of polychaetes is ammonia, but Aphrodite 

 gives off small amounts of urea and uric acid, and small quantities of urea 

 and uric acid have been reported in the coleomic fluid of Arenicola, and 

 uric acid in Chaetopterus (Tables 7.1 and 7.2). 



Other predominantly ammonotelic groups are sipunculoids, amphipods, 

 isopods, decapod crustaceans, echinoderms, lamellibranchs, cephalopods, 

 many marine gastropods and ascidians. Nearly all species examined excrete 

 small quantities of urea, and frequently uric acid as well. Concentrations of 

 urea and uric acid in the blood and body fluids of various species are given 

 in Table 7. 1 . The small amounts of urea and uric acid produced by am- 

 monotelic animals are derived largely from degradation of nucleic acids 

 (8, 10, 15, 15a, 54). 



Some species, although predominantly ammonotelic, excrete consider- 

 able quantities of amino-acids. Thus, echinoderms (starfish, urchins and 

 cucumbers) lose amino-acids in quantities that may equal the ammonia 

 excreted. Crustacea excrete surprisingly large amounts of amino-acids, 

 up to 10 % of total non-protein nitrogen in some species. Part of the amino- 

 acid nitrogen is discharged through the excretory organs proper, but there 

 is a certain amount of leakage across the body surface as well. Excretion 

 of amino-acids represents loss of potentially useful metabolites, and points 

 to inefficiency of conservation mechanisms (2). 



Uricotelism in Vertebrates 



Uric acid is the main nitrogenous end-product of protein metabolism 

 in two groups of higher vertebrates, namely Squamata (lizards and snakes) 

 and birds. This mode of excretion is a specialization adapting these 



