SKELETONS, SHELTERS AND SPECIAL DEFENCES 635 



6-7 % (Table 15.1). The bones of marine fishes contain only about 60 % as 

 much CaC0 3 as found in higher vertebrates, apparently owing to lower 

 carbonate reserves in the body fluids of fishes. Much of the calcium phos- 

 phate is probably arranged in the form of the mineral apatite (10Ca:6PO 4 ) 

 (6, 59, 85). 



Phosphatases are involved in ossification and deposition of calcareous 

 matter. Alkaline phosphatase appears at the time of ossification, and 

 probably catalyses the local deposition of P0 4 ions in insoluble form. At 

 the same time Ca ++ is liberated from the blood and calcium phosphate is 

 laid down in crystalline form in the organic matrix. Calcifying cartilage of 

 the selachian Scyliorhinus contains phosphatase, as does developing bone 

 of teleosts and higher vertebrates. Differences in the properties of true bone 

 (teleost) and calcified cartilage (elasmobranch) arise from different methods 

 of ossification (deposition of calcium phosphate) (73, 74). 



Scales (Fig. 12.9) are somewhat similar to bone in structure. The teleost 

 scale consists of an organic base of collagen and an albuminoid ichthyle- 

 pidin. About 25 % of the scale is dry matter. Menhaden scales (Brevoortia) 

 contain 59 % organic matter and 41 % ash. Collagen forms 76 % of organic 

 matter, ichthylepidin 24 %. Selachian denticles and scales of certain teleosts 

 (Mola, Spheroides) have an organic framework of collagen alone. The 

 mineral matter is largely calcium phosphate (Ca(H 2 P0 4 ) 2 ) (44, 110). 



In temperate and arctic waters seasonal changes in growth and meta- 

 bolism of teleost fishes are also attended by variations in deposition of 

 skeletal material. The annual growth rings on scales and otoliths, consisting 

 of alternating bands of rapid summer and slow winter growth, are well 

 known. Similar growth rings also occur in the bones of many species. They 

 are, for example, well marked in the dragonet Callionymus (20, 35, 

 83). 



Variations in scale pattern can be noted only briefly. In some species, 

 e.g. conger eel, scales are greatly reduced or absent. In others, the scales 

 are enlarged or strengthened to form an external armour. Armoured 

 teleosts include needle-fishes (Centriscidae), in which the body is enclosed 

 in a bony cuirass ; knight-fish (Monocentridae) and trunk-fish (Ostracionti- 

 dae), which bear thick scales so as to enclose the body in a kind of box. 

 Spicules and spines are a common form of external armament. In puffers 

 or globe-fishes (Tetraodontidae) the body is naked except for numerous 

 small movable spines set in the skin. By means of a large sac connected 

 with the gullet, the puffer-fish can inflate itself with air or water until the 

 body is blown out like a balloon and spines stand erect. Porcupine-fishes 

 (Diodontidae) are covered with fixed or movable spines. In trigger-fishes 

 {Batistes) the first spine in the dorsal fin is strong and hollowed out 

 posteriorly to receive a bony knob at the base of the second spine. By this 

 mechanism the first spine remains firmly erect until the second spine, 

 which acts after the manner of a trigger, is lowered. An account of venom- 

 ous mechanisms associated with spines is given in a later section. 



Exoskeletons. Many protozoa are protected externally by plates, tests or 



