634 



THE BIOLOGY OF MARINE ANIMALS 



in magnesia than those from cold water. This is revealed in the accompany- 

 ing table (15.2) showing selected data for crinoids from Clarke and 

 Wheeler (23). Since the magnesium and calcium content of sea water is 

 universally constant, we must look to some effect of temperature on 

 physiological mechanisms to account for this phenomenon (21, 22). 



TABLE 15.2 

 Percentage of Magnesium Carbonate in Ash from Crinoids 



The physiology of skeleton-formation in adult echinoderms has not been 

 investigated. In the larvae, spicules are formed by mesenchyme cells after 

 gastrulation (sea-urchin, starfish). Salts for spicule formation are derived 

 from sea water. Mineral matter is absorbed from sea water throughout 

 development: ash increases from 1-5% (dry weight) at fertilization to 

 16-8 % at 40 hours (pluteus). Sea-urchin and starfish eggs fail to develop in 

 sea water lacking Ca ++ and CO = ions and spicule-formation is delayed 

 in Ca ++ -poor media; Mg++ and SO7 ions are also necessary for spicule- 

 formation. However, Ca ++ and certain other ions are required for tissue 

 differentiation and growth apart from skeletal formation, as evidenced by 

 requirements of Beroe and Ciona larvae. At the time of spicule formation 

 the pH of sea-urchin larvae rises owing to incorporation of CO 7 ions 

 in the skeleton. At the same time HCO7 ions are absorbed from the 

 environment to supplement those derived from respiratory C0 2 (60, 76, 

 82, 87, 91 a, 111, 132). 



Endoskeleton of Fishes. The internal skeleton of cyclostomes and fishes 

 consists of cartilage or bone. Cartilage contains chondromucoid (gluco- 

 protein), elastin and collagen. In elasmobranchs the cartilage is to some 

 extent calcified. This takes the form of a deposition of crystalline plates of 

 calcium phosphate over the surfaces of the cartilage. 



Bone consists of an organic base impregnated with mineral matter. The 

 water content is about 50 %. Dry matter consists of 60 % organic and 40 % 

 mineral material. The organic substratum contains collagen, glycoprotein 

 and protein. Most of the mineral matter is calcium phosphate, which forms 

 about 83% of total mineral matter in teleost bone; CaC0 3 forms some 



