SKELETONS, SHELTERS AND SPECIAL DEFENCES 631 



fibres are classed with collagen. Amino-acid composition is markedly 

 different from both keratin and gelatin (81). 



Mineral spicules and organic fibres are secreted by cells termed sclero- 

 blasts. Both siliceous and calcareous spicules are laid down around an 

 axial thread of organic matter. More is known about spicule formation in 

 calcareous sponges. At first intracellular, the spicules frequently become 

 larger than the scleroblasts secreting them. In this event the scleroblasts 

 become apposed to the sides of the spicule, on which they continue to 

 deposit CaC0 3 extracellularly. Carbonate ions are necessary for formation 

 of spicules. Larvae reared in carbonate-free sea water fail to produce 

 spicules, and spicules of young sponges regress in such media, probably 

 due to increased acidity occasioned by action of respiratory C0 2 . Spongin 

 fibres are also secreted by mesenchyme cells. These arrange themselves in 

 rows and each gives rise to part of an elongated fibre (23, 61, 99, 111, 132). 



Echinoderms. The majority of echinoderms are protected by an endo- 

 skeleton of calcareous plates termed ossicles. These are embedded in the 

 skin and frequently bear spines which are movably articulated with the 

 plates. When the ossicles are scattered the integument has a leathery tex- 

 ture; when closely apposed they encase the animal in armour. 



In echinoids the skeleton usually forms a compact cuirass except for a 

 space about mouth and anus. The ossicles bear large movable spines. A 

 peculiarity of these animals is the presence about the oesophagus of a 

 calcareous framework known as Aristotle's lantern, which supports a set 

 of teeth. In asteroids there is a well-developed system of ossicles, which are 

 not united, however, into a continuous shell. Ophiuroids have the body 

 covered with closely-set plates, those of the arms articulating so as to 

 afford free movability. Large ossicles occur on the aboral surface, in the 

 arms and in the stalk of crinoids. In holothurians the ossicles are greatly 

 reduced and scattered through the integument. They have varied shapes, 

 such as wheels, anchors and crosses. The planktonic Pelagothuria lacks 

 ossicles. Psolus has the dorsal ossicles enlarged to form a complete mail of 

 plates. 



The skeleton forms a high proportion of total body material in starfish 

 and brittle stars. Starfish contain about 30 % dry matter and brittle stars 

 over 50%, of which some 34% is protein and 42% CaC0 3 . 



Echinoderm skeletons contain a small proportion of organic matter 

 (10-20%), which presumably forms a substrate on which mineral matter is 

 deposited. Amino-acid residues have been determined for skeletal protein 

 of Arbacia, and X-ray diffraction studies have revealed the presence of 

 collagen-type fibrils (105, 112). Although preponderantly calcareous the 

 skeletons also contain much magnesia. CaC0 3 , as calcite, occurs in amounts 

 ranging from 78-95% (mineral matter); average calcareous content lies 

 around 88% (starfish, sea-urchins, holothurians, crinoids). MgC0 3 is 

 present in all groups (5-16% of skeletal material, Table 15.1). As in certain 

 other marine organisms, the proportion of MgC0 3 in the skeleton appears 

 to be a function of temperature, animals from warmer waters being richer 



