SKELETONS, SHELTERS AND SPECIAL DEFENCES 



649 



15.5). The hinge is divided into two main layers, inner and outer, both of 

 which consist of tanned protein. The inner layer also contains CaC0 3 and 

 the outer layer deposits of lipoid. The internal cavities of the gills are 

 lined by a supporting substance which is a fibrillar protein resembling the 

 untanned protein of the byssus (9, 12, 105, 112). 



Internal Spicules. Apart from exoskeletons, the tissues of certain 

 molluscs are strengthened by spicules of various kinds. These are charac- 

 teristic especially of gastropods which have lost the external shell. In 

 nudibranchs the tissues of the body wall contain numerous calcareous 



Inner layer 

 Outer lobe 

 of mantle 



Middle lobe 

 of mantle 



Outer layer 



Vacuolated zone 

 of middle layer 



40^ 



Fig. 15.5. Section Across the Mantle Edge of Mytilus, 

 Showing the Periostracum. (After Brown (12).) 



spicules. According to Odum (97) these consist of amorphous CaC0 3 and 

 form 50% of dry weight {Archidoris). The spicules are responsible for the 

 high levels of calcium reported for tissues of nudibranchs. Remarkably 

 large quantities of magnesium and fluorine also occur in the body wall of 

 Archidoris (7 and 3% of dry matter). Strontium content is almost 0-5%. 

 These last three elements may be associated with the skeletal spicules. 

 When Accmthodoris is placed in sea water activated with 90 Sr, the strontium 

 becomes concentrated about calcium concretions in the mantle. Strontium 

 and calcium are probably taken in through the body wall as well as the gut 

 (37, 40, 77, 78, 132). 



Integumentary spicules are not always calcareous. Marine pulmonates 



M.A. — 21* 



