SKELETONS, SHELTERS AND SPECIAL DEFENCES 



647 



stracum are usually formed by the mantle edge alone. Under certain 

 circumstances, e.g. during shell regeneration, the periostracum may be 

 formed by the epithelium of the general mantle. 



The CaC0 3 used in shell formation is liberated by certain cells of the 

 mantle edge in colloidal form between the epithelium and periostracum. 

 Subsequent crystallization of CaC0 3 from this colloidal gel takes place 

 outside the epithelial cells. Calcium ions enter through the general external 

 surface, mantle, gills and especially the gut. Experiments with mussels show 

 that 90 Sr and 45 Ca are absorbed on mucous feeding sheets from whence 



Iron 

 granules 



Nucleus 



Ca 

 granules 



Lumen of 

 acinus 



^'^Ltme 



ceil 



Vacuole , 

 containing Col t 

 and mucoprobein 



Fig. 1 5.4. Section through Acinus of Digestive Gland of Haliotis tuberculoid, 

 Showing Glandular Cells Containing Calcareous Granules. 



(After Manigault (80).) 



they enter the body through gills, mantle and gut. In Aplysia 90 Sr passes 

 through the general body surface, especially the gill. Phosphate ions 

 (measured by uptake of 32 P) are absorbed principally by the gills (oyster) 

 (40, 62, 75, 104, 106, 111). 



It is well known that terrestrial gastropods (pulmonates) accumulate 

 calcium reserves in special lime cells of the digestive gland, and are able 

 to draw upon this supply for shell growth and regeneration. Lime cells are 

 found in the digestive gland of chitons, and calcium stores in the connec- 

 tive tissue and digestive gland of Littorina, Haliotis and Nucella (Fig. 15.4). 

 The significance of these tissue deposits in an environment rich in calcium 

 is not clear. Marine lamellibranchs do not store calcium, and find ade- 



