648 THE BIOLOGY OF MARINE ANIMALS 



quate supplies immediately available in the surrounding sea water (39, 

 80, 131). 



Shell formation involves two phases — elaboration of the fibrous organic 

 framework, followed by concentration and deposition of mineral salts. 

 When specimens of Isognomon, with broken shell edges, are placed in cal- 

 cium-free sea water, the newly regenerated shell shows a normal organic 

 matrix but lacks Ca ++ . Carbonic anhydrase is found in mantle as well as 

 other tissues of many molluscs, and is probably involved in shell formation 

 (conversion of C0 2 to carbonate). During calcification, crystals of calcium 

 phosphate are deposited initially in the conchiolin layer, to be replaced 

 later by crystals of calcite (or aragonite). Mantle tissue contains much 

 Ca 3 (P0 4 ) 2 , whereas the shell is composed mainly of CaC0 3 . The mechan- 

 ism of conversion of Ca 3 (P0 4 ) 2 to CaC0 3 has still to be explained. Heavy 

 concentrations of alkaline phosphatase are found in mantle tissue, and 

 may be concerned with mobilization of calcium and mineralization of the 

 shell (4, 5, 80, 120, 134). 



In typical lamellibranchs two kinds of shell can be distinguished on the 

 basis of surface markings. In the first type there are a number of well- 

 marked rings on the shell, which are laid down in winter when growth is 

 retarded; the wider areas between the rings are formed when growth is 

 rapid. Each ring, therefore, represents a winter period. In the second type 

 of shell, growth rings are poorly marked. Bivalves which show pronounced 

 growth rings are oyster Ostrea, cockle Cardium, etc. Tellina tenuis is an 

 example of a species without well-marked winter rings. In some gastropods, 

 e.g. the periwinkle Littorina littorea, shell formation takes place throughout 

 the year, but is slowed down in winter and accelerated during spring and 

 summer. In the limpet Patella vulgata, shell growth may be arrested in 

 winter and resumed in spring (42, 67, 84). 



Structural Proteins in Molluscs. In lamellibranchs, such as Mytilus, 

 structural proteins are present in the periostracum, hinge, byssus and in 

 the supporting material of the gills. With the exception of the latter, these 

 structures contain quinone-tanned protein (conchiolin). Analyses of amino- 

 acid residues point to the existence of distinct species of conchiolins in 

 prismatic and nacreous layers, and X-ray diffraction pictures reveal 

 collagen-type fibrils. Under electron microscopy, thin layers from decalci- 

 fied shells appear riddled with fine pores, the patterning of which varies 

 with the species (gastropods, lamellibranchs, pearly nautilus) (45, 46). 



The byssus threads of lamellibranchs are formed in the posterior groove 

 of the foot from the secretions of two glands, a "white" gland which 

 supplies most of the protein of the thread and a "purple" gland which 

 supplies the aromatic material responsible for the tanning {Mytilus). A 

 polyphenol oxidase is present which converts polyphenol to the quinone 

 tanning agent. Similar reactions take place in the hardening of the perio- 

 stracum, hinge and conchiolin ground substance of the shell. The perio- 

 stracum is secreted by the mantle edge; it originates as a single homo- 

 geneous sheet, which soon becomes differentiated into three layers (Fig. 



