646 THE BIOLOGY OF MARINE ANIMALS 



shell of the argonaut is a peculiar and specialized structure. In the following 

 account we shall be concerned chiefly with the shells of gastropods and 

 lamellibranchs, which have been most investigated. 



Shell Composition. The shells of gastropods and lamellibranchs con- 

 sist of an outer organic membrane of conchiolin, the periostracum, and a 

 series of crystalline layers containing CaC0 3 as aragonite or calcite in a 

 conchiolin base. Tables and lists showing the mineralogical form in which 

 CaC0 3 is present in molluscan shells are available (75, 80, 120). Con- 

 chiolin i<; predominantly protein, with traces of polysaccharide. In gastro- 

 pods and lamellibranchs the amount of organic matter is small, ranging 

 from 1-10% (dry weight). CaC0 3 constitutes 98-99 % of inorganic matter. 

 Small amounts of MgC0 3 occur in many species, and reach levels of 1-2% 

 in shells of Nassarius, Pinna and Pec ten (Table 15. 1). Analyses of scaphopod 

 (Dentalium) and amphineuran (Mopa/ia) shells reveal similar values (5, 12, 

 21, 42, 108, 124, 129). 



The lamellibranch shell typically consists of two valves closely apposed 

 along the dorsal side where they articulate through interlocking teeth. 

 A ligament, which may be internal or external, occurs along the mid-dorsal 

 line. This ligament is mechanically opposed to the adductor muscles of the 

 shell, and by its elasticity it holds the valves open when the muscles are 

 relaxed. Typically the shell consists of several layers, an external perio- 

 stracum and two or three calcareous layers. The ligament is largely organic 

 and consists of inner and outer regions. The outer layer is subject to tensile 

 strain transverse to the longitudinal plane of the shell, and this strain is 

 increased when the valves are closed. The inner layer is elastic to com- 

 pressional stresses which it normally experiences during closure of the 

 shell (130). 



In marine prosobranchs the shell has essentially the same layered struc- 

 ture, but shows much variation in finer composition. Externally, there is 

 an organic periostracum, and beneath this usually two calcareous layers, 

 of which the outer is frequently homogeneous and prismatic, and the 

 inner nacreous (la). 



Origin of Shell Material. Most of the calcareous material of the shells 

 of marine molluscs is absorbed directly from sea water as calcium ions, 

 which are secreted as CaC0 3 at the edge of the mantle. Metabolic C0 2 is 

 the primary source of the carbonate radicle. 



Shell growth in oysters can occur in sterile sea water in the virtual 

 absence of food. From analysis of oyster shell it has been calculated that 

 some 75 g of shell are laid down in 12 months; this corresponds to 

 about 25 g of Ca ++ , and a deposition-rate of 70 mg Ca ++ /day. Direct 

 measurements of calcium deposition, using 45 Ca, give a rate of 30 mg/day 

 for Crassostrea virginica (25-26°C). Shell is formed throughout the year, 

 even during the colder winter months when the oyster is not feeding and 

 deposition of soft tissues is at a standstill (42, 124, 135). 



Formation of Shell. The innermost layer of the shell (nacreous layer) 

 is formed by the epithelium of the entire mantle; the outer layers and perio- 



