The Shell-builders. 



form a double shell in which to dwell. Amongst them 

 not only are there many which produce most exquisitely 

 coloured and gracefully shaped shells, but some which 

 go further than the secretion of a shelly protection, and 

 which are weavers, masons, and miners. 



Largest of all the bivalve Molluscs is the Giant 

 Clam, which inhabits the Indian Ocean. The great 

 shell often weighs upwards of 500 Ibs., while the animal 

 which lives within and formed these immense valves 

 attains a weight of 20 Ibs. They are frequently to be 

 seen in the quiet lagoons of coral islands, or atolls, 

 with their great valves partly opened; and Darwin, 

 describing his visit to Keeling Atoll, says : " We stayed 

 a long time in the lagoon, examining the fields of coral 

 and the gigantic Clam shells, into which if a man were 

 to put his hand he would not as long as the animal 

 lived be able to withdraw it." The exterior of the 

 Clam shell is deeply grooved and moulded, presenting 

 a very handsome appearance. 



Another interesting bivalve inhabitant of the coral 

 reefs is the Spondylus or Thorny Oyster, which, al- 

 though never reaching the gigantic proportions of the 

 Clam, is a fairly large and handsome Mollusc. The 

 exterior of both upper and lower valves is covered 

 with spines, which on the centre and towards the apex 

 are more or less sharply pointed and slightly curved 

 like thorns, while as they approach the outer edges of 

 the valves they increase in length and broaden out in 

 foliaceous expansions. An interesting peculiarity of 

 the Spondylus is that with advancing age the shell does 

 not increase in size, but becomes thicker in its interior 

 by trre addition of inner layers of shell, which are dis- 



37 



