40 Guide to (he Mollusca. 



foreign bodies between fche mantle and the shell, or are secreted 

 over areas invaded by boring animals such as Sponges {Cliona 

 cclata) and worms. 



The Chinese obtain pearls artificially from a species of fresh- 

 water Mussel (Dipsas pliaita). In order to do this they keep them 

 in tanks and insert between the shell and the animal either small 

 shot or small round pieces of mother-of-pearl, which soon receive 

 regular coatings of nacre and assume the appearance of ordinary 

 pearls. They also insert small metal images of Buddha, which 

 also soon become covered with pearl and firmly cemented to the 

 shell. Specimens illustrating this practice, as well as shells of the 

 Ceylon Pearl Oyster containing small fish and crabs coated over 

 with nacre, are exhibited in the Pearl Oyster Series (Wall-case F). 



The Spondylidae, or Thorny Oysters, closely resemble the 

 Scallops, but their shells are heavier, more spiny, and united by 

 interlocking teeth ; many are very brightly coloured. The 

 Pectinidae, Scallops or Fan-shells, are well known for their 

 beautiful colours and sculpture. The eyes, which are borne on 

 the mantle, approach the Vertebrate eye in complexity. The 

 young move through the water by opening and shutting their 

 valves. The species are numerous and are found at all depths. 



Order 3. EULAMELLIBRANCHIA. 



{Cases 153 to 204.) 



The Eulamellibranchia have the gill-plates joined together by 

 vascular interlamellar junctions and very much folded. The 

 edges of the mantle are united at one or two points. 



The Oysters (Ostraeidae, Case 154) are the most familiar 

 members of this order ; since early times the cultivation of these 

 animals has been practised in various parts of the world. A series 

 of shells of different ages from the ' spat ' to the adult is shown in 

 Case 154. During the months of May, June, and July, the eggs 

 are discharged into the gills, where they remain until hatched ; 

 during this period and until the eggs are hatched in September, 

 Oysters are ' out of season '. The Common Oyster passes through 

 a sexual cycle to which reference has been made already (p. 12). 



The Japanese Ostraea gigas is said to grow to a length of 3 feet, 

 and some species of Pinna reach a length of 2 feet. The latter 

 are found embedded in sand with the pointed end downwards. 

 The threads of the large silky byssus can be used for weaving or 

 knitting. 



The Carditidae and Astartidae (Cases 160, 161) have strong, solid 

 shells, frequently ornamented with radiating or concentric 

 ribbing, and usually coated with a dark epidermis. They have 

 the general appearance of certain Veneridae ; the animal has no 



