PELECYPODA. 



37 



Cases 154- 

 155. 



The order is very extensive, comprising nearly sixty families, of 

 which only the more important can be referred to. 



The Limas (Limidce) are very like the Pectens, but the inner Case 153 

 edge of their mantle is fringed with very long thread-like filaments. 

 The shells are always white, generally more or less oblique, and 

 radiately ridged. They appear to be found in most seas, and either 

 swim about freely like the young Scallops by flapping their valves, 

 or attach themselves by a byssus, sometimes forming a sort of nest, 

 consisting of pieces of coral and shell or small stones, in which they 

 are completely concealed. 



The Ostreldce, or Oysters, undoubtedly take the first rank among 

 molluscs as regards usefulness to mankind as an article of food. 

 They have no foot ; the mantle is entirely open, with double edges, 

 each being bordered by a short fringe, and the labial palps are large 

 and somewhat triangular. There are on each side a pair of simple 

 gills, which appear closely striated ; the single adductor muscle is 

 large and nearly central (see Fig. 29). The Oyster is, except in the 

 very young state, entirely incapable of locomotion, and always 

 attached by the deeper valve to other shells, rocks, or other sub- 

 stances. The common British species is not full-grown until it is 

 about five or seven years old. A series of different ages, from the 

 "spat" to the adult form, is exhibited in Case 154. During the 

 months of May, June, and July the eggs are discharged into 

 the gills, where they remain until hatched ; and it is during 

 this period that oysters are " out of season." In the American 

 Oyster (0. virginicd), on the contrary, the eggs are said to be 

 hatched outside the parent shell. Oysters of different kinds are 

 found on nearly every shore. The gigantic 0. fjigas is said to 

 grow to the length of three feet in the Bay of Taichou, Japan, 

 where it is commonly eaten. About two hundred fossil species have 

 already been described. 



Some specimens of Pinna attain to a length of two feet. They 

 are found imbedded in the sand with the narrow pointed end down- 

 wards. They form a large silky byssus, which can be woven or 

 knitted into gloves, socks, etc. (see side table-case B). 



The CarditidcB and AstartidcB have strong solid shells, frequently 

 ornamented with radiating or concentric ribbing, and usually are 

 coated with a dark epidermis. They have the general appearance 

 of certain VeneridcB ; but the animal has no prolonged siphons, but 

 merely a fringed opening in the mantle. One very remarkable species, 

 Thecalia concamerata (Case 161), has an internal cup-like process 



Cases 156- 

 160. 



