BIVALVES. 



409 



MARGIN OF MANTLE OF PECTEN, SHOWING TENTACLES 



and eyes (somewhat magnified). 



be found. They are irregular in their growth, and consequently the deter- 

 mination of the species is a matter of difficulty. The pectens or scallop-shells 

 (Pectinidce) are remarkable for the variety and beauty of their coloration and 

 sculpture. In most species the shells are nearly equivalve ; but in a few, of 

 which the common edible scollop is an example, the right valve is convex, and 

 the left flat. One species (Pecten 

 jacobceus) of the Mediterranean 

 was worn as a badge by pilgrims 

 who had been to the Holy Land. 

 Most of the pectens are orna- 

 mented with radiating ribs, but a 

 few are smooth. Some swim 

 freely by flapping their valves, 

 others live permanently attached 

 by a byssus. The animal has 

 the mantle free, and frequently bears a row of brightly coloured eyes on the 

 margin. The foot is small, the gills are extremely delicate, and the single adductor 

 muscle in the adult is excentric. They are generally hermaphrodite, but sometimes 



the sexes are separate. 

 More than a hundred 

 species from all parts 

 of the world and all 

 depths have been 

 described. Ten occur 

 on the British coasts, 

 and fossil species are 

 numerous in all forma- 

 tions, from the Carboni- 

 ferous. The file -shells 

 (Limidce) somewhat 

 resemble the scallops, 

 but are nearly always 

 white, and the edges of 

 the mantle, which have 

 no eyes, are furnished 

 with long tentacular 

 filaments. Some swim 

 freely by flapping their 

 valves, others attach 

 themselves by a byssus, 

 or, as in the accompany- 

 ing figure, construct a 

 nest of broken shells, 

 stones, and other substances held together by a network of byssal threads. 

 The two largest species occur off the coasts of Norway and Japan. The recent 

 species of the chief genus (Lima) are not numerous, and some occur at 



file-shell (Lima) in its nest (nat. size). 



