108 THE STORY OF ANIMAL LIFE 



Pecten, the Comb-shell, 'probably refers, not to 

 the shape of the gills, which is somewhat peculiar, 

 but to the marking of the shell, which presents 

 raised ridges, side by side. Anyone familiar with 

 shells will see at once that this is an unusual pat- 

 tern. There are plenty of bivalve shells with 

 concentric ridge markings, comparatively few 

 with radiating ridges. We shall see presently 

 that there is a good reason for this. The specific 

 name "opercularis," lid-like, refers to .the neat 

 round shape of the shell. Each half of the shell 

 has a pair of " ears," so-called. The person who 

 first gave this name to these flaps of shell, three 

 of which are three-cornered and the fourth non- 

 descript, must have been familiar in his youth 

 with books afflicted with the "dog's-ear" disfig- 

 urement; for certainly there is no other kind, of 

 ear which greatly resembles these. The notch 

 beneath the irregularly shaped ear is called the 

 "byssal notch": many Pectens spin a byssus or 

 thread, like that spun by the common Sea-Mussel, 

 and thus anchor themselves to fixed objects for a 

 time ; this notch is the place where threads of 

 this kind leave the shell. 



The two valves of the shell differ in depth, 

 one being flatter than the other; and the "ears" 

 of the two valves differ in shape. The inside of 

 the shell shows muscular impressions, but these 

 cannot be seen in a photograph. The picture, 

 however, shows the strong hinge-ligament which 

 joins the halves of the shell, and the difference 

 in depth and shape of the two valves. The valve 

 on which the animal usually lies is the lighter in 

 colour of the two, and has one ear much longer 

 than the other. 



The creature swims by means of the " mantle," 



