4O2 THE OCEAN WORLD. 



its half-opened valves and forcibly expelling the water, it moves back- 

 ward by a sort of recoil ; this action, repeated many times, compels the 

 animal to move almost in spite of itself, and enables it to avoid 

 danger, or directs its steps towards the spot it wishes to reach. 



The Pectens, of which 176 species are described, are inhabitants 

 of every known sea. Twenty species belong to Europe, among 

 which we may mention P. opercularis, represented in Fig. 176; P. 

 glaber(V\g. 175), and P. mvea. Fig. 177 represents the White-mantled 



if ffl O) 



Fig. 176. Pecten opercularis (Linnaeus). 



Pecten (P. plica, Linn.) of the Indian Ocean, and Fig. 178, the Con- 

 centric Pecten (P. japonicd) of the Japan seas. 



Among the Ostreadse the shells of several species of the genus 

 Spondylns are distinguished for their variety of form and the brilliant 

 colours with which they are decorated. This makes them much 

 sought after by amateur collectors, and procures for them a high 

 price. The shell of Spondylus is solid and thick, with unequal 

 adherent valves, nearly always bristling with spines, forming a very 

 peculiar kind of ornamentation to the valves ; the hinges have two 

 very strong teeth. The animals which inhabit this shell resemble 

 the oyster in many respects, but they still more closely resemble the 

 Pectens. The edges of the mantle are provided with two rows of 

 tentacles, the exterior row being, many of them, furnished at their 

 extremities with coloured tubercles. As examples, we note several 



