402 THE OCEAN WORLD. 
its halfopened 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 
ef every known sea. Twenty species belong to Europe, among 
which we may mention P. ofercularis, represented in Fig. 176; £, 
glaber (Fig. 175), and P. nivea. Fig. 177 represents the White-mantled 
Fig. 176.—Pecten opercularis (Linnzus). 
Pecten (P. plica, Linn.) of the Indian Ocean, and Fig. 178, the Con- 
centric Pecten (P. japonica) of the Japan seas. 
Among the Ostreadz the shells of several species of the genus 
Spondylus 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 
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