M 



NATURAL HISTORY. 



ends [n the scallop the structure is more complex, a regular lens being 



deduced. These eyes can see but little. It is impossib e that by then 



i(1 the mollusc sees a , we see; all that is possible is merely a recognition 



;,,- j. , ht and darkness. In the higher molluscs, as we shall see later, the 



eye is much more perfect, and able to recognize the shape of objects 



While on this subject of sense-organs, - it may be well to refer to 

 th e ears of the acephals, for all are provided with them. These, like 



the eyes, are very simple, but they are more 

 permanent in position, being situated in or 

 near the foot. Each consists of a sac filled 

 with fluid in which one or more solid particles 

 are suspended. The sac is lined with ciliated 

 cells, and has a nerve supplying it. The mech- 

 anism is simple; a noise jars the water, and 

 sets the granules in the ear in motion, and 

 these, hitting against the cilia with which the 

 sac is lined, affect the cells, and these in turn 

 communicate the motion to the nerves. 



Scallops and some allied shells have a pecu- 

 liar style of locomotion, — a sub-aquatic flight 

 it has been termed. They rapidly open and 

 close the calves of the shell, and thus swim through the water with con- 

 siderable rapidity. 



The thorny oysters {Spondijlus) are large forms, with very thick and 

 heavy shells, one valve being much larger than the other. The name 

 'thorny oyster' has reference to the spines with which the outer surface 

 of the shell is ornamented, and these in some species are very long and 

 slender. One of the thorny oysters (Spondylus regius) is considered as a 

 rnv shell, and perfect specimens are extremely rare, so easily are the 

 spines broken off. Indeed, during the present century a perfect specimen 

 was sold in Paris for several thousand francs, and even in 1876* the same 

 specimen brought thirty-six dollars. One species of thorny oyster produces 

 pearls of value. 



It is, however, to the true pearl-oysters that we must turn for the 

 supply and most valuable pearls. They are natives of tropical 

 seas, and occur in the Indo-Pacific and the Western Atlantic, beds of con- 

 siderable size being found on the shores of Madagascar, the Persian Gulf, 

 Ceylon, the western coast of Mexico and Central America, and in the West 

 [ndies. Externally the shells themselves are far from beautiful, but the 

 pearly surface of the inside of the valves, shading off into dark brown or 

 black at the edges, makes them very attractive. The shells themselves 



I to. 79.— Ear of a mollusc (Pterotra- 

 ch( ■< I, greal ly enlarged. The concre- 

 i ion in the C< nire is set in vibration 

 by the water, and hitting the hairs 

 li'ninu the cavity, conveys the sensa- 

 tion to the nervous centres. 





