262 



LOWER INVERTEBRATES. 



but the eyes have not yet been discovered. Whether or no they are really absent has 

 not yet been decided, but the recent investigations of Dr. Benjamin Sharp show that 

 light-perceiving organs exist in many forms where their presence was not previously 

 suspected, and so it may be here. Lima hians, the species figured, swims with great 

 ease, and in the same manner as do the scallops. It also spins a byssus, and the adults 

 not infrequently build a rude nest or burrow, by cementing together bits of coralline, 

 shells, and sand. " The species of Lima usually live quietly at the bottom, with the 

 valves widely extended, and thrown flat back, like the wings of certain butterflies 

 when basking in the sun ; but when disturbed, they start up, flap their light valves, and 



move through the water 

 by a succession of sud- 

 den jerks. The cause 

 of alarm over, they 

 bring themsehes to an 

 anchor by means of 

 their provisional bys- 

 sus, which they seem 

 to fix with much care 

 and attention, previ- 

 ously exploring every 

 part of the surface with 

 their singular, leech- 

 like foot." 



The species of Spon- 

 dylus are known as 

 thorny oysters. The 

 unequal valves are usu- 

 ally armed with spines, 

 which not infrequently 

 are very long and flat. 

 The right valve is the 

 largest, and is attached 

 at the beak, the hinge 

 ligament is internal, 

 and the hinge is pro- 

 vided with two teeth in either valve. By the process of growth, the hinge area of 

 the lower valve becomes converted into a triangular space furro\ved down the 

 centre by the groove for the hinge ligament. Slight ears are present at the hinge line. 

 The ocelli on the margin of the mantle are numerous. The species are all inhabitants 

 of tropical and sub-tropical seas. In the "West Indies occurs ^S". americana. S. gcedero- 

 pus, of the Mediterranean, is said to produce pearls. Most noted of all is the Spoii- 

 dyltts regiiis of the East Indies, which is classed among the rare shells. In times past 

 perfect specimens have brought immense prices, and no longer ago than 1876 a specimen 

 sold for thirty-six dollars. The long and delicate spines are so easily broken that per- 

 fect specimens are comparatively rare. 



The AvicuLiD^ is the family of the true pearl oysters, and although many otbei' 

 molluscs produce pearls valued as ornaments, it is to this family that the world Qwes 

 the largest proportion of these so-called precious stones. The family is characterized 



Fig. 273. — Lima hians in its nest. 



