CRUSTACEA. 



143 



the market in vast numbers, large ponds being devoted to the purpose. 

 To catch them for the market, bundles of twigs are sunk in the ponds, and 

 the cray-fishes seem to delight in crawling into the interstices, and are thus 

 drawn to the surface. One of our American species deserves mention from 

 the fact that it is found in the streams in the caves of Indiana and Ken- 

 tucky, Mammoth Cave having furnished large numbers of specimens. 

 Living in these absolutely dark abodes, eyes are of no necessity to it, and 

 so these organs have become obsolete through disuse. Instead of the 

 highly developed eye of their relatives, here barely more than the eye- 

 stalks remain, while the optic nerve has dwindled to a mere rudiment. 



In the plate opposite page 138 there is figured a spiny lobster, a form 

 which usually occurs in warmer seas than the true lobsters. As will be 

 n, the differences between the two forms is considerable ; in the one 

 now under discussion the body is covered with strong spines, while the 

 feet instead of being armed with pincers terminate in simple claws. In 

 their development the dif- 

 ferences are even more 

 marked than in the adult. 

 The young lobster bears 

 a considerable resemblance 

 to the adult, but who 

 would suspect that the 

 form figured here was the 

 young of the spiny lob- 

 ster? It is a flattened form 

 scarcely thicker than a 

 sheet of paper, and so 

 transparent as to warrant 

 the name glass-crab which 

 is applied to it. Natural- 

 ists were for a long time 

 unaware that these forms 

 were the young of the spiny lobsters, and it was not until 1857 that the 

 true state of affairs was even suspected. The spiny lobsters frequent rocky 

 bottoms, and in Europe and our southern states they are extensively eaten ; 

 the flesh and the eggs being esteemed as delicacies in the countries sur- 

 rounding the Mediterranean. On the Pacific coast, where the true lobster 

 is unknown, species of spiny lobsters take their place, and in speaking of 

 Aiem. the adjective spiny is almost invariably dropped. 



Possibly the most interesting of all crustaceans are those forms known 

 3 hermit-crabs. The hinder part of their body lacks the hardened integu- 



Fig. 128. — Glass-crab or leaf -crab, natural size. This crab, which 

 is scarcely thicker than the paper on which this volume is printed, 

 is the young of the spiny lobster (Palinurus) shown at page 139. 



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