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246 



STALK-EYED CRUSTACEA. 



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on the coast of Florida extend from a few fathoms off shore to 2000 fathoms. 

 Considerations like these will put one on his guard against such an assump- 

 tion as that above alluded to, Pourtales was surprised at the bulk and 

 massive type of the West Indian Tertiary simple corals as compared with 

 their modern representatives which he dredged in the Caribbean Sea. But 

 this difference Is explicable if we suppose that the ancient forms were littoral. 

 Under the enormous pressure which exists in deep water^, great size is pos- 

 sible only when accompanied by a soft and freely permeable texture. The 

 calcareous shells and corals from deep water are generally small, or if large, 

 extremely thin and fragile. The deep-sea Crustacea, as a rulc; lack the 

 rigid calcareous coat which protects their littoral relatives. Species living 

 under a pressure of a ton or more to the square inch are often so limp and 

 delicate that it is difficult to secure a perfect specimen. By casting away 

 their armor, their battle with the abyss was won. 



The small number of ancient types of Crustacea preserved in the great 

 depths of the ocean is not a subject for wonder if we bear in mind the fact 

 that most of the fossil Crustacea known to us are probably littoral, or from 

 the present point of view, shallow-water forms. The changes in environ- 

 ment to be met and overcome by a highly specialized littoral species in 

 adapting itself to life at great depths are presumably as great, and lead to as 

 much structural modification as those encountered by the littoral descend- 

 ants of ancient species through the vicissitudes of the shores. As a concrete 

 example, I will instance the family Galateida). This family has a very 

 extensive vertical distribution, being represented at all depths from the 

 littoral zone to below 2000 fathoms. But this great distance is apportioned 

 in a rough way among the different genera of the family. In the shallower 

 waters from tlie shore to 25 fathoms the genus Galatea prevails, in the 

 deeper, but not abyssal belt Munida comes to the front, while in the greater 

 depths below 500 fathoms the family is represented chiefly by the blind 



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Now MM. Milne Edwards and Bouvicr * 



have shown, in their interesting memoir on this group, that as we pass from 



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generalized, or Macruran type. 



In some instances the more primitive types of Crustacea flourish in the 



* Considerations Geiicniles sur la Famillo des Galatlu'ides, Par MM. A. Milne Edwards ct E. L. 

 Bouvicr. Ann. Sci. Nai., ZooL, 7^™" S6r., XVL 315-317, 1891 



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