THE IMPENETRABLE SEA 



nearly all of them are less than an inch long, in fact most 

 of them are less than half an inch; but some of the 

 crustaceans measure several inches, and many authorities 

 include giant jelly-fish five or six feet across. For clearer 

 understanding of the creatures one might divide them 

 into temporary and permanent planktonic animals ; the first 

 class consisting of the babies of numerous sea creatures 

 which pass out of the planktonic stage as they become 

 adults and resist the tides and currents as burrowers, 

 crawlers or swimmers. The permanent planktonic animals, 

 on the other hand, spend their entire lives at the mercy 

 of the waters, drifting around unresistingly, so that they 

 never have fixed habitations or localities, and are only at 

 rest when the waters permit them to be. 



The copepods are the main group in the second class, 

 although not all of them are planktonic, for some of them 

 attach themselves to the sea-floor, or to rocks or sea- 

 weeds. 



Many early students of sea creatures did not realize, 

 as they studied the tiny babies of fishes and other sea 

 animals, that they were looking at the oflfspring of 

 creatures already known to them. So they sometimes 

 gave names to the babies of parents whom they had 

 already named. One of these they named ''zoea" — it was 

 a strange little shrimp-like creature, which developed 

 into something half-way between a lobster and a crab, 

 and was classified as two diflferent species. We know today 

 that both creatures are immature crabs in stages through 

 which they must pass to become adults. 



Such mistakes in classification are easy to make, 

 especially when studying planktonic animals, for they 

 are extremely difiicult to keep alive in captivity and for 

 a curious reason. In the sea they are "cushioned" by the 

 waters themselves and seldom come into confined spaces. 

 But in captivity they often bump into the glass walls of an 

 aquarium or case and injure themselves. Others die 

 because it is difficult to know what food to give to 



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