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 COMMON KNOWLEDGE OF THE CRAYFISH.

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My purpose, in the present work, is to exemplify the general truths respecting the development of zoological science which have just been stated by the study of a special case; and, to this end, I have selected an animal, the Common Crayfish, which, taking it altogether, is better fitted for my purpose than any other. It is readily obtained,* and all the most important points of its construction are easily deciphered; hence, those who read what follows will have no difficulty in ascertaining whether the statements correspond with facts or not. And unless my readers are prepared to take this much trouble, they may almost as well shut the book; for nothing is truer than Harvey’s dictum, that those who read without acquiring distinct images of the things about which they read, by the help of their own senses, gather no real knowledge, but conceive mere phantoms and idola. It is a matter of common information that a number of our streams and rivulets harbour small animals, rarely more than three or four inches long, which are very similar to little lobsters, except that they are usually of a dull, greenish or brownish colour, generally diversified with pale yellow on the under side of the body, and some¬ times with red on the limbs. In rare cases, their the description of the former, in almost all points ; but the gills and the abdominal appendages present differences ; and the last thoracic somite is united with the rest in the lobster. (See Chap. Y.)
 * If crayfish are not to be had, a lobster will be found to answer to