COMMON KNOWLEDGE OF THE CRAYFISH. 5 
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 
* If crayfish are not to be had, a lobster will be found to answer to 
the desvription 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. V.) 
