CHAPTER THIRD. 



NORTH AMERICAN C T E N P II R ^E . 



SECTION I . 



THE GEXUS PLEUROBRACIIIA AXD ITS SPECIES. 



All our knowledge of the animal kingdom being derived from a study of 

 individuals, and our insight into their various relations growing out of the fullest 

 comparisons between them, it is natural, that, in describing animals, we should at 

 times dwell more extensively ujjon the results of such comparisons, and at other 

 times turn our attention more especially to the conditions under which they live. 

 Investigations neglecting either the one or the other side of the sulyect must, 

 from the nature of things, remain imperfect. It is only when the study of the 

 structure and functions of an animal draws to a close, and Ave have made ourselves 

 perfectly familiar with its mode of life, that we may l^egin a systematic survey 

 of its various connections, and ascertain its position in the system, as determined 

 by its anatomical peculiarities, its embryonic growth, the period of its appearance 

 upon our globe, and its geographical distribution upon its surface. But science 

 cannot take for granted mere results, presented in a dogmatic form : it requires 

 the fullest evidence of every statement and the fullest demonstration of every 

 inference. Special descriptions must therefore he full and minute : they must, 

 moreover, l)e comparative, and even embrace the widest range of comparisons, that 

 no doubts may remain in the mind of the reader respecting the correctness of 

 the information offered him. I have on that account thought it desirable to 

 reproduce here such parts of the descriptions of the North American Ctenophora3 

 published in the Memoirs of the American Academy for 1849 as may throw 

 additional light upon the subjects discussed in the preceding chapters. I begin 

 with our Pleurobrachia, a small, rounded, transparent body with two long threads 



