vi PREFACE. 



thcin as known quantitic's. Besides tliis, tlicy utronled in themselves a rieli field for extensive 

 comjiarisons witii numerous other elasses, with wliieh some of their numl>er liad been asso- 

 ciated at diiTerent times. And again, their- remarkal)le modes of development could not 

 fail to hrhvj; to a test the value of the changes which tliey undergo during their growth 

 for the pur]H)se of ascertaining the aliinities and relative rank of animals. The circumstance 

 that the study of this class has received less attention in America than any other, had 

 also nuich weight with me, as it gives me an oj)portunity of making our students more 

 intimately acquainted with those naturalists, who, in Euro])e, have given a new aspect to 

 Zoology since the days of C'uvier, and who, not Ijeing ornithologists, entomologists, or 

 conchologists, are hardly known here as they should be ; for it is much to be regretted, 

 that, with the Anglo-Saxons, Zoiilogy has now become too much a descriptive or too much 

 a speculative science. 



To the general reader the first part of this volume may be of some interest, inasmuch 

 as it presents a general account of the progress of Zoology since the time of Aristotle to 

 the present day with special reference to the class of Acalephs, including, besides, such 

 generalizations as may be deduced from a comparison of these animals with tiie repre- 

 sentatives of other elasses. To the professional naturalist I venture to reconnnend the 

 second i)art as containing additional information respecting the struc'ture of the Ctenojthora; 

 not to be found in jirevious contrilmtions to their natural history, and I ask especial 

 attention to the discussions in which the value of the natural groups admitted in that 

 Order is considered in detail. 



In the preparation of this part of my work I have received nmch valnaljle assistance 

 from my friend and colleague, Professor H. J. Clark, who has traced with me, for more 

 than nine years, the metamorphoses of our Acalephs, and especially those of the Hydroids ; 

 besides which he ha.s investigated for himself some sjiecial points of their structure, which are 

 noticed as his contributions in the proper place : l)ut I would particularly call attention to 

 the description of iiie lasso-cells of the C'tenophora^ on page 2o7, and to the investigation 

 of the structure of the eye of our Aiu-<'lia, which will be pul.)lished in tlii' next volume, 

 and the illustrations of which, drawii by him, Pis. XK., and XI"'., are issued with this 

 volume. 



Most of the plates were drawn from nature and on stone by Mr. Sonrel ; and it is 

 but justic(.' to him tt) sav, that I do not know representations of Acalephs executed with 

 greater accuracy, patience, and skill. Only those fully conversant with the whole range 

 of our literature on this subject can do complete justice to their great merit; and I can 

 truly say, that, without tlic aid of his ])ersevering zeal, I could not have acconqjlishcd 

 what I aimed at in this volume. 



I have, further, derived much assistance in my work from the liberality of the Smith- 

 sonian Institution, in lending me books not to be found in the libraries of Boston and 



