VI PREFACE. 



appears to have entered upon the general question of the 

 existence of an analogous course of development through- 

 out the lower classes of animals, until the author of the 

 present work, who was already favorably known among 

 natm^alists by other publications. He has taken up the 

 subject with the view principally of tracing its general 

 connexion between the various classes. Whilst doing 

 this, however, he has made some very important and 

 highly interesting special observations on the development 

 of the trematode entozoa. In this he has undoubtedly 

 opened up a most extensive field for futm^e inquiry ; and 

 there is every reason, from what he states, for believing 

 that numerous and important additions will be made to 

 facts, serving for the foundation of a sufficient theory of 

 alternating generations, from investigations relating to 

 the multiform class of animals comprised under the en- 

 tozoa. 



In the present work, owing to the paucity of the ma- 

 terials afforded by previous observations, the speculations 

 of the author may, at first sight, appear bold, and his 

 conclusions in some places hastily drawn ; but, in many 

 respects, there must be conceded to them the character of 

 great ingenuity and considerable originality. Most of 

 the observations themselves are not brought forward as 

 new J but the attempt at giving these phenomena a natural 

 explanation, and at showing their mutual analogies, is, 

 to a great extent, as novel as the full elucidation of the 

 subject will be beneficial to the progress of invertebrate 

 zoology. 



With respect to the use of several new terms in this 

 translation, the author's apology will apply as well to 

 their use in this version as to their original adoption. 



