via AUTHORS PREFACE 



invertebrate animals may require an explanation, and perhaps 

 also an excuse. We have been guided exclusivelj by prac- 

 tical considerations, especially the fact that the comparative 

 embryology of Vertebrates has very recently been compre- 

 hensively treated in an excellent manner, and further the 

 reflection that, with the limitation of our field to the In- 

 vertebrates, the treatment of them might be so much the 

 more thorousfh. 



We take the liberty of expi'essing here our best thanks to 

 Herrn Geheimrath F. E. Schulze, who has aided us in the 

 most amiable manner, both by his advice and by his assist- 

 ance in procuring literature; and likewise to our publisher, 

 who has made it possible to issue the book in its present 

 form. 



THE AUTHORS. 



