PBEFACE. 



The present volume on the Anatomy of Invert ebrated 

 Animals fulfills an undertaking to produce a treatise on 

 comparative anatomy for students, into which I entered 

 two-and-twenty years ago. A considerable installment of 

 the work, relating wholly to the Invertebrate, appeared in 

 the Medical Times and Gazette for the years 1856 and 

 1857, under the title of " Lectures on General Natural 

 History.' 1 But a variety of circumstances having con- 

 spired, about that time, to compel me to direct my atten- 

 tion more particularly to the Vertebrata, I was led to in- 

 terrupt the publication of the " Lectures " and to com- 

 plete the Vertebrate half of the proposed work first. This 

 appeared in 1871, as a " Manual of the Anatomy of Verte- 

 brated Animals." 



A period of incapacity for any serious toil prevented 

 me from attempting, before 1874, to grapple with the im- 

 mense mass of new and important information respecting 

 the structure, and especially the development, of Inverte- 

 brated animals, which the activity of a host of investiga- 

 tors has accumulated of late vears. 



That my progress has been slow will not surprise any 

 one who is acquainted with the growth of the literature 

 of animal morphology, or with the expenditure of time 

 involved in the attempt to verify for one's self even the 

 cardinal facts of that science ; but I have endeavored, in 



