PREFACE. 



As latterly, Zootomists have given much greater attention to the 

 invertebrate animals than formerlj ; and as, with these investigations 

 they have united, as much as possible, others upon the generation 

 and development of these animals, such a mass of material, composed, 

 in part, of entirely new and very remarkable facts, has accumulated, 

 that the manuals of Zootomy hitherto published are of a scale quite 

 inadequate to receive them. It is unnecessary, therefore, for me to 

 offer further reason for the task I have undertaken of arranging these 

 materials and reducing them to a systematic form. But the order 

 in which I have disposed them may not meet with general approval, 

 for, hitherto, in works of comparative anatomy, the organs, and not 

 the zoological classes, have served as the basis of the order pursued. 



But, in the present state of Science, and at least provisionally, it 

 appears to me that the anatomical order should not be followed, for, 

 the types, which, until now, have been recognized in the develop- 

 mental series of the several organs, appear no longer valid and 

 permanent. Indeed, extended researches made upon a great number 

 of animals, have shown that these types, hitherto regarded as express- 

 ive of fundamental laws, may almost be taken as the exceptions. 

 Such genera as Hydra, Lumbricus, Hlrndo, Unio, Astacus, &c., 

 can now no longer be regarded as the representatives of certain 

 animal classes or orders, for their organization is far from affording 

 the requisite type of that of allied animals. It appears now clearly 

 determined that the types of the development and disposition of the 

 various organs of the Invertebrata are more numerous and varied 

 than hitherto supposed, and that, in this respect, a rule wholly differ- 

 ent from that of those of the Vertebrata must here be applied. But as 

 the numberless details which we now possess upon the organization of 

 the Invertebrata, have not been thoroughly worked out and system- 

 atized in all the orders, it is really a task too difficult to here 

 distinguish the rule from the exception, and the type from that which 

 is only a secondary modification. 



