PREFACE. 
As latterly, Zootomists have given much greater attention to the 
invertebrate animals than formerly; 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 toa 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mental series of the several organs, appear no longer valid and 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, Hirudo, Unio, Astacus,.&s , 
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. 
