INTRODUCTION. Xlll 



true that there is not a single known species in which all 

 these segments are found in a distinct and tangible con- 

 dition there being in all the forms, more or fewer of 

 them so inseparably united together as to offer no other 

 means by which to predicate their existence, than those 

 already alluded to yet, .on the other hand, there is not 

 one which may not be found distinctly formed in some or 

 other of the species. The appendages, too, which have 

 already been slightly mentioned, are no less subject to the 

 most extraordinary variation both of form and office ; 

 many of them serving in one case the purposes of loco- 

 motion, in another the reception and preparation of the 

 food, in another the attachment of the branchiae, in 

 another the support and pjotection of the eggs. When, 

 therefore, we consider the almost endless diversity of 

 form, under which the species composing this class of 

 animals appear, the astonishing discrepancy which exists 

 in the forms and relative proportions of the different 

 regions of the body, and other parts of their organization, 

 for the performance of offices and functions equally various, 

 and see that all these diversities are produced only by mo- 

 difications of a typical number of parts, we cannot but be 

 struck by so remarkable and interesting an illustration of 

 the great economical law, as it may be termed, that 

 the typical structure of any group being given , the different 

 habits of its component species or minor groups are provided 

 for, not by the creation of new organs or the destruction of 

 others, but by the modification, in form, structure, or place, 

 of organs typically belonging to the group. 



Of this law numerous examples will be exhibited in the 

 course of this work, in the structural characters of every 

 order and of every family ; but for the sake of offering 

 a single comprehensible illustration, the various modifi- 



