4 PREFACE. 



the last chapter, to supply the most important recent ad- 

 ditions to our knowledge, respecting the groups treated of 

 in those which have long been printed. 



When I commenced this work, it was my intention to 

 continue the plan adopted in the " Manual of the Anatomy 

 of Vertebrated Animals," of giving a summary account 

 of what appeared to me to be ascertained morphological 

 facts, w^ithout referring to my sources of information. I 

 soon found, however, that it would be inconvenient to 

 carry out this scheme consistently ; and some of my pages 

 are, I am afraid, somewhat burdened with notes and ref- 

 erences. 



I am the more careful to mention this circumstance as, 

 had it been my purpose to give any adequate Bibliography, 

 the conspicuous absence of the titles of many important 

 books and memoirs might appear unaccountable and in- 

 deed blameworthy. 



My object, in writing the book, has been to make it 

 useful to those who wish to become acquainted with the 

 broad outlines of what is at present known of the morphol- 

 ogy of the Invevtehrata ; though I have not avoided the 

 incidental mention of facts connected with their physiol- 

 ogy and their distribution. On the other hand, I have ab- 

 stained from discussing questions of aetiology, not because 

 I underestimate their importance, or am insensible to the 

 interest of the great problem of Evolution ; but because, 

 to my mind, the growing tendency to mix up setiological 

 speculations with morphological generalizations will, if 

 unchecked, throw Biolo^cv into confusion. 



For the student, that which is essential is a knowledge 

 of the facts of morphology ; and he should recollect that 

 generalizations are empty formulas, unless there is some- 

 thing in his personal experience which gives reality and 

 substance to the terms of the propositions in which these 

 generalizations are expressed. 



