4 PREFACE. 



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

 ditions to om- knowledge, respecting the groups treated of 

 in those which have long been printed. 



^^hen I commenced this work, it was mv intention to 

 continue the plan adopted in the " Manual of the Anatomy 

 of Tertehrated Animals,'' of giving a summary account 

 of what appeared to me to be ascertained morphokgical 

 facts, without referrincr to mr sources of information. I 

 soon found, however, that it would be inconvenient to 

 carrv out this scheme consistentlv : and some of mv pao^es 

 are, I am afi'aid, somewhat bm'dened with notes and ref- 

 erences. 



I am the more careful to mention this circumstance as, 

 had it been my pui^ose to give any adequate Bibliograj^hy, 

 the conspicuous absence of the titles of many important 

 books and memoirs might appear unaccountable and in- 

 deed blame worth V. 



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 moiiDhol- 

 02T of the Invertehraia : though I have not avoided the 

 incidental mention of facts connected with their physiol- 

 osTT 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 ^etiological 

 speculations with morphological generalizations will, if 

 unchecked, throw Biology 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. 



