PREFACE TO THE THIRD EDITION 



IN the eleven years which have elapsed since the publication 

 of the second edition of this Text-book the science of 

 Zoology has made such advances that it has become necessary 

 to re-write considerable portions of this book. The brilliant 

 work of Dr Goodrich on the nature and development of 

 nephridia has caused us to reconsider our position with regard 

 to the nature of these organs and we have now confined the 

 name nephridium to the excretory organs of Platyhelminthes, 

 Nemertinea, Rotifera, Annelida and Amphioxus, and have no 

 longer applied the term to the excretory organs of Arthropoda, 

 Mollusca and Brachiopoda. 



The chapter on Protozoa has had to be radically changed 

 and we are indebted to Mr Dobell for valuable hints on this 

 subject. The newer discoveries in the laws of inheritance are 

 dealt with in the Introduction. 



The chapters dealing with Platyhelminthes, Nemertinea, 

 Rotifera and Nematoda have been moved to a position follow- 

 ing Coelenterata and preceding Annelida. A short chapter 

 on Gephyrea has been added and the chapter on Arthropoda 

 has been largely rewritten. In the second edition detailed 

 accounts of a type of Arachnida and of Insecta were given 

 to these we have now added a detailed account pf the anatomy 

 of the Crayfish as a type of the Crustacea, as experience 

 has shown that only by the detailed study of types can the 

 elementary student form clear images of Animals as " going 

 machines." Indeed throughout the book we have endeavoured 

 to avoid describing structure unless function was indicated at 

 the same time. 



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