preface. t 



Preface to the Second Editioit. 



The most important discovery made since this book was 

 published is that the two lowest mammals, i.e., the duck- 

 bill and Echidna, both lay eggs which are introduced into 

 the mammary pouch, where the young are hatched in a 

 very rudimentary condition; the eggs have a soft jiarch- 

 ment-like shell, and in the case of the spiny ant-eater, or 

 Echidna, are nearly an inch {\ij-2\ cm.) in length. Refer- 

 ences to these points are incorjjorated in the text. More- 

 over, the nervous system of Echinoderms has been found 

 to consist of a delicate sheet lying under the soft integu- 

 ment, the thickenings seen by the naked eye forming the 

 nervous ring and radial branches heretofore regarded as 

 forming the nervous system of these animals. These and a 

 few other corrections have been made in the present edition. 



Providbnce, March, 1885. 



Preface to the Third Edition. 



Recent discoveries, now generally accepted, have ren- 

 dered necessary the following important changes in this 

 work: The Tunicates are placed in the same sub-kingdom 

 ( Gliordata) as the Vertebrates; the Merostomata and Trilo- 

 bites are regarded as together forming a class of Arfchrop- 

 oda called Podostomata; the sub-kingdom Arthropoda is 

 subdivided into six classes; the Malacopoda, MjTiopoda, 

 Arachnida, and Insecta being regarded as classes, instead 

 of sub-classes, as in the former edition. The orders of in- 

 sects have been increased from eight to sixteen. ISTumerous 

 minor corrections have also been made. 



Peovide::icb, Jmie, 1886' 



