/ 7.7.7 .U . 



Preface to the Second Edition. 



The in"-' important discover) made since this book was 

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

 bill and Echidna, both lay egga which are introduced into 

 tin- mammary pouch, where the young are hutched in a 

 very rudimentary condition; the eggs have a soft parch- 

 ment-like Bhell, and in the case of the -piny ant-eater, or 

 Echidna, are nearly an inch (11 2 j cm.) in length. B 

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

 over, the nervous Bystem of Echinoderms has been found 

 toe<' ■!' a delicate sheet lying under the soft integu- 



ment, the thickenings seen by the naked eve forming the 

 nervous ring and radial branches heretofore regarded as 



forming the nervous 83 Bte I' t hese animals. These and a 



few other corrections have been made in the present edition. 



Pbovtdrnce, March, 1 



Preface to the Third Edition. 



liYeent diseo\erie^, now generally accepted, have ren- 

 dered necessarj the following important changes in this 

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

 (Chordata) ;is the Vertebrates; the Merostomata and Trilo- 

 bites are regarded as together Forming a class of Axthrop- 

 oda called Podostomata ; the sub-kingdom Arthropods is 

 subdivided into six classes; the Malacopoda, Myriopoda, 

 Arachnida, and tnsecta being regarded as classes, instead 

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

 sects have been increased from eight to sixteen. Numerous 

 minor corrections have also been made. 



Providence, June, 1886' 



