PMEFACE. 
V 
Preface to the Second Edition". 
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 parch- 
ment-like shell, and in the case of the spiny ant-eater, or 
Echidna, are nearly an inch (li-2| cm.) in length. Refer- 
ences to these points are incorporated 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. 
Providence, March, 1885. 
Peeface to the Third Editioist. 
Eecent discoveries, now generally accepted^ have ren- 
dered necessary the following important changes in this 
work: The Tunicates are placed in the same sub-kingdom 
{Cliordata) as the Vertebrates; the Merostomata and Trilo- 
bites are regarded as together forming a class of Arthrop- 
oda called Podostomata; the sub-kingdom Arthropoda is 
subdivided into six classes; the Malacopoda, Myriopoda^ 
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. Numerous 
minor corrections have also been made. 
Providence, June, l^S^. 
