VI PREFACE. 



Preface to the Sixth Edition. 



Besides some verbal and a few other corrections and 

 additions, no radical changes have been made in this 

 edition, except the addition of lists of the most important 

 works and essays, both on the general subject and on 

 the different branches and some of the more important 

 classes. 



Recent studies show that the Echinoderms have origi- 

 nated from some primitive worm in which there was a 

 body-cavity and a vascular system. The worm-like, foot- 

 less Holothurians such as Synapta, and other Apoda, were 

 the first to be evolved, and from these may have developed 

 the normal Holothurians, which were succeeded by the 

 Echinoids, the starfish, and perhaps finally the Crinoids, 

 whose radiate shape was due to their fixed mode of life. 

 If these views should prove correct the branch of Echino- 

 dermata should be placed between the Vermes and Mollusca, 

 and the succession of orders given on pp. 41-46 should be 

 reversed. 



The researches of Claus, and the close resemblance be- 

 tween the legs of Phyllopods and the swimming appendages 

 of certain Annelid worms, tend to show that the Crustacea 

 originated from an Annelid-like worm and that the Phyl- 

 lopods are perhaps the most generalized Crustacea. Hence 

 on pages 86 to 94 the sequence of the first three orders of 

 Crustacea should be as in the synoptical table on p. 85. 



Pkovidence, January, 1892. 



