vi 
PltEFACE. 
Preface to the Sixth Edition'. 
Besides some verbal and a few other corrections and 
additions, no radical clianges liave been made in this 
edition, except the addition of lists of the most important 
works and essjiys, botli 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 Chius, 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. 
Providence,- January, 1892. 
