THE METAMORPHOSIS OF ECHINODERMS. 47 
I am indebted to the kindness of Professor McIntosh, who 
most generously placed at my disposal the resources of the 
Marine Laboratory at St. Andrews, where these larve are ex- 
ceedingly abundant. 
Of course it is not possible within the limits of this paper 
to give a detailed account of the development of every larva ; 
and indeed so much is already known of most of them as to 
render this unnecessary; I have therefore, in most cases, 
given only such a brief outline of the facts as would render 
the metamorphosis intelligible and bring into prominence 
those features to which I attach theoretical importance. This 
method has indeed the disadvantage of making my work appear 
somewhat sketchy and superficial, but this seemed preferable 
to loading the pages with a number of facts having no con- 
nection with the theoretical views expressed in the second part 
of this paper, or with a mere recapitulation of observations 
originally made and recorded by other writers. In the case 
of Holothurians, however, it has been necessary to describe in 
some detail the metamorphosis of Synapta, as almost all the 
points which seem to bear on the origin of this class have been 
overlooked by previous observers. 
To this alone is due the prominent position of the Holothu- 
riaus in the ontogenetic portion of this paper—the order in 
which the various groups are taken being throughout purely 
a matter of convenience, and wholly independent of all phylo- 
genetic considerations. 
Part I.—OnvToceny. 
A. HOLOTHURIANS. 
Metamorphosis of Synapta. 
The earliest stage of Auricularia with which we need here 
concern ourselves is one in which the full size is already 
attained, and the eleven pouches of the hydroccel (five large 
tentacles, five as yet small radial vessels, and one polian 
vesicle) distinctly visible. The right and left body-cavities 
are still small, and have hitherto been symmetrically arranged 
