THE DEVELOPMENT OF ECIlINOC'AliDlUM CORDATUM. 481 
has grown in size so much that it not only takes up the entire 
left side of the larva, but extends round on to the dorsal 
surface and also on to the ventral surface of the larva, 
displacing the anus {a., fig. 15) to the right side. It becomes 
thickly studded with conical adult spines, which fit so closely 
together that the whole presents the aspect of a tesselated 
pavement (fig. 11). In two points the larva differs con- 
spicuously from the larva of Echinus esculentus at the 
corresponding stage. (1) The larva of Echinus develops a 
series of plates on its right side which bear small four-sided 
clavate spines and three pedicellarite, whereas the larva of 
Echinocardium shows no trace of these extra-amniotic 
spines and pedicellarige. (2) The echinus-rudiment in the 
larva of Echinus grows in size but is confined to the left 
side and it develops only four adult spines in each interradius, 
whereas in the larva of Echinocardium the Echinus-rudi- 
ment develops many spines and extends far over both on to 
the dorsal and ventral surfaces of the larva (figs. 14 and 15). 
At some time during the fourth week the critical epoch in 
the development of Echinocardium cor datum is reached 
and the metamorphosis of the larva into the imago takes 
place with startling rapidity. I have often watched a larva 
with all its processes except the aboral spike fully developed, 
change into an urchin inside half an hour. The roof of the 
amniotic cavity, which we may term the amnio tic veil, 
becomes torn, and the tentacles and spines of the Echinus- 
rudiment emerge. The epithelial skin of the larval arms flows 
back into the body leaving the supporting rods to project as 
bare spines. I use the word flow advisedly for this is the 
appearance which is presented : the epithelial covering often 
becomes massed into drops, and it is at once evident that the 
separation of its component cells one from another is a mere 
temporary and superficial phenomenon, and that the epithelium 
as a whole behaves like a film of fluid. The projecting spines 
soon become broken off and the little urchin becomes launched 
on its career. The adult mouth appears to be open from the 
beginning, whereas in Echinus it is covered with an epi- 
