THE METAMORPHOSIS OF ECHINODERMS. ee 
into the dorsal sac (fig. 30) often shrivels up and forms, with 
the adjacent mass of yellow cells, a compact knot of tissue 
seemingly composed mainly of nuclei; even this, however, is 
instructive, since it presents a close resemblance to the dorsal 
organ as it projects into the dorsal sac and axial canal in post- 
larval stages. 
The water-tube les throughout its course close against the 
stomach, and finally enters the water-vascular ring adradially— 
being, when the animal is viewed from the right (future aboral) 
side, on the left side of its interradius. 
When the time for metamorphosis is reached, the hydroccel 
seems to have moved even beyond the middle of the left side, 
and the ambulacral surface is directed somewhat backwards, 
as in Bipinnaria asterigera; but at the same time the 
original apex is pushed over to the right, and the mesentery 
separating the right and left body-cavities becoming somewhat 
oblique, its plane still continues to be about parallel to that 
of the ambulacral surface, and at right angles to the axis of the 
adult cesophagus. 
The larva at this stage creeps about on the bottom by 
means of its five primary tentacles, the larval arms elevated, 
and the thin membranous “amnion ” spread out like an um- 
brella, supported on the spines of the young Echinid. 
The actual metamorphosis is accomplished very rapidly ; 
the “amnion” contracts and is absorbed, while the spines 
which were embedded in it become erect ; then the larval cwso- 
phagus is absorbed, and the spicules of the larval arms are 
(usually) broken off by the force of the accompanying con- 
tractions of the ectoderm—so that in less than an hour a 
perfect Pluteus is transformed into a small rounded Echinid, 
in which radial symmetry entirely replaces the bilateral sym- 
metry of the larva. This young Kchinid is usually rendered 
extremely opaque by a species of histolysis, which begins in 
the Pluteus with the proliferation of cells into the cavity of 
the stomach, and afterwards extends to other tissues, render- 
ing the examination of the internal organs extremely difficult 
Exactly the same thing usually occurs in the larva of Antedon, 
