THE METAMORPHOSIS OF ECHINODERMS. 1238 
the origin of the Pelmatozoa are of an extremely speculative 
character, and will require a great deal of evidence to support 
them ; but a too rigid adherence to the apparent teachings of 
the larva of Antedon is really open to equally strong objections. 
In opposition to those who, like Seeliger, regard the ontogeny 
of this larva as a safe guide to phylogeny, it cannot be too 
strongly urged that at present we only know the development 
of one Pelmatozoan larva, and that we have no reason for 
regarding this as specially primitive ; on the contrary, the very 
early loss of bilateral symmetry in the arrangement of the 
body-cavities, as well as the entire absence, before fixation, of 
either oesophagus or intestine, point most conclusively to its 
being a much altered form. 
Origin of Holothurians. 
Until phenomena similar to those which I have described 
in Synapta have been observed in other Holothurians, it 
would be rash to attempt more than a cautious suggestion as to 
the origin of this class; Even in Synapta my investigations 
are unfortunately far from complete, but so far as they go 
they appear to teach a fairly definite lesson. 
In the light of our knowledge of other Echinoderms, we are 
justified in regarding the asymmetrical movement of the atrial 
aperture, and the formation of the mesentery of the water-tube 
by the left body-cavity (a portion of which grows round the 
cesophagus for this purpose), as indications that in the ancestor 
of Holothurians also the movement of the esophagus into the 
left side has taken place. But, on the other hand, although 
we cannot determine the exact limits of the larval body-cavities 
in adult Holothurians, we can certainly assert that the left 
cavity is not symmetrically disposed about the cesophagus, and 
that the mesentery of the water-pore is not, as in other 
Echinoderms, at right angles to the mesentery of the stomach 
and intestine (dorsal mesentery of the larva), but nearly in a 
straight line with it. These facts, coupled with the observed 
migration of the atrial aperture towards the anterior pole, 
