THE METAMORPHOSIS OF ECHINODERMS. 105 
ties have already shifted their position in Antedon, so that the 
relation of the pore to the ‘* Medianebene ” of the larva as a 
whole no longer has any morphological importance. 
If we compare Antedon in the “Cystid” stage with an 
KEchinid, we shall see that in both the water-pore may be stated 
to lie in the same interradius as the cesophagus, alongside 
which runs the mesentery holding the water-tube. From this 
point in both cases the alimentary canal runs round the disc 
till it again reaches this same interradius. It is true that only 
in Antedon does the intestine end here, but remembering that 
evidence which Echinids afford of the variability of the anal 
interradius, this is of small importance; and it is a bold thing 
to deny that the interradius of the water-pore is not homo- 
logous in these two forms, yet Echinid Plutei have the water- 
pore even more nearly in the “‘ Medianebene”’ than the Holo- 
thurians on which Seeliger relies. A further argument might 
be derived from the skeletal plates, but it is unnecessary to 
pursue it; it is enough for my purpose to show, not the impos- 
sibility of Seeliger’s assumption, but the extreme weakness of 
the evidence. In the larve of all Echinozoa the water-pore 
lies at the anterior end of the dorsal mesentery, though usually 
somewhat to the left. In Bipinnaria the mesentery becomes 
oblique (fig. 22), and the pore is consequently pushed to the 
left of the ‘‘ Medianebene” ; in Antedon this mesentery is still 
more oblique at the time the pore appears, and consequently 
the latter is situated further still over on the left side—even, 
indeed, on the ventral surface. This, at least, appears to me 
to be a far easier explanation of the phenomena than Seeliger’s 
supposition that the pore has changed its interradius. 
A change of position of the union of the water-tube with 
the water-vascular ring would be a good deal more difficult to 
detect ; but in the entire absence of any evidence that it has 
taken place, we are not justified in assuming it as the cause 
of the straight course of the former, so long as any other 
explanation is possible. That another cause is not only con- 
ceivable, but actually supported by ontogeny, I shall now 
endeavour to show. 
