Fewkes.] 
104 
[Dec. 5, 
ambulacrals or adambulacrals originate in Asterina as in Asterias 
between the marginal extremities of the ambulacral rafters, and 
differ from other plates of the body and arms. I think we can 
associate them together and regard them as serially homologous, 
and suggest the following explanation for their difference in form. 
They are different parts of a supposed ring, surrounding the water 
vessel, and are calcified in different regions. While in the case 
of the ambulacral rafters the portion above the tube is calcified, in 
the adambulacrals the lateral regions are formed into plates. 
It has been shown that the oral plates of Asterina are formed 
like those of Asterias and that there is no fundamental difference 
between them great enough to base a classification upon. As we 
have seen, the manner of development of the mouth plates of both 
Asterias and Asterina, is similar and we may conclude that in both 
the mouth parts are homologous and. that in the one case they as- 
sume the form of adambulacrals and in the other of ambulacrals, 
but that in both instances they are serially homologous calcifica- 
tions. 
It will be noticed that in both Asterias and Asterina the adam- 
bulacrals form between the distal ends of the ambulacrals. While 
they are serially to be compared, the adambulacrals simply differ 
from ambulacrals in their time of development and their limitation 
to the border of the ray. They have spines which ambulacrals 
want, but there seems no good reason to regard their homologies 
as distinct. We have, in other words, in the starfish arm, alternate 
pairs of ambulacral plates on the under side of the arms. Some 
of these extend to the median line and form the true ambulacrals, 
while others remain on the periphery and form the so-called ad- 
ambulacrals, or interambulacrals, of authors. 
If now we turn to the sea-urchin we find the case just reversed. 
The alternate plates which in the starfish are on the periphery are 
here extended to the median line below or outside 1 the water- vas- 
cular system, while the ambulacrals of the starfish are wanting in 
the adult sea-urchin. The marginal plates in both instances are 
largely developed and in the sea-urchins form the so-called inter- 
ambulacrals. It seems, therefore, since ambulacrals of starfishes, 
VTt may of course be asked, How do you know that the ambulacrals, so called, of the 
urchins, are not simply the lower (outside) half of the ring of which the ambulacrals of 
the starfishes are the inside ring? We do not know that such is not the fact, but it is 
thought the spherical development of the urchin can best be explained on the supposition 
that the ambulacrals of the urchin are homologous with the adambulacrals of starfishes . 
