1888.] 
105 
[Fewkes. 
and interambulacrals of sea-urchins are homologous, that the diffi- 
culty is very much lessened in our comparisons of sea-urchins and 
starfishes. 
The former may be known as echinoderms in which serially ho- 
mologous plates in one case grow externally to, in the other in- 
ternally to, the water system. 
In the starfish we have ambulacral rafters joining at the middle 
line, while the so-called interambulacrals retain an embryonic con- 
dition and in that they do not approach the middle line, remaining 
at the border of the starfish ray as the adambul acral. In the 
sea-urchin, however, the condition is just reversed, for the ambu- 
lacral rafters are here wanting, and are only represented in a mouth 
plate known as the auricle, while the alternate ambulacrals have 
grown to join at the middle line outside the water-vascular system 
of tubes. It will thus be seen that I regard the ambulacral rafters 
of the starfishes as unrepresented in the sea-urchin, while the ad- 
ambulacrals are homologous to those plates which are ordinarily 
called the ambulacrals. The adambulaerals in the starfishes are 
only ambulacrals, which alternate with ambulacral rafters, and are 
serially homologous with them. From the fact that they lie at first 
between the ends of the ambulacrals the old name of interambula- 
cral is a convenient one by which to designate these plates, re- 
membering, however, that they are the same as the ambulacral. 
The plates in the Echinoids called adambulaerals which lie be- 
tween the system of plates generally known as ambulacral are 
regarded as the same as the marginal plates of the starfish. An 
Echinus-like condition of these plates is seen in the pentagonal 
larva of Asterina. 
We know from Ludwig’s paper the early form of the oral plates 
in Asterina. Although my studies of later larvae differ somewhat 
from his, yet still they agree as to the place of formation of the 
earliest formed plates. We know also the history of the growth 
of the oral plates in Asterias . 1 We have enough observations to 
compare the manner of growth of these plates in genera widely dif- 
ferent and representatives of the two great groups into which star- 
fishes have been divided. 
The mode of development of the oral plates of both Asterias and 
Asterina seem to me to show that both adambulaerals and ambu- 
lacrals are simply modifications of the same elements, and that in 
1 The relationship of these plates in Asterias will be seen in a paper on the plates of 
this genus. Bull. Mus. Corap. Zool., Yol. xm, 3. 
