STUDIES IN THE EMBRYOLOGY OE ECHINODERMS. 433 
admits that this statement rests only on the relatively small 
size of the terminals in the young pentagonal embryo. On p. 
132 he further states that the first and second adambulacral 
plates appear before the terminals, but probably after the 
radials. I have myself worked out the first appearance of 
these plates in Amphiura squamata, and am convinced 
that the plates in the bilateral stage, which Fewkes took for 
the adambulacrals ( 10 , p. 131, figs. 7, 8, and 10), are really 
terminals ; but as the whole subject is far more easily studied 
in the various forms of Plutei, I shall begin with a description 
of these. 
Soon after the formation of the water-tube (stone-canal), 
and shortly before metamorphosis, ten skeletal plates make 
their appearance simultaneously in the mesoderm surrounding 
the posterior enteroccels; five plates accompany each cavity, 
and are arranged along it in a straight line antero-posteriorly, 
three being dorsal and two ventral, as shown for the left side 
in the diagram (fig. 2) ; it will be convenient to state at once 
that those on the right side are the radials, and those on the 
left the terminals. In some few cases the terminals appeared 
before the radials, and several times the dorsal plates of both 
series appeared before the ventral. It is possible that these 
peculiarities may be constant for certain forms of Plutei, but 
of this I have not sufficient evidence. Sometimes simul- 
taneously with, but usually some hours later than, the radials 
and terminals a plate appears in the middle of the right side, 
which is destined to form the dorso-central. Later again than 
this there arises another plate on the left side, just in front of 
the water-pore; it is the madreporic plate, or first oral (fig. 3). 
These twelve plates are all that were ever observed in the 
bilateral larva. It is not until after metamorphosis has com- 
menced that the adambulacral plates make their appearance. 
The development of the plates in Amphiura squamata 
so closely resembles that already described, that the details of 
it will be postponed to a future paper. The opacity of this 
larva, the excessive development of the larval skeleton, and 
certain irregularities to which its skeleton seems peculiai’ly 
