STUDIES IN THE EMBRYOLOGY OF EOHINODERMS. 435 
tentacle ”) is about to be embraced. Between the first and 
second terminals, the second adambulacral plates are now 
visible ; the first pair belonging to this interradius, and both 
pairs of other interradii lie at too deep a level to be shown in 
this drawing. 
If my description has been followed up to this point, it will 
be an easy matter to follow the plates, which I have identified 
as the radials and terminals, into the stage represented in 
fig. 6, and the correctness of the identification will then have 
been sufficiently proved. 
Asterids. — In the small Bipinnaria seen in fig. 14 the five 
terminal plates are already present, though the lobes of the 
hydrocoel are not yet developed. In Asterina, however, the 
primary tentacles are formed before any skeletal plates appear, 
and certain plates in connection with the water-vascular ring 
appear as early as, if not earlier than, the terminals. These, 
however, need not detain us now. 
Fig. 14 shows clearly that the terminals are developed round 
the left enterocoel, as in Ophiurids, and I have supplemented 
this view of the whole larva by sections through this and 
another form of Bipinnaria (B. asterigera), in both of which 
this relation of the plates to the enterocoel was quite evident. 
The case of Asterina, which offers some difficulties, will be 
considered presently. 
Up to the present time it has, I believe, been invariably 
assumed that the terminals of Asterids and Ophiurids belonged 
to the right enterocoel. No attempt, so far as I know, has 
been made to prove this for the latter group, but for Asterids 
we have the authority of Agassiz (2, p. 32), and it is worth 
while to spend a few moments in explaining how his mistake 
arose. At the time when he wrote, the ultimate fate of the 
two primary enterocoel pouches was still very imperfectly 
understood, and he believed that the left pouch (“ left water- 
tube,” as he calls it) 1 gave rise solely to the water-vascular 
1 It will be noticed that Agassiz’ use of the term “ water-tube ” is very 
different from that adopted in this paper. Here it is substituted for the 
