1888 .] 
109 
[Fewkes. 
developraent which is known as the brachiolaria, and they preserve 
their relatively large size after the Asterias has taken on a stellate 
form. 
Among true starfishes these spines are not peculiar to Asterias, 
but occur also in Asterina, in the genus Leptasterias, which does 
not have a true free-swimming brachiolaria, and in one or two others. 
Among young ophiurans also, as in Ophiocoma, Ophiothrix and 
Ophiopholis the first-formed spines are relatively very large and in 
the genera mentioned have a peculiar shape which differs very con- 
siderably from that of later formed appendages of this name. 
Nor is this fact of the relatively large size of the primary spines 
confined to the starfishes and ophiurans, for in the sea-urchins we 
find a similar condition. In the “ sand dollar,” Echinarachnius, 
where the spines of the adult are very small, we find that the first 
formed spines are very conspicuous and relatively of monstrous size. 
Here as in Asterias also these appendages are very early formed in 
the development of the animal, and before the arms of that larval 
condition known as the pluteus are absorbed, the beginnings of these 
structures can readily be seen in the body of the growing sea-ur- 
chin. 
In Arbacia also the first- formed spines are very different in form 
from the adult spines and have relatively a larger size. In this ge- 
nus the first-formed spines are spatulate and form a ring about the 
rim of the disk. 
In the ordinary starfish, as pointed out by other investigators 
these early formed spines are more or less fan-shaped, while in 
Ophiocoma they have a hook-shaped form. In Asterias they are 
flattened and spatulate. In all cases they differ in form markedly 
from the later-formed appendages, found on the body of these gen- 
era. None of the spines which appear in the late larval stages of 
starfishes approach them in relative magnitude, and none have the 
characteristic shapes peculiar to those first formed. 
It is believed that the form and size of these early formed spines 
are so characteristic that we are justified in regarding them as 
embryonic in their nature. What the fate of these structures is, 
whether they become permanent body spines losing their predom- 
inance by the increase in the size of the plates to which they are 
connected, or whether they are absorbed and their place taken by 
others, observation has not yet taught us. In some instances, prob- 
ably the latter ; in others probably the former, is the case. At all 
