1888 .] 
115 
[Fewkes. 
bacise may have fastened to some floating body, but why not Echi- 
narachnius in the same way ? It looks as if Arbacia may be free 
swimming even after the absorption of the pluteus. In those Arba- 
ciffi which were raised in aquaria I have never seen one rise in the 
water from the bottom of the glass, after absorption of the pluteus, 
and I have never seen a flapping motion of its spatulate spines, 
but the fact that in confinement it clings to the bottom is offset by 
the observation that I have often found it in numbers in pelagic 
fishing where the soundings were very deep. 
The larval Arbacia is thought to resemble the ancestral form 
from which have sprung the stellate and echinoid groups of eeliino- 
derms. This problematical ancestor (Archiarbacia) may be sup- 
posed to have a disk-shaped body on the periphery of which are 
placed spatulate or lappet-like flaps supported by a calcareous axis. 
These marginal flappers are separated into five zones by the water 
tubes which extend beyond the margin of the disk in the form of 
embryonic tentacles. The calcareous spines which support the lap- 
pets are articulated with calcifications in the body. The mouth is 
situated in the centre of the under side . 1 Motion is produced by 
a flapping of the marginal lappets. While this problematical an- 
cestor of stellate and echinoid echinoderms is marvellously like 
the Epliyra or ancestral form of the Acraspeda, it differs from it 
in the presence of calcifications. These calcifications become so 
heavy that the animal soon falls to the ocean floor, and later the 
likeness to the ancestor is lost. I am not prepared to defend the 
proposition that the problematical young of the stellate and echi- 
noid is a direct descendant of the Ephyra. I think it is, but there 
are many difficulties to be explained. I suggest the name Archi- 
arbacia for this ancestral stage. 
Among other echinoderms, as in the ophiurans, we have the same 
primary spines as in the starfishes and echinoids, which although 
homologous in position have possibly another function. 
It is a well known fact that the young of certain ophiuran gen- 
era bear at the tip of the rays certain hook-like spines, which differ 
very considerably in form from the ordinary spines of the genus 
upon which they are found. Such spines occur in Ophiocoma and 
Ophiothrix and have been well figured and described by several au- 
thors. They seem to be confined to those genera of ophiurans 
1 A primary fundamental difficulty is the coelenterate character of the Ephyra, while 
this larva has a body cavity. 
