THE DEVELOPMENT OF ASTERINA GIBBOSA. 385 
arises at the base of the terminal tentacle of the radial canal ; 
two stages in its development are given in PI. 28, figs. 142 and 
143. In the first we see a simple ectodermic involution; in 
the second we see a pit surrounded by columnar cells, pro- 
bably retinal, and filled up by closely fitting polygonal cells, 
which correspond to the layer of vitelligenous cells in an 
Arthropod eye. The existence of these cells has been vigor- 
ously denied by Cuénot (8), who maintains that we have only 
polygonal cuticular plates. My sections, however, remove all 
doubt on the subject; they show with perfect clearness that we 
have to do with cells, and their nuclei can be made out. This 
pit is the first only of the numerous pits which cover the “eye ”’ 
of the adult, which is really essentially a small rounded 
swelling at the very tip of the radial nerve. The method 
of preservation employed seems to have dissolved the pigment. 
The remaining sense-orgaus are the tips of the 
tube-feet and the terminal tentacle. A longitudinal 
section of a tube-foot is given in Pl. 28, fig. 150. This is 
taken from a specimen in which R equals ‘4 millimetre, but 
it holds true for specimens of a radius of a millimetre or more,— 
that is, for probably the first two months after the metamor- 
phosis. Comparing it with fig. 149, a similar section taken 
from a larva in Stage F, we see that the ectoderm at the tip 
has become thickened, and underneath it we can make out on 
each side a mass of nerve-fibrils. A powerful nerve leaves the 
radial nerve-cord to supply each sense disc; it would be more 
correct to speak of these branches as actual prolongations of 
the nerve-cord with its cells and fibrils; they are, indeed, the 
only conspicuous branches which it gives off. Some of the 
ectoderm cells of the sense dise have a peculiar regular cylin- 
drical form, which recalls that of the retinal cells. 
The facts above related justify the view that the whole radial 
canal with its tube-feet is to be looked on as one large branched 
tentacle, the main function of which was probably originally 
prehensile and therefore also sensory; and since a plexus of 
nerve-fibrils is in the adult found under the ectoderm all over 
the body, the central nervous system may be said to be a local 
