THE DEVELOPMENT OP ASTEEINA GIBBOSA. 267 



arises at the base of the terminal tentacle of the radial canal; 

 two stages in its development are given in PI. XXI, 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 Cuenot (3), 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-organs are the tips of the 

 tube-feet and the terminal tentacle. A longitudinal 

 section of a tube-foot is given in PI. XXI, 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 j 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 ofi'. Some of the 

 ectoderm cells of the sense disc 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 



