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 ; 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 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 



