THE DEVELOPMENT OP ASTERINA GIBBOSA. 269 



the aboral side of the nerve-cord, and Hamann (8) has shown 

 that the nerves for the ambulacral muscles arise entirely from 

 these. 



Now it has been for a long time suspected, and Cuenot has 

 finally proved it (4), that there is a similar but feebler develop- 

 ment of what we may call "coelomic nervous tissue" takes 

 place in the Asterid. None of my specimens were old enough 

 to show this, though one can see (fig. 141) that the perihsemal 

 epithelium has come into intimate connection with the nervous 

 matter. PI. XXII, fig. 155, represents a transverse section of 

 the nerve-cord of a young Asterias; we see in it that this 

 epithelium has become thickened on each side of the median 

 septum ; one requires, however, a section of the nerve of a 

 fully grown adult to see the ccelomic nervous fibrils. So we 

 may say that from their aboral wall the perihsemal spaces give 

 rise to muscles, and from their oral wall to the corresponding 

 nervous tissue. I ought to mention in this place that 

 Cuenot describes a canal leading from the perihsemal space 

 into the coelom at the level of each ambulacral ossicle ; also 

 five pores leading from the outer perihsemal ring to the coelom. 

 If these communications exist, they are certainly secondary, 

 as there is no trace of them in my specimens; but as Cuenot's 

 results were founded on injection I am exceedingly sceptical 

 as to the existence of such openings. 



I have said above that the increasing importance of the 

 ambulacral muscles is the explanation of the evolution of 

 Ophiurids from Asterids. The Ophiurids have substituted 

 the quick powerful movements of these muscles for the slow 

 motions of the tube-feet. In correlation with this change the 

 nervous system has become better developed, the radial cords 

 becoming gangiiated, and the whole is removed from the ex- 

 terior by invagination, and thus the subneural space is really 

 a neural canal. The ambulacral ossicles have become firmly 

 united, each to its fellow, to form a series of vertebrae, and thus 

 the cavity of the arm is reduced, and this, with the simpler food, 

 accounts for the disappearance of the pyloric caeca. 



We have already pointed out that the lessened activity of 



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