THE DEVELOPMENT OF ASTERINA GIBBOSA. 387 
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 Cuénot 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 perihemal 
epithelium has come into intimate connection with the nervous 
matter. Pl. 29, 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 coelomic nervous fibrils. So we 
may say that from their aboral wall the perihzmal spaces give 
rise to muscles, and from their oral wall to the corresponding 
nervous tissue. I ought to mention in this place that 
Cuénot describes a canal leading from the perihemal space 
into the celom at the level of each ambulacral ossicle; also 
five pores leading from the outer perihemal ring to the celom. 
If these communications exist, they are certainly secondary, 
as there is no trace of them in my specimens; but as Cuénot’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 the nervous system 
has become better developed, the radial cords becoming gan- 
gliated, and the whole is removed from the exterior by invagi- 
nation, 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 vertebre, and thus the cavity of the 
arm is reduced, and this, with the simpler food, accounts for 
the disappearance of the pyloric czca. 
We have already pointed out that the lessened activity of 
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