THE DEVELOPMENT OF ASTERINA GIBBOSA. 381 
subsequently showed that in the stone-canal of Holothu- 
rids and KEchinids the direction of the current is inwards. 
He examined the stone-canal cut out of the living animal; I 
have confirmed his result by a somewhat more satisfactory 
method. I kept Amphiura squamata living for several days 
in sea water, carrying in one case carmine, and in another 
lamp-black in suspension; and on cutting sections I found 
these particles in the pore-canal, and in some cases apparently 
ingested by the cells lining it. In view of Ludwig’s researches 
Cuénot comes in a later paper (4) to what I believe to be the 
correct solution of the question of function. He there suggests 
that the flagella lining the stone-canal are always tending to 
produce an inward current, and that thus the turgidity of the 
whole water-vascular system is kept up. [This is practically 
the old view; except that he does not assert a continuous 
inward current.— December, 1895. | 
It is obvious from the structure of the valves of the tube- 
feet that, in consequence of the ambulatory movements, there 
must be a slow loss of fluid. The ampulla and the tube-foot 
are shut off from the canal leading into the radial water-vascu- 
lar canal by a pair of valves opening only inwards. Conse- 
quently during the contraction of either ampulla or tube-foot 
the two act together as a closed system, since no fluid can escape 
into the radial canal. The existence of the valves however shows 
clearly that fluid occasionally enters the tube-foot, and this can 
only be rendered possible by a slow loss of turgidity owing to 
the osmosis of the contained fluid when under pressure. This 
is confirmed by considering the case of Ophiurids, where, the 
tube-feet having lost their ambulatory function, the madre- 
porite has only one or at most two pores, and the calibre of the 
stone-canal is exceedingly narrow. 
The dermal branchiz arise when the star-fish has reached a 
diameter of about 1°5 millimetres (R equal *85 millimetre). 
We see that the branchia is only a very thin piece of the body- 
wall produced into a finger-like process (Pl. 23, fig. 98). 
Around the base of the branchia is a peribranchial space lined 
by flattened epithelium: this space, as Cuénot has rightly 
