THE DEVELOPMEKT OP ASTBEINA GIBBOSA. 263 



subsequently showed that in the stone-canal of Holothu- 

 rids and Echinids the direction of the current is inwards. 

 He examined the stone-canal cut out of the living animal j 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 

 Cuenot 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 (except 

 in the Astrophytidse), the tube-feet having lost their ambulatory 

 function, the madreporite has only one or at most two pores, 

 and the calibre of the stone-canal is exceedingly narrow. 



The dermal branchise arise when the star-fish has reached a 

 diameter of about IS 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 XVI, fig. 98). 

 Around the base of the branchia is a peribranchial space lined 

 by flattened epithelium: this space, as Cuenot has rightly 



