AMBULACEA OF THE EECENT DIADEMATID^. Ill 



The following are ttie descriptions given of the ambulacra : — 



" In the ambulacral areas the primary tubercles, arranged in 

 only two vertical rows, increase regularly in size towards the 

 ambitus, where they, as well as the interambulacral tubercles, 

 are largest, and while occupying there nearly the whole of the 

 ambulacral plates between the poriferous zones, become reduced 

 on the abactinal surface to small secondary tubercles placed in 

 the centre of the plates, which carry, besides, a few small 

 miliaries or granules, occurring irregularly on the plates." 



" The poriferous zone is of nearly uniform width, from the 

 actinal edge to the apical system. There is no tendency to 

 expansion of the poriferous zone at the actinostome." It had been 

 noticed in dealing with the generic details, that " the poriferous 

 zone is narrow, the pores are in pairs arranged in. two vertical 

 rows." 



After studying the ambulacra of two dry specimens at the 

 British Museum, I am able to add to this description. At the 

 ambitus, the non-crenulate but perforated large tubercles are on 

 alternate plates of the same ambulacral zone, the intermediate 

 plates are smaller and carry from one to three large granules. 

 Towards the apex tliere is a tubercle on every plate, and one is 

 arger than the other. Near the radial plate the crowding of 

 the ambulacral plates soon begins to be seen ; and in the speci- 

 mens examined there is, on the contrary, a simple single row of 

 pairs of pores close to the peristome. There is no crowding 

 there whatever. 



As A. Agassiz has so well shown, the pairs of pores present a 

 remarkable appearance in being placed nearly throughout the 

 zones in two rows — an outer and an inner, and close together. 

 See the drawings on pi. vii. figs. 4 & 5, ' Challenger' E-eport. 



In order to examine the meaning of these two rows, which at 

 first would seem to have no relation to series of triplets, part of 

 ambulacrum II, near the ambitus, but above it, was chosen. It 

 became evident, after the application of benzole, that every 

 compound ambulacral plate is in relation with three of the pairs 

 of pores ; and that whilst the tubercle-bearing or larger plate has 

 two pairs of the outer row of pores and one pair of the inner row 

 associated with it, the plate immediately actinally, and which 

 has not a large tubercle, has two pairs of the inner row and 

 one pair of the outer row connected with its component plates. 

 (PI. Y. fig. 110 



