STRUCTURE OF THE CYSTIDEiE. 



21 



to permit of a considerable amount of dilation. The ambulacral 

 orifices are ten in number, and enter the cup at the base of the arms. 

 They do not however immediately penetrate into the cavity of the 

 body, but ascend towards the top of the ventral elevation by five 

 tunnel-like passages, which lie under the external plates and extend 

 nearly to the apex of the dome. These passages are floored by a 

 series of plates, which form an elongated arch under them. They do 

 not reach the centre at the summit, but are discontinued at about 

 two-thirds the distance from the base of the arms. 



Fig. 7. A specimen of ^. rugosus, shewing the course of the ambulacral 

 channels from the orifices towards the summit. The exterior 

 plates have been removed, leaving the floors of the channels 

 visible. They terminate below the summit, at the points 

 indicated by the lines diverging from a. 



Fig. 8. View of the opposite side of the same specimen. 



In order to ascertain whether these tubes might exist in other 

 individuals, I procured several other specimens from the dealers in 

 fossils and, upon removing the outer plates, found the same structure 

 in all. Upon grinding down the summit of one of these, I find that 

 the tubes do not reach the top of the vault, but terminate at a short 

 distance below. 



Fig. 9. 



