28 



STRUCTURE OF THE CYSTIDE^. 



Figure 9 is a view of a section made transversely through the 

 upper half of the ventral dome of a specimen. The unshaded border 

 shews the great thickness of the plates of the vault. The tubes and 

 visceral cavity are filled with soft light-brown rock, m is the mouth, 

 which is cut through ; at 1 is a tube, the floor of which has thinned 

 out so, that it already communicates with the visceral cavity; a2 is 

 another, with the floor remaining ; 3 and 4 are in communication 

 with the interior, although in consequence of the truncation being 

 a little oblique, they are exposed at a lower level than the others ; 

 5 is one with the external plates removed, leaving the bottom of 

 the channel exposed down to the arm. The floor extends upwards 

 four lines and a-half, and thins out just before it reaches the level of 

 the truncation. 



The important additional fact established by these specimens, and 

 first ascertained by Mr. Huxley, is that at least in this species the 

 ambulacral vessels, after entering the body, turn upwards towards 

 the centre of the summit. 



Fig. 10. 



Fig. 11. 



Fig. 12. 



5. Actinocrinus stellaris (DeKoninck) has twenty ambulacral orifices 

 which enter the body from the second or third plates of the tertiary 

 rays. The ventral dome is very large, and at its base forms five 

 projecting lobes, in each of which are four of the orifices. Figure 10 

 represents one of the lobes of a specimen in my possession. One 

 of the orifices (fig. 10) is perfect, and shews that the margin of the 

 upper half is formed of five small plates, while the lower side lies 

 in the excavation in the tertiary radial plate. This excavation is 

 simply the ambulacral groove, which is here continued into the 

 body. Fig. 11 is an enlarged view of another orifice. A fragment 

 of the vault of this species, which is empty, exhibits a broad rounded 

 furrow, bounded on each side by two angular ridges running towards 

 the summit (fig. 12). It occupies the position of one of the internal 

 channels in A. amphora, and no doubt, as in that species, served to 



