ANNULOIDA : ECHINODERMATA. 



173 



of the arms to the mouth. The action of the cilia gives rise to 

 a constant current of sea-water, bearing organic matter in solu- 

 and this current proceeds from the brachial grooves to 



tion 



Fig. &),. Platycrinus tricontadactylns. Carboniferous. The left-hand figure shows 

 the calyx, arms, and upper part of the stem, and the figure next this shows the 

 surface of one of the joints of the column. The right-hand figure shows the 

 proboscis ; and on p. 172 is a magnified figure of part of one arm with its pinnulse. 



the mouth. In this way the animal obtains its food. As the 

 bases of the arms are separated from the mouth by an inter- 

 vening space, it follows that the brachial grooves are continued 

 over the ventral surface of the calyx, till they reach the oral 

 opening. 



There is no doubt that it is by the above arrangement that 

 the living Crinoids obtain their food, and the mechanism seems 

 to have been essentially the same in many extinct species. In 

 the Palaeozoic Crinoids, however, there seems to have been a 

 modification of this arrangement. In these forms, the arms 

 have much the structure of those of the recent Crinoids, and 

 are deeply grooved on their ventral surfaces. The ventral 

 surface of the calyx, however, exhibits no central aperture, but 

 only a proboscidiform tube, which arises from one of the inter- 

 radial spaces (i.e., one of the intervals between two of the 

 arms). This tube is probably anal, but good observers regard 

 it as discharging the functions of both mouth and amis. 

 However this may be, the brachial grooves are certainly not 



