210 MANUAL OF ZOOLOGY. 



branches or " pinnulae " on both sides ; and they contain the 

 so-called "coeliac" and " subtentacular " canals, these being 

 tubular extensions of the cavity of the body. The upper 

 surface of the arms and pinnulae is covered with a soft mem- 

 brane, and below this are placed the re- 

 productive organs. The generative organs 

 are therefore not placed within the calyx, 

 and it follows of necessity that there is no 

 generative opening or "ovarian aperture" 

 in the walls of the calyx. The ventral sur- 

 faces of the arms and pinnulae are furnished 

 with grooves, which in the living species are 

 seen to be covered with vibratile cilia. The 

 brachial grooves coalesce till they constitute 

 five primary grooves, which are continued 

 from the bases of the arms to the mouth. 

 The action of the cilia gives rise to a con- 

 stant current of sea-water, bearing organic 

 matter in suspension ; and this current pro- 

 ceeds from the brachial grooves to the 

 mouth. In this way the animal obtains its 



Fio2.-Portion of an ' As the baSCS f . 



arm of Piatycrinus, rated from the mouth by an intervening 



showing the ralpm- ^^ it ^ follows ^ thfi 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 sur- 

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

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

 ter-radial spaces (/". e. t one of the intervals between two of the 

 arms). This tube is almost certainly anal, but good observers 

 regard it as discharging the functions of both mouth and anus. 

 However this may be, the brachial grooves are certainly not 

 continued over the ventral surface of the calyx, but stop short 

 at the bases of the arms. Hence they are continued as covered 

 passages or tunnels to a central point in the ventral surface of 

 the disc. Here is placed the mouth, concealed by the cal- 

 careous plates of the perisome. 



The dorsal surface of the " calyx " of the Crinoidea is com- 



