A MONOGRAPH OF THE EXISTING CRINOIDS 59 



fairly well at a temperature of 15° C. when the sea water had a salinity of 30 per 

 thousand. If the animals are to do well Gisldn says it is absolutely necessary to sup- 

 ply the bottom of the aquarium with small pebbles or shells to which they can cling 

 with their cirri, otherwise they fall over and soon die. 



In the crinoids, as in other echinoderms, there occur on the surface of the body 

 ciliary currents that serve the purpose of keeping the animal clean. These are most 

 pronounced in the interradial and interbrachial areas of the disk. On the median 

 parts of the interradial areas run centripetal (adoral) currents. These are checked 

 at the oral angle by the upright adambulacral fold on the margin of the ambulacral 

 furrow. Circular currents, therefore, running vertically, often arise in the oral angles. 

 Along the sides of this median centripetally directed current the grains of carmine 

 are carried radipetally, and in the area nearest the ambulacra) groove centrifugally. 

 As a result of this all the grains in the interradial area are gradually carried away 

 along the margin of the ambulacral furrow. Where the ossicles in tuis upright margin 

 are rather low, or where there is a gap between them, the grains are sucked down into 

 the centripetal current of the ambulacral groove, and in this way most of the parti- 

 cles, even those falling upon the interradial areas, at last reach the mouth. Some 

 of them, however, are carried out to the sides of the body and fall off the animal. 

 The interbrachial parts of the perisome have ciliary currents going in an oropetal or 

 radipetal direction. The oral pinnules are ciliated slightly toward their tips, and 

 the grains of carmine carried thither thus sink down on to the surface of the disk 

 and usually sooner or later reach the mouth. The anal cone is ciliated only at the 

 base, where weak currents may be observed running upward. The excrement con- 

 sists of the undigested particles from the intestine held together by a jellylike substance 

 and formed into large yellow mucus balls. These balls are too large to be transported 

 by the ciliary currents. They are removed from the surface of the disk, where they 

 usually fall, by the movements of the animal when moving to another place, or by 

 stronger movements of the water caused by the animal flexing its arms, etc. The 

 cirri and the dorsal side of the animal are not ciliated but are kept clean by the move- 

 ments of the animal. 



Gislen said that the currents described above refer to Antedon petasus, but 

 those of Hathrometra tenella var. sarsii are similar except that the marginal currents 

 directed centrifugally may be followed further out on the sides of the arms. In 

 Rhizocrinus the interradial currents are centripetal only. 



When not irritated a hungry Antedon sits immovable with the arms outstretched 

 and slightly curved upward. The pinnules extend out from the arm almost at right 

 angles, and the tentacles, arranged in groups of three on the pinnules, are stiffly 

 extended. 



If a plankton sample or a few grains of carmine with some crab liver be put into 

 the water of the aquarium the arms and pinnules at once become very active. The 

 tentacles all beat rapidly inward toward the ambulacral furrow. This, the margins 

 of which usually lie tightly pressed together, opens as soon as any grains fall upon it, 

 with a wavy movement proceeding toward or away from the mouth. The mouth, 

 previously only a narrow slit, opens to its widest extent and becomes circular. After 

 some time the tentacles straighten themselves out again, but now and then when 

 tiny grains fasten on the papillae the tentacles are whipped with rapid nervous move- 



