MONOGRAPH OF THE EXISTING CRINOIDS. 593 



the anterior and the right anterior rays. This migration of the mouth is accom- 

 panied by the shoving of all the ambulacral grooves outward to the periphery of 

 the disk, where they form a liorseshoe-shaped furrow, the opening of the horseshoe 

 being first between the two posterior rays and later, as a result of the disappear- 

 ance of the ambulacral grooves from the two posterior rays, between the right and 

 left anterior rays. If the mouth moves to the right the opening of the horseshoe, 

 because of the loss of the ambulacral structures on the opposite (left posterior) ray, 

 is between the left anterior and right posterior rays, and if the suppression of 

 the ambulacral structures is carried to an extreme, between the anterior and right 

 anterior rays. 



By these changes the bilateral symmetry on either side of a plane passing 

 through the anterior ray, the mouth, the anal tube, and the middle of the posterior 

 interradius, in most types affecting only the disk, first becomes extended to all the 

 other structures by the dwarfing of the two posterior rays, and then changes over 

 into a bilateral symmetry on either side of a plane passing through the middle of 

 the right anterior interradius, the mouth, the central anal tube, and the atrophied 

 left posterior ray. 



Occasionally comatulids are found which are six-rayed, with the additional 

 ray inserted behind the left posterior. This variation is especially common in 

 Tropiometra picta. In two genera, Promachocrinus and Thaumatocnnus, an 

 additional ray is normally inserted to the left of all five rays, so that a 10-rayed 

 animal results ; but the basal structures remain pentamerous, so that the basal rays 

 lie under the center of alternate radials. 



HABITAT OF THE LITTORAL CRINOIDS. 



Though a detailed discussion of the relation between the recent crinoids as a 

 whole and their physical, chemical, and ecological environment will be reserved 

 until later, it seems advisable here to indicate the conditions under which the 

 mmierous littoral species are found along the shores, in order first of all to em- 

 phasize the importance of these animals in the littoral fauna of the present day, 

 and secondly to show in what diverse surroundings they exist. 



Except on sandy and exposed mudtly shores littoral crinoids occur in all 

 possible situations. Their one essential requirement is pure, well-aerated water 

 having a relatively high minimima salt content and well provided with minute 

 plankton organisms; and wherever this condition is met within the range of the 

 littoral species they may be looked for in the water just below the low-tide mark, 

 or in the tide pools ; sometimes, even, they occur in situations left bare at low tide. 



Along the shores of the Indian Ocean from southeastern Africa, Madagascar, 

 and Mautitius to Suez, India, and the Malay Archipelago, along all the coasts of 

 Australia, especially in the north, and thence northward to Fokien and southern 

 Japan, littoral comatulids of many species are abundant, particularly on reefs and 

 rocky shores, less commonly in sheltered situations and in eelgrass, though their 

 occurrence is commonly more or less local, and they are frequently not to be found 

 in apparently ideal places. 



