MONOGRAPH OF THE EXISTING CRINOIDS. 



517 



are now represented, with the exception of the pinnules. The orals form a low 

 pyramid, at the apex of which is the mouth. They have been reduced by rela- 

 tively one-third from their former size and have become separated from the 

 radials by the extension of a band of interradial plates, which tend gradually to 

 pass over into the perisomic plates. The tegmen thus is now approximately 

 comparable to that of Holopus, Ptilocrinus, and the young of Thaumatocrinus 

 renovatus as described by P. H. Carpenter. 



From this point onward the resorption of the orals proceeds rapidly, and 

 concurrently there is seen a progressively increasing differentiation between the 

 interradials and the ventral integument, the perisomic plates of which become 

 gradually resorbed. This resorption also involves the radianal, which has now been 

 left far behind by the protruding anal tube and is actually as well as relatively 

 diminished in size. The same relative decrease is true of all the dorsal elements 

 of the calyx, which have leaned outward, approaching more and more a horizontal 

 position, so that now they collectively serve merely as a shallow saucer for the 

 support of the visceral mass. 



The pentacrinoid is now almost ready to discard its column, the cirri having 

 developed to a length of 7 or 8 segments, with strong terminal claws. Pinnules 

 of considerable size have appeared. The ventral perisome, still further expanded, 

 is now studded with a multitude of indefinite plates from which the interradials 

 are well differentiated, though reduced in number and evidently in process of 

 resorption. The ambulacral grooves have appeared, traversing the disk from the 

 arms to the mouth and passing in between the orals, which are now reduced to 

 very small triangular plates. The anal tube has become still more conspicuous, and 

 rises high above the general level of the disk. The radianal is now entirely above 

 the radial circlet and no longer forms a part of the dorsal cup. The perisome at 

 this stage is covered with a sort of velvety coating in which the numerous small 

 plates are difficult to distinguish, and it is evidently on the verge of transformation 

 into the granular skin of the adult stage, from which all plates ventrally situated, 

 including interradials and radianal, have been completely resorbed. The radianal, 

 after being left far behind by the posterior end of the gut as the latter is carried 

 upward with the anal tube, is resorbed, together with the interradial plates from 

 which in the later stages it scarcely differs. The basals become transformed into 

 a rosette, just as do the basals of Antedon. 



According to Mr. Springer the outstanding differences in the development of 

 this type from that of Antedon lie in the position, size, and much greater sweep 

 of migration of the radianal and in the prominence of the anal tube, both attesting 

 the great importance of the movements of the posterior portion of the intestinal 

 canal in inducing, or retarding, changes in the form and in the composition of 

 the calyx. 



COMASTEKID PENTACRINOIDS. 



At station 3809, off Honolulu, Hawaiian Islands, in 51 to 125 fathoms, the 

 Albatross dredged a number of pentacrinoids, some of which were attached to 

 a cirrus probably of a species of Comactiniinse. 



