4 SMITHSONMAN INSTITl'TIOX 



PLATE B 

 Devklopment of Radianal in Larvae of Existing Crinoids 



PAGE 



Figs. i-TO. Comactinia meridionalis (A. and E. C. Agassiz ) 80 



Recent. Coast of Yucatan, Mexico. 



Fig. I. Pre-brachial stage. Calyx composed almost exclusively of basals and closed orals, with 

 a few elongate stem ossicles ; the plates consisting of a thin calcareous reticulation, 

 translucent, and showing by transmitted light the oesophagus, stomach, and dextrally 

 coiled intestine to which the radianal is attached in the right posterior rhombic space 

 between the corners of radials and basals ; patches of stereom smaller than the 

 radianal appear in the other rhombic spaces, which are the incipient radials, but no 

 such plate has appeared in the right posterior space, as yet occupied exclusively by 

 the radianal, which thus preceded the radial in order, and occupies the primitive 

 position at the foot of the right posterior ray. X 95- 



2a. Another specimen in the same stage, with orals slightly parted, between the summits of 

 which the oral tentacles, as yet unbranched, project; the radial of the right posterior 

 ray is still wanting, its place being occupied by the radianal attached to the gut at its 

 anal end. X 95- 



26. Outline sketch of same specimen as seen by transmitted light, showing the alimentary 

 system as in fig. i, except that here the upper margin of the oesophagus is distended 

 by the opening of the orals and therefore smooth instead of wrinkled as in the other 

 specimen. X 95- 



3. Specimen in same stage further advanced, with orals opened and tentacles projecting: the 



right posterior radial has now appeared, with the radianal, still attached to the gut, 

 much larger than the radial and lying obliquely below it; the anal opening of the gut is 

 well shown. X S7- 



4. Incipient brachial stage. Small points of stereom in the ray above the radial are the 



beginnings of the primibrachs ; radials have increased in size until they exceed the 



radianal, which has moved toward the middle of the posterior basal while still strongly 



indenting the inner margin of r. post, radial. X 50. 

 SO. Brachial stage. Primibrachs developed and connected with radials by articulation; the 



anal end of the gut has grown upward to half the height of the orals, carrying the 



radianal to a level slightly above the radials. X -10. 

 Sb. Anterior side of same specimen. The interradials have now appeared as small patches 



in the triangular spaces between orals and radials. X 40. 



6. Specimen somewhat further advanced, to show the complete stem and its mode of attach- 



ment to a cirrus of the host. X I3- 



7. Specimen in about the same stage as last. The radials have now met beneath the inter- 



radials, and the radianal is about median above posterior basal, but still clearly con- - 



nected with the right posterior radial by a curved margin. The radials have become 



larger than the basals. X 30. 

 8a. Specimen further advanced, with radianal carried still higher up by growth of the gut, and 



the two posterior radials almost meeting beneath it. X 30. 

 86. Lateral view of same specimen, showing great protuberance caused by anal structures. 



X30. 



9. Specimen still further advanced. Radials now meet at posterior side beneath the radianal, 



which has been lifted still higher up with growth of the anal tube, and other plates 

 appear at either side of it. X 26. 



10. Specimen in which the radianal is connected with radials only by short margins at their 



shoulders, and is lifted still higher up by the anal tube. X 25. Note the great diminu- 

 tion in size of basal and oral plates in this and next preceding specimens as compared 

 with their condition in figs. I and 2. 



