MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 97 



two investigators have reached the same conclusiou. The point of view from which the subject 

 has been approached is not the same in all cases, but the results obtained by those who have 

 studied the question admit of being reduced to the same basis; that is, the relation of the planes 

 of bilateral sjinmetry in larva and adult. 



CuENOT (4) in his latest work denies the existence of any known relation between them. 



Skmon (13), working on a holothurian, found the two planes in qnestion to coincide, but his 

 conclusion is based on the supposition that the dorsal mesentery of the adult is the same as that 

 of the anricularia larva, which sup])()sition 15URY has since shown to bo incorrect. 



I)i"RV (.5), after working on members of all the groups of echinoderms, concluded that the 

 plane of bilateral symmetrj' of the larval form coincides not with the plane dividing the adult 

 form into two symmetrical halves, but with the plane of radial symmetry. 



McBkide"s (10) observation on a starfish, A.sferiiia {/ibbosa, led him to adopt about the same 

 view as that of liUKY. He found that the plane of radial symmetry of the star makes an angle 

 of 703 plus with the frontal plane of the larva, but may, without error, be considered as 90^, This 

 is equivalent to saying that the plane of radial symmetry of the star is parallel with the sagittal 

 plane or the plane of bilateral symmetry of the larva, and is also reducible to the statement thiit 

 the i)lanes of bilateral symmetry of the larva and adult are at right angles to one another. Thus 

 right and left in the larva become aboral and oral in the adult. 



The ditl'erence in results arrived at by Goto and JIcBeidk are due almost wholly to the 

 stages in the metamorphosis selected in each case for the study of the (juestioii, Goto selecting a 

 very late stage, when the larval body had all but disappeared, while the stage chosen by McBeide 

 is an early one, in which the rudiments of the star are just appearing. 



If the five groups of echinoderms have sprung from a common stem after radial symmetry 

 had been established, then in the metamorphosis which is found in all the groups there should be 

 discoverable a unity of relation between larva and adult. It is bard to conceive of the radial 

 symmetry of echinoderms as having been indei)endently acquired by each group, although it is 

 easy to see how secondary changes may have arisen in the metamorphosis since the .uronps separated. 



The five groups of echinoderms stand isohited from one another almost as completely as does 

 the echinoderm i)hylum from the other phyla of the aninuil kingdom, and it is not my intention at 

 this time to enter into a discussion of the interrelationshii)s of echinoderms. I wish, however, to 

 point out an interesting series of facts i)resented by members of the Asterid, Griuoid, and ( )phiurid 

 groups which may have a bearing upon the subject, and in the same connection I wish to call 

 attention to how well McBride's hypotheti('al ancestor of the Asterids and Grinoids (10, fig. — ), 

 when details are not too closely compared, fits into the facts of the larva of Ophinra hrcvispina. 



In one of the Asterids Goto has shown that toward the end of metamori)hosis the almost 

 comi)lcte star sits as a cap at the posterior end of the larva, with its aboral end posterior, its oral 

 surface anterior, the bivium dorsal, aiul the trivium ventral. 



In Ant«don, like the starfish, the rotation brings the developing crinoid head to the posterior 

 end of the larva, but (liffering diametrically from the starfish in that the oral instead of the 

 aboral surface of the crinoid is posterior; but this difference does not in any way aflect the 

 homologies between the two groups as has been supi)osed. 



In Ophiura brerispina the relation of larva and adult at the time of metamoriihosis is ai)prox- 

 imately the same as is shown iu Stage "F" (fig. 22), in which ventral in the larva is ventral 

 (oral) in the adult. 



Now, if we take an ophiuran larva at Stage "F," and imagine the disk to rotate in such a 

 way as to bring its oral surface away from the larval organ or ])reoral lobe, it f'aiily represents 

 that which takes place in the metamorphosis of Antedon, but if we think of it as rotating in 

 the opposite way, bringing the aboral surface away from the preoral lobe, then it more nearly 

 illustrates the starfish metamorphosis. 



In Antedon, as metamorphosis proceeds, the stem is carried on to the aboral surface, while in 

 the starfish the preoral lobe finally disappears on the oral surface. In (>. brwinpina the place of 

 disai)pearance of the larval organ more nearly recalls the crinoid than any other echinoderm, the 

 larval organ being found in some of my oldest specimens as a small knob near the edge of the 

 aboral surface between arms 1 and V. 



