96 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



As no perceptible change has taken place since "F" in the organs not referred to above, the 

 description of them given in the previous chapter will serve equally as well for "G" as for "F," 

 and the figures of these organs in "G" may be examined in connection with their description 

 in » P." 



Stage " H," 8 Days Old. 



(Figures 33 and 34.) 



In the oldest larva" I have, the metamorphosis has been almost completed. The larval organ 

 has nearly disappeared, that part of it which yet remains being found sticking to the edge of the 

 aboral disk of the young pentagonal star. 



When living the little ophiurids clung to the bottom and sides of the aquaria dishes. Although 

 the ciliated bands were still evident on the disk their free swimming habits had been wholly 

 given up. 



The pore canal still opens on the aboral surface, but with the growth of the latter it is 

 traveling toward the edge of the disk, and by a continuation of this process the oral surface will 

 ultimately be reached. 



As the closure of the grooves over the nervous system took place, circular areas below the 

 tips of the tentacles were left open, the tentacle pores, and through these the tentacles, were able 

 to protrude and withdraw themselves. 



The subneural sinuses which had begun to be formed in "F" have been completed in the 

 eight-day larva (figs. 31 and 34 ss). In "H," then, the nervous system is cushioned below by the 

 subneural sinus and above by the outer perihtemal ring. 



The stomach, which for so long a period has been at a standstill in its development, has begun 

 to grow, its sides pushing out between the epigastric and hypogastric body cavities. The lumen 

 of both stomach and (.esophagus have reappeared (fig. 3-1 s). The glandular structure which makes 

 the walls of the stomach so complicated in the adult has not begun to form in "H," the walls being- 

 simple and one cell in thickness. 



No figure of " H" as a whole object has been made for the reason that the skeletal plates should 

 be included, and material adequate to a complete study of them is at present not in my possession. 



RELATION OF LARVA TO ADULT. 



The hydroccele is the first organ to show radial symmetry in the developing larva of Ophiura 

 brevixpina, and from the time when this organ has completed its rotation about the oesophagus it 

 shows a definite relation to the plane of bilateral symmetry of the larva. 



The hydroccele is not only radially symmetrical, but bilaterally symmetrical, since it is divided 

 into symmetrical halves by the plane which passes through radial canal 3 and through the inter- 

 radius of the stone canal. This plane coincides with the plane of bilateral symmetry of the larva. 

 The other parts of the star are built about the water vascular system; hence it, as a whole, bears 

 a similar relation to the larva as was initiated by the hydroccele. 



No secondary twisting of the various parts of the star occurs, and its relation to the larva 

 remains constant as it began, and throughout the life history of the species the following state- 

 ments hold true: Ventral and dorsal in the larva are equivalent to oral and aboral in the adult. 

 Although no physiological differentiation exists, if we regard that part of the adult as anterior 

 which was anteriorly directed in the free swimming larva, the trivium is anterior, the bivium is 

 posterior. 



In the. foregoing I have confirmed, in an ophiurid, the conclusions drawn by Goto from his 

 studies on a starfish. In his work on the development of Asterias pallida Goto (5) thought he 

 was able to prove the coincidence of bilateral symmetry, which obtains in the adult starfish, with 

 the plane of bilateral symmetry of the bipinnarian and brachiolarian larvae. 



The study of the relation of larva to adult in the starfishes is made most difficult, however, 

 by the independent origin and subsequent twisting of the parts of the star. At the time of their 

 origin no two parts of the star bear the same relation to the larva. The relation of each part to 

 the larva also changes as metamorphosis proceeds. 



The facts just enumerated admit of other conclusions than those deduced by Goto, and no 



