1907. J Development q/Ophiothrix fragilis. 445 



In Asteroidea and Echinoidea, the right hydrocoele is from the beginning 

 close to the dorsal pore, and on this account Masterman (3) and Goto (4) have 

 called it the central coelom and denied its homology with the left hydrocoele. 

 In Ophiothrix fragilis there can be no question on the subject, for the right 

 hydrocoele is, from the beginning, on the right side of the larva, and its dorsal 

 position in the other groups may be explained by supposing that it is right 

 with regard to the adult position of the mouth. As the adult mouth 

 is on the left side of the larva, right with respect to this mouth will be 

 nearly mid-dorsal with respect to the larval mouth. But in Ophiuroidea, 

 where the larval mouth persists as the adult mouth, the right hydrocoele is on 

 the right of it from the beginning, and the pore, originally situated on the 

 left side, moves round so as to assume a mid-dorsal position. 



Little remains to be added in this preliminary account. The arm rudiments 

 are outgrowths of the left posterior coelom and, counting from the dorsal end, 

 arm rudiment No. 1 comes to support hydrocoele lobe No. 2, as in Asterina 

 gibbosa, whilst arm rudiment No. 5 supports hydrocoele lobe No. 1. As in 

 Echinus esculentus, inter-radial ridges alternating with the lobes of the 

 hydrocoele grow out and extend outgrowths to the right and left, so as to 

 roof over what would be the ambulacral grooves and thus form the epineural 

 canals. The perihsemal canals also arise from hollow inter-radial outgrowths 

 of the left posterior coelom, except that the first arises, as in Asterina gibbosa, 

 from the axial sinus. The arms of the larva gradually disappear, except 

 the two postero-lateral, which are only cast off when the larva has sunk 

 to the bottom and metamorphosis is complete. 



LIST OF PAPERS CITED. 



1. MacBride, E. W. "Some Points in the Development of Ophiothrix fragilis ," ' ^roc- 



American Soc. of Zoologists,' Philadelphia, 1903. 



2. Grave, Caswell. " Ophiura brevispi?ia," ' Mem. from Biol. Lab. Johns Hopkins,' 45. 



3. Masterman, A. " The Early Development of Cribrella oculata, with Remarks on 



Echinoderm Development," ' Trans. Roy. Soc. Edin.,' vol. 40, Part 2. 



4. Goto, S. "The Metamorphosis of Aster ias pallida, with Special Reference to the Fate 



of the Body Cavities," ' Contributions from the Zool. Lab. of Mus. of Comp. 



2 K 2 



