10 



HJALMAR THÉEL, NOTES OX THE DEVELOrMENT OF ECHINOCARDIUM AND PARECHINUS. 



However, in 1903 Mac Bride j subjected this part of the evolution of the 

 liydrocoele to a revision, and in a foot-note he pronounced an opinion somewhat 

 different from his former views. His accoimt runa as follows: »I could not with 

 certainty determine how the ring-form is attained. In the preliminarv account of this 

 paper ('Proc. Ro\\ Soc.', vol. 69) I stated that it was caused by the perforation of 

 the disc by the oesophagus. This is a mistake, as I find by careful revision of my 

 sections that the ring-form is attained much earlier. I believe, however, that it is 

 formed as described for Echinus miliaris in a paper by Théel, of which 1 received 

 a copy after the MS. of this paper was sent in. In that paper (»Preliminarv Account 

 of the Development of E. miliaris* 'Proc. Roy. Swedish Akad.', vol. 28) he descri- 



I. — Left Bide view <>i tlio postorior portion <>f a young pluteus of Parechinus mUiariå L.)j reconstructed from 

 BectionR. — ph., pharynx; »t-, Btomach; in., intestine; Ipc, 1 « ■ t" t posterior ooelom almosl surrounding the hydroooele 



hoop; o. exteriör opening of ili !todermic invagination ; ed., ectodermic disc; s. stone-canal ; \-, ampulla >>!' the 



tone-canal lefl anterior coelom of Mac Bribe); p., madreporic porpj v., madreporic vesicle 'i | righl hydroooele 

 df M M Bbidb). The aumerals indicate ti der of the Bections. Highly magnified. 



bes the hydroooele as having at first a pentagonal form, but soon a small aotch 

 appears on the porterior börder of this, which, by the meeting <>!' ils edges, is made 

 into a perforation of the pentagon, and so Uh> ring-form is attained. In Mellita, 

 accordinc to Gravb, the hydroooele is for a long time an open hoop, as in the As- 

 teroidea. — November 6, 1902.» 



I must confess that I was highly astonished when I read this explanation <>l 



1 loc. cit. 1903. p. 301, 



