Page:Proceedings of the Royal Society of London Vol 69.djvu/284

268 " The Development of Echinus esculentus." By E. W. MACBRIDE, M.A., D.Sc., Professor of Zoology in McGill University, Montreal. Communicated by ADAM SEDGWICK, F.RS. Re- ceived November 20, Eead November 28, 1901.

The present communication is to be regarded as a brief preliminary account of an investigation which has occupied my attention for the last three years. The object of this investigation was to examine by means of modern methods the manner in which the various organs of the adult Echinus are fashioned out of the corresponding structures of the Pluteus larva, and this object has been at last successfully accom- plished, and the most important results are briefly described in the present paper.

It was necessary in the first instance to rear the larvae through the later stages of development in considerable quantities, and the means by which this was accomplished have already been described in two short communications.*

The complete development from fecundation until the conclusion of metamorphosis occupies about 45 days, a period which closely agrees with that required for the development of Echinocyamus pusillus as determined by Theel.f

The fertilised eggs become at the end of 24 hours free-swimming ciliated blastulse ; at the end of 48 hours, by the invagination of one side, they are transformed into gastrulse. The larvae now assume a somewhat prismatic form, one side becoming flattened and even concave, whilst the opposite one becomes developed into a more or less rounded hump. On the flattened side, about 3 days after fertilisa- tion, the stomodseum makes its appearance as a wide, shallow invagina- tion. The gut has already been differentiated into the three portions, viz., oesophagus, stomach, and intestine, and from the blind apex of the first of these, which has not as yet met the stomodasum, the coelomic rudiment is given off in the form of a flattened sac. This sac rapidly becomes bilobed and then divided into two, the right and left coelomic vesicles.

The longitudinal ciliated band, which is the organ of locomotion, makes its appearance at this time as a thickening round the edge of the concave side of the larva, and at the same time the adoral band appears. This latter, which has the form of a V-shaped loop, projecting

" Studies in the Development of Echinoidea. I. The Larvae of Echinus esculentus and Echinus miliaris," ' Quart. Journ. Micro. Sci.,' vol. 42, 1899 ; " The Bearing of Echinoderm Larvae," ' Journ. Mar. Biol. Assoc.,' vol. 6, No. 1, 1900.

t " The Development of Echinocvamus pusillus," ' Trans. Roy. Soc. of Upsala,' 1892.