The Development of Ophiocoma nigra

Abstract

1. Cleavage results in a thick-walled blastula and not a morula as in the artificially fertilized eggs of Ophiothrix. 2. The coelomic divisions are thick-walled and their cavities are much smaller than in Ophiothrix. 3. A right hydrocoele arises as a thickening of the right anterior coelom in all the larvae, but it does not assume a fivelobed form in any case. 4. The right hydrocoele does not persist in the adult, but degenerates as metamorphosis progresses. 5. A pericardial vesicle arises from the dorsal surface of the right anterior coelom as in Echinoids and Asteroids. It persists in the adult as a thin-walled vesicle lying beside the pore canal. 6. A heart is formed by the invagination of the ventral wall of this vesicle. Pulsations occur once in about twelve seconds in the ventral wall of the vesicle, in the just metamorphosed forms. 7. During metamorphosis the fifth lobe of the left hydrocoele does not move across the oesophagus to the larval right as in Ophiothrix, but it bends slightly to the right to meet the first lobe which travels towards it after passing round the oesophagus. 8. All the perihaemal spaces arise as pocket-shaped evaginations from the left posterior coelom. 9. The stomach of Ophiocoma is peculiar. The outer ends of the cylindrical cells are vacuolated and stain deep black in osmic acid and Müller's fluid, owing to the presence of fat in them. 10. In post-larval stages the stomach at first appears as a solid mass, but later sends out five projections alternating with the arms. In conclusion, I have pleasure in expressing my indebtedness to Professor E. W. MacBride, F.R.S., under whose guidance the work was carried out, for his constant advice and constructive criticism which have gone a long way in improving this paper. I must also thank Dr. E. J. Allen, Director of the Marine Biological Laboratory at Plymouth, for providing me with the material required, and giving me facilities to carry out those parts of the work which were done in the Plymouth laboratories. I must also thank my friend Mr. J. K. Deuskar to whose skill I owe the excellent figures in this paper.