Page:Proceedings of the Royal Society of London Vol 69.djvu/292

276 gut becomes profoundly altered in chemical composition. It acquires staining properties, but the amoebocytes contained in it remain glassy and clear. At the same time, the gut-cells become more difficult to stain, pointing clearly to the exudation of proteid matter from the gut- cells into the surrounding jelly. From this envelope of altered jelly the mesenteric and anti-mesenteric vessels of the adult are developed.

The metamorphosis commences at about 40 42 days. By that time the amniotic cavity, the hydrocosle, and the underlying left posterior coelom have grown so greatly that the posterior epaulettes and the pedicellaria in the centre of the aboral surface are pushed to one side. In other words, the preponderant growth of the organs of the left side, which in Asterina gibbosa becomes evident at an early period of develop- ment, here makes its appearance only just before metamorphosis. From the anterior end of the left posterior ccelom a cellular rod grows out, burrowing into the jelly lying between the ampulla and the right hydroccele. This is the rudiment of the " ovoid gland," or so-called " heart," which here, as in Asterina gibbosa, is in reality a genital stolon. The anus closes up, and the mouth closes also about the place where the stomodseum joined the larval oesophagus. The Pluteus arms have become shorter and thicker, and a hole has appeared in the roof of the amniotic cavity through which the tube-feet are protruded.

The larva now sinks to the bottom, and in a very short time the ciliated band and the " epaulettes " disappear, being devoured by amosbocytes. The roof of the amniotic cavity is entirely abolished, and the young Echinus creeps about freely on its five tube-feet. The rudiments of a second pair of tube-feet are already present in each radius, but are not yet functional. The arched dorsal surface, with its spines and the three pedicellaria, is at this time greater in extent than the ventral ambulatory surface, so that it may be said that the young Echinus passes through an Asterid stage in development.

The whole ambulacral surface, however, grows rapidly in comparison with the dorsal surface ; the terminal tube-feet become smaller, and are eventually sunken in grooves in the test. They become adherent to the sides of these grooves, and so the long flexible tube-foot of the just metamorphosed Echinus becomes changed into the insignificant ocular nodule of the adult.

Compared with the development of Asterina gibbosa, the development of Echinus esculentus must be regarded as being very much modified; nevertheless, all the main features of the development of Asterina can be recognised in Echinus.