Page:Proceedings of the Royal Society of London Vol 69.djvu/291

Rh coele and amniotic cavity have grown so as to occupy the entire side of the larva ; the tube-feet are long, and show wriggling movements. Spines have commenced to make their appearance ; one or two small quadrangular rudiments are situated at the sides of the posterior pedicellaria, but, in addition, there are ten conical outgrowths of the oral disc of the Echinus, i.e., of the floor of the amniotic cavity, alter- nating with the radial canals.

FIG. 8. Diagrammatic Section of the developing Oral Disc of the future Echinus in a Larva 30 days old.

Names as before. In addition, ep., epineural space ; ep.f., epineural fold ; t.g., tooth, sac (developed from the invagination of coelomic epithelium shown in fig. 7).

The first trace of the adult mouth and oesophagus appears as an imagination of the oral disc, indenting the centre of the hydrocoele. At a later period it completely perforates the hydrocoele, thus convert- ing it from a flattened plate into a ring. The adult oesophagus is thus shown to be a stomodseum. The stomach loses its globular form, and becomes flattened on the side towards the hydrocoele.

About this time the first trace of the so-called " blood-system " becomes apparent. From the beginning the wall of the gut has at no time been in actual contact with the coelomic wall. Always there has intervened a layer of jelly with a few amoebocytes in it, the same sub- stance which exists in much greater quantity between the outer wall of the coelom and the ectoderm.

At about 36 days the jelly intervening between the coelom and the