At the same time the ectodermic invagination becomes completely closed : its floor consists of a thick layer of cells which is moulded over the outgrowths of the developing hydrocoele, but its walls and roof remain very thin. Its cavity will be called the amniotic cavity.
The further development of the Pluteus consists chiefly in the enlargement of the hydrocoele and of its outgrowths, and of course of the ectoderm covering the latter, and of the amniotic cavity into which they project.
At about 23 days five interradial ridges of the ectoderm, which covers the hydroccele, make their appearance, alternating with the five primary tentacles. The sides of these ridges grow out into over- arching lamellae, which soon meet one another, and in this way five epineural canals are formed, opening into a central epineural space (see figs. 5 and 6).
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FIG. 6. Plan of the Hydrocoele (Rudiment of the Water-vascular System) of a Larva about 24 days old.
The wedges (distinguished by cross-hatching) are the epineural folds, which even- tually meet one another laterally and form a complete covering over the hydro- ccele.
At 26 or 27 days the left posterior ccelom has extended to the posterior apex of the hump, and has there met the right posterior coelom, forming a thin vertical mesentery. Just above this mesentery the rudiment of the first pedicellaria appears as a little elevation filled with amoebocytes (fig. 5). A little later the rudiments of two other pedicellariae appear on the right side one situated dorsally, the other ventrally.
The left posterior coelom, which was at first situated entirely posterior to the hydroccele, has by this time grown completely round it in a circle, and in the axis of this circle the stone-canal still connects the " ampulla" or anterior coelom and the hydrocoele (fig. 5).
At 27 28 days the rudiments of the masticatory system of the so- called " Aristotle's lantern " make their appearance. These are five outgrowths of the left posterior coelom, having the shape of inverted
