Page:Proceedings of the Royal Society of London Vol 69.djvu/285

Rh into the oesophagus, can first be discerned at the age of 4 days after the alimentary canal is completed by the opening of the stomodaeum into the oesophagus. It is formed both from the lining of the stomo- deeum, which is of course ectoderm, and from the endoderm of the oesophagus. This is important, in view of the circumstance that it has been compared to the V-shaped endostyle of the larva of Amphioxtis (see fig. 1).

FIG. 1. Diagrammatic View of a Larva about 4 days old, seen from in front. ad., adoral band of cilia ; coe., coelomic vesicle ; ce., oasophagus ; st., stomach ; stom., stomodseum.

By this time, through the formation of outgrowths from the longi- tudinal ciliated band, the first four arms of the Pluteus have been formed. Even at this early period the left coelomic vesicle is wider and more thin-walled than the right, and at 4 days a vertical tubular outgrowth is formed from it, which reaches and fuses with the ectoderm, and in this way the primary pore-canal and madreporic pore are established. These are distinctly structures belonging to the left side. No trace of corresponding structures on the right side was ever observed.

The coelomic vesicles grow backwards along the sides of the stomach ; that on the left side shows an anterior dilatation into which the pore- canal opens, followed by a constriction and a posterior dilatation. When the larva has reached the age of 8 or 9 days, this posterior dilatation divides into two (fig. 2), the front part being the rudi- ment of the hydrocoele, whilst the hind part will form the left pos- terior coelom, out of which a large portion of the adult body-cavity is formed. The anterior dilatation is the so-called "ampulla" of the stone-canal, and the neck connecting it with the hydrocoele is the stone- canal itself (fig. 3).

About this time the rudiments of the 5th and 6th Pluteus arms make their appearance, and shortly afterwards the right coelomic vesicle is divided into an anterior and a posterior part. This occurs when the