. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 220 Mr. G. Lefevre on dorsal mid-line. By this time a lumen has appeared in tlie centre, and around this the cells ,a;radually arrange themselves into an epithelium to form the dorsal tube (fig. 4, G). The Fig. latter fuses later at its anterior extremity with the dorsal wall of the pharynx, and an opening breaks through ; at no time is there any communication posteriorly between the tube and .the peribranchial cavity. The nerve-ganglion is formed by a thickening of the do
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 220 Mr. G. Lefevre on dorsal mid-line. By this time a lumen has appeared in tlie centre, and around this the cells ,a;radually arrange themselves into an epithelium to form the dorsal tube (fig. 4, G). The Fig. latter fuses later at its anterior extremity with the dorsal wall of the pharynx, and an opening breaks through ; at no time is there any communication posteriorly between the tube and .the peribranchial cavity. The nerve-ganglion is formed by a thickening of the dorsal wall of the tube, which eventually becomes constricted off in the manner described by Hjort for BotryUas, although in the latter it is the ventral wall of the tube which gives rise to the ganglion. Summary. 1. By a peculiar process of rotation of the endoderraal vesicle through 90° the thickened right wall of the vesicle is carried down to the ventral side of the bud-rudiment, where it forms the floor of the future pharynx. This process seems to be due to rapid growth and flattening of the cells com- posing the vesicle except in the thickened portion. 2. The pericardial rudiment, which is the first oi'gan to appear, is formed from the free cells of the blood. It arises on the right side of the inner vesicle, and through the rota- tion of the latter is brought down to tlie ventral side. 3. The peribranchial sacs arise asymmetrically. As the rotation takes place, the ventral wall of the inner vesicle is folded up immediately to the right of the point where the stolonic partition joins the vesicle, to form the left peri- branchial sac. The portion of the vesicle thus folded off is continuous with the stolonic partition; the connexion with the latter persists until quite a late stage. A fold appearing on the right side of the vesicle gives rise to the right sac, which then moves ventrally as the rotation continues. 4. No epicardium is present; in this respect PeropJiora diffe
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