Footnote 220: The recent researches of Selenka, _Zeitschrift f. Wiss. Zoologie_, Vol. XXVII. 1876, demonstrate that in Echinoderms the muscles are derived from the cells first split off from the hypoblast, and that the diverticula only form the water-vascular system and the epithelial lining of the body-cavity. There is _one_ outgrowth from the alimentary tract in Synapta; _two_ in Echinoids, Asteroids and Ophiura; _three_ in Comatula, and four (?) in Amphiura. The cavity of the outgrowth usually forms the body-cavity, but sometimes in Ophiura and Amphiura (Metschnikoff) the outgrowths are from the first or soon become solid, and only secondarily acquire a cavity, which is however homologous with the body-cavity of the other groups. In Sagitta[221] the formation of the mesoblast and the alimentary tract takes place in nearly the same fashion as in the Echinoderms. The simple invaginate alimentary cavity becomes divided into three lobes, a central and two lateral. The two lateral lobes are gradually more and more constricted off from the central one, and become eventually quite separated from it; their cavities remain independent, _and form in the adult the body-cavity_, divided by a mesentery into two distinct lateral sections. _The inner layer of each of the two lateral lobes forms the mesoblast of the splanchnopleure, the outer layer the mesoblast of the somatopleure._ The central division of the primitive gastræa cavity remains as the alimentary tract of the adult. Footnote 221: Kowalevsky, "Würmer u. Arthropoden," _Mém. Acad. Pétersbourg_, 1871. The remarkable observations of Kowalevsky[222] on the development of the Brachiopoda have brought to light the unexpected fact that in two genera at least (Argiope and Terebratula) the mesoblast and body-cavity develop as paired constrictions from the alimentary tract in a manner almost identically the same as in Sagitta. Footnote 222: "Zur Entwicklungsgeschichte d. Brachiopoden", Protokoll d. ersten Session der Versammlung Russischer Naturforscher in Kasan, 1873. Published in _Kaiserliche Gesellschaft Moskau_, 1874 (Russian). Abstracted in Hoffmann and Schwalbe, _Jahresbericht f._ 1873.