(366) A. Kowalevsky. _Sitz. d. zool. Abth. d. III. Versam. russ. Naturj._ (Thalassema). _Zeit. f. wiss. Zool._ Vol. XXII. 1872, p. 284. (367) A. Krohn. "Ueb. d. Larve d. Sipunculus nudus nebst Bemerkungen," etc. Müller's _Archiv_, 1857. (368) M. Salensky. "Ueber die Metamorphose d. Echiurus." _Morphologisches Jahrbuch_, Bd. II. (369) E. Selenka. "Eifurchung u. Larvenbildung von Phascolosoma elongatum." _Zeit. f. wiss. Zool._ 1875, Bd. XXV. p. 1. (370) J. W. Spengel. "Beiträge z. Kenntniss d. Gephyreen (Bonellia)." _Mittheil. a. d. zool. Station z. Neapel_, Vol. I. 1879. _Gephyrea tubicola (Actinotrocha)._ (371) A. Krohn. "Ueb. Pilidium u. Actinotrocha." Müller's _Archiv_, 1858. (372) A. Kowalevsky. "On anatomy and development of Phoronis," Pétersbourg, 1867. 2 Pl. Russian. _Vide_ Leuckart's _Bericht_, 1866-7. (373) E. Metschnikoff. "Ueber d. Metamorphose einiger Seethiere (Actinotrocha)." _Zeit. f. wiss. Zool._ Bd. XXI. 1871. (374) J. Müller. "Bericht üb. ein. Thierformen d. Nordsee." Müller's _Archiv_, 1846. (375) An. Schneider. "Ueb. d. Metamorphose d. Actinotrocha branchiata." Müller's _Arch._ 1862. CHAPTER XV. CHÆTOGNATHA, MYZOSTOMEA AND GASTROTRICHA. The present chapter deals with three small isolated groups, which only resemble each other in that the systematic position of all of them is equally obscure. _Chætognatha._ The discoveries of Kowalevsky (No. 378) confirmed by Bütschli (No. 376) with reference to the development of Sagitta, though they have not brought us nearer to a knowledge of the systematic position of this remarkable form, are nevertheless of great value for the more general problems of embryology. The development commences after the eggs are laid. The segmentation is uniform, and a blastosphere, formed of a single layer of columnar cells, is the product of it. An invagination takes place, the opening of which narrows to a blastopore situated at the pole of the embryo opposite that at which the mouth subsequently appears (fig. 164 A). The simple archenteron soon becomes anteriorly divided into three lobes, which communicate freely with the still single cavity behind (fig. 164 B). The two lateral lobes are destined to form the body cavity, and the median lobe the alimentary tract of the adult. An invagination soon arises at the opposite pole of the embryo to the blastopore and forms the mouth and oesophagus (fig. 164 B and C, _m_). [FIG. 164. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.