(362) R. Leuckart. _Die menschlichen Parasiten (Hirudo)_, Vol. I. p. 686, et seq. (363) H. Rathke. _Beit. z. Entwicklungsgesch. d. Hirudineen._ Leipzig, 1862. (364) Ch. Robin. _Mém. sur le Développement embryogenique des Hirudinées._ Paris, 1875. (365) C. O. Whitman. "Embryology of Clepsine." _Quart. J. of Micro. Science_, Vol. XVIII. 1878. [_Vide_ also C. Semper (No. 355) and Kowalevsky (No. 342) for isolated observations.] CHAPTER XIV. GEPHYREA[149]. [149] The following scheme shews the classification of the Gephyrea adopted in the present chapter: I. Gephyrea nuda. { (1) _Inermia._ { (2) _Armata._ II. Gephyrea tubicola. (Phoronis). It is convenient for the purposes of embryology to divide the Gephyrea into two groups, viz. (1) Gephyrea nuda or true Gephyrea; and (2) Gephyrea tubicola formed by the genus Phoronis. GEPHYREA NUDA. _Segmentation and formation of the layers._ An embolic or epibolic gastrula is characteristic of the Gephyrea, and the blastopore appears, in some cases at any rate (Phascolosoma, Thalassema), to become the mouth. Bonellia. In Bonellia (Spengel, No. 370) the segmentation is unequal but complete, and, as in many Molluscs etc., the ovum exhibits before its commencement a distinction into a protoplasmic and a yoke pole. The ovum first divides into four equal segments, each of them formed of the same constituents as the original ovum. At the animal pole four small cells, entirely formed of protoplasm, are next formed by an equatorial furrow. They soon place themselves in the intervals between the large spheres. Four small cells are again budded off from the large spheres and the eight small cells then divide. By a further continuation of the division of the existing small cells, and the formation of fresh ones from the large spheres, a layer of small cells is eventually formed, which completely envelops the four large spheres except for a small blastopore at the vegetative pole of the ovum (fig. 160 A). The large spheres continue to give rise to smaller cells which however no longer take a superficial position but lie within the layer of small cells, and give rise to the hypoblast (fig. 160 B). The small cells become the epiblast, and at the blastopore they curl inwards (fig. 160 B) and give rise to a layer of cells, which appears to extend as an unbroken sheet between the epiblast and hypoblast, and to form the mesoblast. The blastopore now closes up, but its position in relation to the parts of the embryo has not been made out. [FIG. 160. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.) A. Stage when the four hypoblast cells are nearly enclosed. B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore. _ep._ epiblast; _me._ mesoblast; _bl._ blastopore.]