638 MOLLUSCA [SCHEMATIC MOLLUSC, mately the following history. By division of the egg-cell (fig. 3, A, B ; fig. 4, A, B ; and fig. 5) a mulberry-mass of embryonic-cells is formed (Morula), which dilates, forming a one-cell-layered sac (Blastula). By invagination one Fio. 6. Development of the Oyster, Ostrea edulis (modified from Horst, 16). A. Blastula stage (one-cell-layered sac), with commencing invagination of the wall of the sac at bl, the blastopore. B. Optical section of a somewhat later stage, in which a second invagination has commenced namely, that of the shell-gland sk; bl, blastopore ; en, invaginated endoderm (wall of the future arch-enteron) ; ec, ectoderm. C. Similar optical section at a little later stage. The invagination connected with the blastopore is now more contracted, d ; and cells, me, forming the mesoblast from which the coelom and muscular and skeleto-trophic tissues develop, are separated. D. Similar section of a later stage. The blastopore, bl, has closed ; the anus will sub sequently perforate the corresponding area. A new aperture, ro, the mouth, lias eaten its way into the invaginated endodermal sac, and the cells pushed in with it constitute the stomodseum. The shell-gland, sk, is flattened out, and a delicate shell, s, appears on its surface. The ciliated velar ring is cut in the section, as shown by the two projecting cilia on the upper part of the figure. The embryo is now a Trochosphere. E. Surface view of an embryo at a period almost identical with that of D. F. Later embryo seen as a transparent object, m, mouth ; ft, foot ; re, anus ; e, intestine ; st, stomach ; tp, velar area of the prostomium. The extent of the shell and commencing upgrowth of the mantle-skirt is indicated by a line forming a curve from a to P. A T .B. In this development, as in that of Pisidium (figs. 150, 151), no part of the blastopore persists either as mouth or as anus, but the aperture closes, the pedicle of invagination, or narrow neck of the invaginated arch-enteron, becoming the intestine. The mouth and the anus are formed as independent in-pushings, the mouth with stomodteum first, and the short anal proctodseum much later. This interpretation of the appearances is contrary to that of Horst (16), from whom our drawings of the oyster s development are taken. The account given by the American naturalist Brooks (19) differs greatly as to matter of fact from that of Horst, and appears to be erroneous in some respects. portion of this sphere becomes tucked into the other as in the preparation of a woven night-cap for the head (fig. 6, B ; fig. 7, A). The orifice of invagination (blastopore) narrows, and we now have a two-cell-layered sac, the Diblastula. The invaginated layer is the enteric cell-layer or endoderm ; the outer cell-layer is the deric cell-layer or ectoderm. The cavity communicating with the blastopore and lined by the endoderm is the arch-enteron. The blas topore, together with the whole embryo, now elongates. The blastopore then closes along the middle portion of its extent, which corresponds with the later developed foot. At the same time the stomodseum or oral invagination forms around the anterior remnant of the blastopore, and the proctodseum or anal invagination forms around the posterior remnant of the blastopore. There are, however, variations in regard to the relation of the blastopore to the mouth and to the anus which are probably modifications of the original process described above. An examination of figs. 3, 4, 5, 6, 7, and of others illustrative of the embryo logy of particular forms which occur later in this article, is now recommended to the reader. The explanation of the figures has been made very full so as to avoid the FIG. 7. Development of the River-Snail, Paludina vivipara (after Lankester,. 17). dc, directive corpuscle (outcast cell) ; ae, arch-enteron or cavity lined by the enteric cell-layer or endoderm ; bl, blastopore ; v r, velum or circlet of ciliated cells ; dv, velar area or cephalic dome ; sm, site of the as yet un formed mouth ; /, foot ; mes, rudiments of the skeleto-trophic tissues ; pi, the pedicle of invagination, the future rectum ; shgl, the primitive shell-sac- or shell-gland ; m, mouth ; an, anus. A. Diblastula phase (optical section). B. The Diblastula has become a Troehosphere by the development of the ciliated ring vr (optical section). C. Side view of the Trochosphere with commencing formation of the foot. D. Further advanced Trochosphere (optical section). E. The Trochosphere passing to the Veliger stage, dorsal view showing the formation of the primitive shell-sac. F. Side view of the same, showing foot, shell-sac (shgl), velum (vr), mouth, and anus. N.B. In this development the blastopore is not elongated ; it persists as the anus. The mouth and stomodseum form independently of the blastopore. necessity of special descriptions in the text. Internally, by the nipping off of a pair of lateral outgrowths (forming part of the indefinable "mesoblast") from the enteric cell- layer the foundations of the coelomic cavity are laid. In some Coelomata these outgrowths are hollow and of large size. In Mollusca they are not hollow and large, which is probably the archaic condition, but they consist at first of a few cells only, adherent to one another ; these cells then diverge, applying themselves to the body-wall and to the gut-wall so as to form the lining layer of the ccelomic cavity. Muscular tissue develops from deep-lying cells, and the rudiments of the paired nerve-tracts from thickenings of the deric-cell layer or ectoderm. The external form meanwhile passes through highly char acteristic changes, which are on the whole fairly constant throughout the Mollusca. A circlet of cilia forms when the
embryo is still nearly spherical (fig. 4, F ; fig. 6, E ; fig. 7,