MOLLUSCA 



101 



mately the following history. By division of the egg-cell 

 (fig. 3, A, B ; fig. 4, A, B ; and fig. 5) a mulberty-mass of 

 embryonic-cells is formed (Morula), which dilates, forming 

 a one-cell-layered sac (Blastula). By invagination one 



Fia. 6. — Development of the Oyster, Oslrea edulis (modified from Horst, 16). 

 A. Blastula stage (one-cell-layered sac), witli commencing invagination of 

 the waU of the sac at W, the blastopore. B. Optical section of a somewhat 

 later stage, in which a second invagination has commenced — namely, thai: 

 of the shell-gland 8%; W, 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, Tree, 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, 6Z, has closed ; the anus will sub- 

 sequently perforate the corresponding area. A new aperture, m, the mouth, 

 has eaten its way into the invaginated endodermal sac, and the cells pushed 

 in with it constitute the stomodaeum. The shell-gland, sh, is flattened out, 

 and a delicate shell, 5, 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. P. Later embryo seen as a 

 transparent object, m, mouth ; /if, foot ; a, 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 

 toF. 



N.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 stomodseum 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 tiie 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 



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 vnth 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 stomodaeum 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 



ayi. 



Pig. 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 ; U, blastopore ; vr, velum or circlet 

 of ciliated cells ; d/o, velar area or cephalic dome ; sm, site of the as yet un- 

 formed mouth ; f, foot ; meSy rudiments of the skeleto-trophic tissues ; pi, 

 the pedicle of invagination, the future rectum ; shgl, the primitive shell-sac 

 or shell-gland ; m, mouth ; au, anus. A. Diblastula phase (optical section). 

 B. The Diblastula has become a Trochosphere 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 MoUusca 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 coelomic 

 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 MoUusca. A circlet of cilia forms when the 

 embryo is stiU nearly spherical (fig. 4, F ; fig. 6, E ; fig. 7, 



