104 THE ANATOMY OF IXYERTEBRATED ANIMALS. 



Fig. n.—Ascfitfa primordialis (after Haeckel). 



I. A mature Ascetta,v&vx of one side of the body of which ig removed: o, the exhal- 

 ent aperture ; 7;. inhaleut oorep in the wall of the body ; i, endoderm ; e, ecto- 

 derm ; q. ova. The triradiate spicnla are seen imbedded in the ectoderm. 



II. A portion of the endoderm, with two pores (p>; i, endodermal cells— those round 

 the margins of the pores have their cilia directed inward ; e, ectodermal syncy- 

 tium: g, ova ; z, sperm-cells. 



III. A monadiform enrlodermal cell. 



IV. An endodermal cell, with retracted cilium, and having the characters of an 

 Ammha. 



V. The cliiated emhryo of Aacetta mirnbUis. 



VI- The same embryo in optical longitudinal section: e, epiblast : i, hypoblast ; v^ 



