366 E. RAY LANKESTER; 



ozus and of Phallusia, pointed out that the inner series of 

 cells which give rise to the alimentary canal in those animals 

 take up their position as the result of an invagination of part 

 of the wall of an original multicellular sac. I found the 

 same mode of origin for the primitive alimentary canal or 

 endoderrn of the Lamellibranch Pisidium, of the Pulmonates 

 Limax and Lymnceus, of the Nudibranchs Tergipes and 

 Polycera. This led me to compare the early development 

 of members of other groups of the animal kingdom, the 

 Planulee of Sponges and Coelenterates, the frog's embryo 

 with Rusconian anus, «&;c., and I was thus led to infer that 

 in this simple double-walled sac, composed of ectoderm and 

 endoderm, we have the transitory indication of a primaeval 

 ancestor of all the higher groups of the animal kingdom, 

 whose essential structure is permanently retained in the 

 corals and polyps (Coelenterata), but is in the course of 

 development improved upon by the evolution of a body- 

 cavity, and an additional third or intermediate mass of em- 

 bryonic cells, giving rise to muscular and vascular structures 

 in worms, molluscs, arthropods, star-fishes, and vertebrates. 

 I proposed to call this developmental form the Planula, its 

 immediate predecessor (the multicellular sac) a Polyblast, 

 and indicated three large divisions of the animal kingdom — 

 Homoblastica, Diploblastica, and Triploblastica — correspond- 

 ing respectively to a lower stage than the Planula, to the 

 Planula itself, and to a higher development of the essential 

 Planula-structure.^ 



Almost simultaneously Professor Haeckel, of Jena, arrived 

 at similar conclusions, which he first made known in his 

 * Monograph of the Calcareous Sponges,' and subsequently 

 developed in the essay entitled " TheGastraea-Theory," which 

 has been translated by Professor Perceval Wright for the 

 April and July numbers of this Journal. The terms Gastrula 

 and Gastraea, introduced by Professor Haeckel, are preferable 

 to the term Planula which I had adopted ; and I may 

 further take this opportunity of admitting to some extent 

 the justice of his criticisms on my use of the term Triplo- 

 blastica. It appears to me more and more certain that (as 

 he has definitely pointed out) the third layer, or those masses 

 of cells which in the embryos of Triploblastica are regarded 

 as belonging to such a layer, are phylogenetically derived 

 either from one or other of the two primitive cell-layers, and 

 only appear by suppression of the historical steps of de- 

 velopment as an intermediate and independent layer. 

 Nevertheless, the fact that they do so appear, and that there 

 > 'Annals and Mag. Nat. History/ June, 1873. 



