Calcispongiae in the Animal Kingdom. 255 



the gastrula possesses exactly the same structure. In all, its 

 simple, rounded elongate, uniaxial body contains a simple 

 central cavity (^omachal cavity), which opens by an orifice at 

 one pole of the axis. In all the thin wall of the cavity consists 

 of two layers of cells or lamellse : — an inner lamella of larger, 

 darker cells — the entoderm, gastral lamella, inner, trophic or 

 vegetative germ-lamella ; and an outer lamella of smaller, 

 generally vibratile, paler cells — the exoderm, dermal lamella, 

 external, sensorial or animal germ-lamella. From this identity 

 of the gastrula in representatives of the most various animal 

 stocks from the Sponcfes to the Vertebrata I deduce, in accord- 

 ance with the hiogenetic fundamental law, a common descent of 

 the animal Phyla from a single unhnoion stock form, Gastrsea, 

 which wets constructed essentially like the gastrula'^ . 



6. The, Body-cavity and Intestinal Cavity of Animals. 



If the preceding comparisons are correct, and consequently 

 the two primordial germ-lamellffi are homologous throughout 

 the animal kingdom from the Sponges to the Vertebrata 

 inclusive, it follows immediately and as a matter of course 

 that the Zoophyta or Coelenterata caiuiot jjossess a hody -cavity ^ 

 and that all the internal cavities of their body (leaving out of 

 consideration the intercanal system of certain sponges) belong 

 to the gastro-canal system, and are parts or diverticula of the 

 intestinal cavity. All these gastro-canals are originally lined 

 hy the entoderm, the gastral lamella, or intesti no-glandular 

 lamella, as is the case with the intestinal canal and its 

 appendages in all the higher animals. Perhaps it will be of 



* Only the Protozoa are exchided fi'om this common descent. For 

 them I assume for the most part an independent polyphyletic descent, 

 especially for those so-called " Protozoa " which might equally well be 

 regarded as plants or animals, and are therefore best grouped as neutral 

 Protista. Other Protozoa undoubtedly belong partially to the direct 

 progenitors of the Gastrula, as especially the Amoeboea and Monera. 

 The scruples which may arise against the homology of the gastrula in all 

 the different animal stocks I will refute elsewhere. The most important 

 objection seems to consist in the fact that the Gastrula is supposed to 

 originate in two perfectly different ways from the Morula : — sometimes (in 

 the Sponges, Hydroida, some Vermes, &c.) by the central excavation of the 

 Morula, and the breakinff thronr/h of the stomachal cavity thus formed ; 

 sometimes (in other Vermes, Ascidia, Echinodermata, Aniphio.vus) by the 

 formation of a germinal vesicle {Blastosphcera), a hollow sphere, the wall 

 of which consists of a layer of cells, and by the inversion of this gci-minal 

 vesicle into itself. This difference, which is apparently so essential, re- 

 quires, however, to be more accurately investigated with regard to its 

 meaning and diffusion ; and as it occurs in very nearly allied forms of the 

 same stock (e. y. the Hydroida and Medusa) ), I regard it (supposing it to 

 be real !) as quite unessential, originating by secondary counterfeiting of 

 the ontogenesis. In both cases the result is exactly the same. 



