THE GASTBAEA-THEORY, ETC. 155 



physiological condition of the gastrula-form throws the 

 clearest light on the monophyletic genealogical tree of the 

 animal kingdom. If anybody wished to construct a jt?nori 

 as simple an animal form as possible which should possess 

 that most important primitive animal organ, the intestine, 

 and the two primary germ-lamellse, he would arrive at the 

 same form which the gastrula actually represents. 



I have fully described the arrangement and structure of 

 the gastrula in the Ontogenesis of the ' Calcareous Sponges ' 

 (loc. cit., pp. 333 — 337). It recurs in all three families of this 

 group of animals, and always in the same form, in the 

 Ascones {Ascuhnis armata, t. xiii, figs. 5, 6) ; in the 

 Leucones {Leuculmis echinus, t. xxx, figs. 8, 9) ; in the 

 Sycones {Sycyssa Huxleyi, t. xliv, figs. 14, 15). It every- 

 where displays the same essential structure, and only differs 

 in quite unimportant proportions. The uniaxial unsegmented 

 body is sometimes globular, sometimes egg-shaped or oval ; 

 more rarely sphseroidally flattened or lens-shaped. The 

 diameter mostly measures from O'l to 0'2 millimetre. The 

 primitive stomachal cavity, or the primitive intestine {pro- 

 gaster), is of the same conformation as the body, and opens 

 at one pole of the longitudinal axis by a simple oral cavity 

 (the primitive mouth, prostoma). The two cell-layers or 

 lamellae, which compose the wall of the stomach, become 

 differentiated in a very characteristic manner. The inner 

 cell-layer, the entoderm or gastral-layer, which corresponds 

 to the inner or vegetative germ-layer of the higher animals, 

 consists of large, dark, globular, or subsphseral-polyhedric 

 cells, which differ in little from the furrowed cells of the 

 morula, and average 01 millimetre in diameter. The 

 outer cellular layer, the exoderm or dermal layer, which 

 represents the outer or animal germ-layer of the higher 

 animals, consists of smaller, paler, cylindrical, or prismatic 

 cells, each of which carries a long cilium, or vibratile body, and 

 is 0"02 millimetre long and only 0"004 millimetre thick. 

 (In the schematic representations of the gastrula, on PI. VII, 

 figs. 1 — 8, which belongs to this section, the cilia of the 

 exoderm are purposely omitted.) 



In the section of the plant-like animals (Zoophyta or 

 Coelenterata) the same gastrula-form occurs not only 

 in the most dissimilar sponges, but also widely distributed 

 in the Acalephse,^ in the Hydroid polypus and Medusae, in 



' The gastrula of the plant-like animals has already been more or less 

 plainly described and figured in many of the older and newer works on 

 sponges, Hydromedusge, &c. Compare Kowalevsky's remarks "On the 

 Development of the Coelenterata" (' Gottinger Nachrichten,' 1868, p. 154) j 

 also the works of Agassiz, AUman, &c. 



