THE DOCTRINE OF TYPES. 247 



my Gastrcea Theory ,^^ on which I base the monoj)hyletic 

 genealogy of the animal kingdom, and which I am con- 

 vinced must supersede the Theory of Types which now 

 prevails. According to this Gastr?ea Theory, which I 

 enunciated in the " Monograph on the Chalk Sponges '' 

 (vol. ii. pp. 465-467), the seven types or tribes of the animal 

 kingdom possess an entirely different significance and an 

 entirely unequal value. Only the four higher tribes 

 — Vertebrates, Arthropods, Molluscs, and Echinoderms — 

 are types in the sense of Cuvier and Baer, and even these 

 only in a limited sense, not as originally meant by the 

 authors of the theory. On the other hand, the lowest 

 type, that of the Primitive-animals, is not really a " type," 

 but the sum of all the lowest animals ; it was from a 

 branch of the Primitive-animals that the Gastr^ea developed. 

 The two remaining types, the Plant-animals and the Worms, 

 stand between the Primitive-animals and the four higher 

 types. They are more specialized and typical than the 

 Primitive-animals, and less typically organized and charac- 

 terized than the four higher tribes. 



The Gastrsea Theory is founded on the fact that we 

 have proved the two primary germ -layers to be the rudi- 

 mentary bodily-structure common to the six higher groups 

 of animals. But it is also proved that a single original 

 organ is of the same use, or homologous, in all these 

 animals; this is the intestine {protog aster), the primitive 

 intestinal or stomach cavity, in its most simple form. In 

 the Gastroea itself, and in the extant Gastreads {Haliph.y- 

 se^ma, Gastrophysema), the entire, simple, spherical or oval 

 body consists only of this simple primitive cavity, open at 

 one pole of the axis (the primitive intestine and primitive 



