THE DOCTRINE OF TYPES. 247 



my Gastrcea Theory, 24 on which I base the monophyletic 

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

 vinced must supersede the Theory of Types which now 

 prevails. According to this Gastrsea 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 Gastrsea 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 (protogaster), the primitive 

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

 the Gastrsea itself, and in the extant Gastreads (Haliphy- 

 sema, 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 



