THE 6ASTRAEA-THE0RY, ETC. 151 



older acoelomatous worms the higher worms (Caelomati) 

 have secondly developed themselves by the formation of a 

 coelom and of a circulatory system (depending thereon). Four 

 divergent descendants of theeoelomatous worms form the four 

 typical most highly developed races of animals : the animal 

 stems or phylee of the Mollusca, Echinodermata, Arthropoda, 

 and Vertebrata. 



The firm foundation for this Gastraea-theory, and for the 

 widely extending consequences which we are about to deduce 

 from it, is explained in my monograph of the Calcareous 

 sponges (1873). In the preparation of this monograph I 

 chiefly endeavoured — firstly, to give as thorough and com- 

 prehensive a representation as possible of all the biological 

 relations of this interesting little group of animals ; and, 

 secondly, to attempt, upon the ground of their extreme 

 plasticity of form, " an analytical solution of the problem of 

 the origin of species," to give an analytical proof of the truth 

 of the theory of descent. But besides these special objects, 

 the developmental history of the calcareous sponges, the 

 discovery of their gastrula form, as well as the question of 

 their natural affinities, and their place in the animal system, 

 necessarily, and of itself, led me on to the general question of 

 the homology of their germ-lamellse with those of the higher 

 animals, and thus further on to that series of ideas whose 

 nucleus, in a word, forms the Gastraea-theory. The leading 

 ideas, which will be developed subsequently, are all con- 

 tained already in the monograph of calcareous sponges, but 

 in it there was neither room nor fitting opportunity to 

 develop them further. In giving here this explanation of 

 the Gastraea-theory, I must refer throughout to the mono- 

 graph of calcareous sponges for the series of special observa- 

 tions which serve me as a sure empirical basis.^ 



The surest possible ground from external evidence was 



' The following passages in the first volume of the ' Calcareous Sponges ' 

 are especially to be compared : — Doctrine of individuality (pp. 89 — 124!), 

 histology (pp. 130 — 180), organology of the canal-system (pp. 210 — 292), 

 development (pp. 328 — 360), adaptability (pp. 381 — 391), inheritance 

 (pp. 399 — 402), and philosophy of the calcareous sponges (pp. 453 — 484). 

 In the last division are the reflections on the primordial form of the sponges 

 (p. 453), the germ-lamella theory, and the genealogical tree of the animal 

 kingdom (pp. 464, 465), the biogenetic principle (p. 471), and the causes of 

 the production of form (p. 481), are of special importance with respect to 

 the Gastraea-theory. lu order to avoid useless repetitions, I must again 

 refer to these passages in the first volume (' The Biology of Calcareous 

 Sponges'). Many relative observations are specially detailed in the second 

 volume (' The System of Calcareous Sponges '). Illustrative figures are to 

 be found in the sixty plates which form the third volume (' The Atlas of 

 Calcareous Sponges'). 



