SOME PROBLEMS OF COELENTERATE ONTOGENY 531 
as to the likeness of the diblastic tissues of coelenterates and the 
mucous and serous layers of the embryo (’49). Let it be noted, 
however, that Huxley does not designate these as homologous, 
but rather as analogous. Ten years after his first utterance he 
remarks ‘‘It by no means justifies the assumption that the Hydro- 
zoa are in any sense arrested developments of higher organisms. 
All that ean be justly affirmed is, that the Hydrozoon travels for 
a certain distance along the same great highway of development 
as the higher animals.’”’ (Oceanic Hydrozoa, p. 2.) 
Interestingly enough, the embryological researches of the time, 
led by Kowalevsky, Gegenbaur, Haeckel and others, centered 
about this pregnant conception of Huxley and led Haeckel to 
formulate his famous Gastraea Theory, with all its far-reaching 
implications as to the homology of the germ layers of all embryos, 
‘‘from the lowest sponge to man.” Of course, the gastrula at 
once sprang into a position of commanding importance in embry- 
ology, and as the prototype of Haeckel’s hypothetical gastraea 
became a focal factor in embryological thought for a whole gen- 
eration. It is not strange, therefore, that the Coelenterata, 
as the distinctively diploblastic phylum of the animal world, 
should early come in for a more than usual measure of interest 
and concern; and as the theoretical ancestral phylum from which 
all higher metazoa must have arisen, should have at once 
assumed a unique and dominant phylogenetic importance. 
When, however, it is clearly known that in only a single class 
of coelenterates does gastrulation occur, and that in no case is 
the gastrula, as an embryo, known, it seems remarkable to the 
point of surprise that the theoretical postulate should still be 
cherished by not a few students of phylogeny. Current liter- 
ature, however, furnishes abundant evidence of just such adher- 
ence to tradition. 
b. The planula. As is well known the planula is the distinc- 
tive larva of the entire phylum, including also the sponges. It 
has generally been assumed that the planula is a specialized gas- 
trula, and that in some early species its enteron must have been 
formed by gastrulation. In this again there is involved the fur- 
ther inference and implication of the dominance of the biogenetic 
