1895.] Embryology. 65 
such case is known, unless it be the anomalous mode of hydroid forma- 
` tion in Epenthesis MeCradyi as described by W. K. Brooks. 
n an important discussion of the morphology of sponges, the 
author, basing his conceptions provisionally upon the generalizations of 
Haeckel and of Schultze, infers from the comparative anatomy of the 
group that there was a common ancestor, the Olynthus, which passed 
into the Sycon state by the outgrowth of radial tubes and this again 
into the Leucon by the growth of the radial tubes into flagellate 
chambers and by the growth of new entodermic diverticula, The non- 
calcareous have come from Leucon-like types. The afferent system 
of canals is ectodermic; the efferent entodermic. 
In the embryology, on the other hand, many abbreviations and’ 
other coenogenetic changes have obscured the record of the past. The- 
entoderm and mesoderm must be regarded as not as yet sharply differ- 
entiated from one another. 
Both sponges and coelenterates probably had a common solid 
ancestor, the Parenchymella. The blastopore cannot be regarded as 
an ancestral mouth and so its position is not of much weight in decid- 
ing how far the cavities of sponges and coelenterates are homologous. 
