358 WILSON. [Vou. IX. 
Sycon are coenogenetically replaced by flagellated chambers. 
The rhagon of Oscarella is formed as an invaginate gastrula, 
which attaches mouth down. The gastrula mouth closes, and 
the osculum is a new formation. The flagellated chambers 
arise as true diverticula of the central cavity. The adult 
Oscarella, the canal system of which is not far removed from 
that of Plakina monolopha, is very probably formed from the 
rhagon, by the development in the latter of a number of simple 
diverticula from the central cavity. These diverticula are the 
efferent canals into which open the flagellated chambers. The 
ectodermic spaces between the efferent diverticula become 
the afferent canals. The adult Oscarella, like Plakina mono- 
lopha, is directly comparable with a simple Leucon. The 
development of Oscarella in large measure confirms the con- 
clusions drawn from comparative anatomy, and may therefore 
be considered as phylogenetic. 
The development of Plakina monolopha (Schulze) has already 
been described. The sac with its single layer of flagellated 
chambers opening into it, is a rhagon, and may be taken as 
representing the Sycon stage. The adult Plakina itself is the 
Leucon stage. 
In Reniera filigrana (Marshall 18) there is a solid swimming 
larva, which after attaching acquires a central cavity with an 
apical osculum. The flagellated chambers arise as diverticula 
from this cavity. Thus in this sponge also there is a rhagon 
stage. But in one matter we strike upon a coenogenetic modi- 
fication. The afferent canals, instead of being ontogenetically 
formed from the ectoderm, as they seem to have been phylo- 
genetically, are really formed from endodermic diverticula, 
which grow outwards, meeting the surface epithelium. 
In Chalinula fertilis (Keller 10) there is also a solid larva 
in which a central cavity is hollowed out. But in this sponge 
the flagellated chambers of the rhagon stage do not arise as 
endodermic diverticula, but are formed independently from 
solid groups of mesoderm cells. This origin of the flagellated 
chambers must be regarded as coenogenetic. The fact that 
the mesoderm may take upon itself the function of forming 
organs ordinarily formed by the entoderm would seem to 
