358 WILSON. [Vol. IX. 



Sycon are coenogenetically replaced by flagellated chambers. 

 The rhagon of Oscarella is formed as an iiivaginate 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 



