ELISHA MITCHELL SCIENTIFIC SOCIETY. 41 



In Reniera Jiligrana^^ there is a solid swimming larva, 

 which after attaching acquires a central cavity with an 

 apical osculum. The flagellated chambers arise as diverti- 

 cula from this cavity. Thus in this sponge also there is a 

 rhagon stage. But in one matter we strike upon a coeno- 

 genetic modification. The afferent canals instead of being 

 ontogeneticall}- formed from the ectoderm, as they seem to 

 have been phylogenetically, are really formed from endo- 

 dermic diverticula, which grow outwards, meeting the sur- 

 face epithelium. 



In Chalinula fertilist 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 flagel- 

 lated 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 indicate that the two layers are of much the 

 same nature. This essential similarity between the tw^o 

 layers has always been maintained by Metschnikoff", not 

 only on the ground of dev^elopment, but for physiological 

 reasons as well. Thus in young Spongillas when the water 

 became bad he witnessed the entire disappearance of the 

 flagellated chambers, the sponge then consisting of ectoderm 

 and mesoderm alone. With a fresh supply of water the 

 chambers re-appeared. J Again, after feeding carmine in an 

 excessive amount to Halisarca poutica^ he found that the 

 canals and chambers entirely disappeared, the whole 

 body of the sponge inside the ectoderm consisting merely 

 of a mass of amoeboid cells full of carmine iibid.^ p. 272). 



^Marshall. Die Ontogenic von Reniera filigrana. Zeit. fur Wiss. Zool. Bd. 37. 

 •Keller. Stud, uber die Organisation und die Entwicl 

 iss. Zool. Bd. 33. 

 jMetschnikoff. Spong. Stud. Zeit. fur Wiss. Zool. Bd. 32. 



tKeller. Stud, uber die Organisation und die Entwick der Chalineen. Zeit. fur 

 Wiss. Zool. Bd. 33. 



