40 JOURNAL OF THE 



The evidence from the calcareous sponges (Sycandra passes 

 through Olynthus stage) has already been given. Several 

 of the non-ca\cai-eo\\s sponges {Osca7^e//a /od^{ /a ris^ Reniera 

 filigrana^ Chalinulafertilis^ Plakina ;;^^;;/<r;A;/>/^^) run tli rough 

 a stage known as the rhagon (Sollas), which it is permissible 

 to regard as the ontogenetic representative of the Sycon type. 



The rhagon of Oscarella* is a three-layed sac with a 

 terminal osculum. The flagellated chambers form a single 

 layer round the central cavity opening into it by wide 

 mouths, and opening on the surface by pores. Regarding 

 this form, as seems best, as equivalent to the Sycon type, 

 it will be noticed that the radial tubes of the 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 rise as true diverticula from 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 P, nionolopha, is directly comparable 

 with a simple Leucon. The development of Oscarella, in 

 laree measures, confirms the conclusions drawn from com- 

 parative anatomy, and may therefore be considered as 

 phylogenetic. 



The development of Plakina monolopha (Schulze, /. r.) 

 has already been described. The sac with its single layer 

 of flagellated chambers round a central cavity is a rhagon, 

 and may be taken as representing the Sycon stage. The 

 adult Plakina itself is the Leucon stage. 



*Heider. Zur Metamorphose der Oscarella lobularis. Arb. Zool. Inst. Wien. Bd. 6. 



