No. 3-] DEVELOPMEXT OF MAREXE SPONGES. 355 



are certain indications in the silicious sponges (p. 8) that in the 

 adult, osciila may be developed almost anywhere. Such facts 

 make it impossible to fix upon the number of component indi- 

 viduals in any sponge. Perhaps the nearest approach made in 

 other groups to the formation of colonies, in which the person- 

 ality of the component individual is so nearly lost, is found in 

 corals like Maeandrina, in which the united gastric cavities of 

 the polyps form continuous canals, perforated at intervals by 

 mouths. 



We therefore reach the conclusion that the higher sponges 

 (Non-Calcarea) have been derived from colony producing, sym- 

 metrical forms, in which the evaginations of the primitively 

 simple paragastric cavity had already taken the form of effer- 

 ent canals and flagellated chambers, that is from forms allied 

 to the existing Leucons. And further we come to the con- 

 clusion that the subdermal cavities and afferent canals are 

 homologous with the intercanals of Sycons, and hence, phylo- 

 genetically at least, are infoldings of the ectoderm. The 

 whole efferent system, canals and flagellated chambers both, 

 on the contrary is homologous with the same system in the 

 calcareous sponges, and is endodermic. 



This conclusion as to the parts played by the germ layers in 

 producing the adult non-calcareous sponge, is the one enun- 

 ciated by Schulze in his classical paper on the Plakinidae 

 (p. 438). In this little family of silicious sponges Schulze 

 finds a genus, Plakina, the three species of which form links in 

 a chain of increasing complexity, showing quite as plainly as 

 do the calcareous sponges that the afferent system is derived 

 from ectodermal infoldings, and the efferent from endodermal 

 outfoldings. 



The Plakinidae are Tetractinellids. The three species of 

 Plakina are small encrusting sponges found in the Mediterra- 

 nean. They all adhere to the under side of stones, shells, etc. 

 A vertical section of the simplest species, Plakina monolopha, 

 is given in PL XXV, Fig. 7. There is a continuous basal cavity 

 crossed by strands of tissue, in which lie developing eggs. 

 From the cavity run vertical efferent canals {ef. c), which arc 

 simple or very slightly branched, and into which open the 



