No: 3.) DEVELOPMENT OF MARINE SPONGES. 355 
are certain indications in the silicious sponges (p. 8) that in the 
adult, oscula 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, ec. 
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 are 
simple or very slightly branched, and into which open the 
