314 WILSON. [Vo. IX. 
The three methods employed for the production of flagellated 
chambers in the recently attached sponge may be regarded as 
fundamentally the same, the second and third being modifica- 
tions of the first. The third method I have already reduced to 
the second. The second method comes into existence because 
of the precocious division of the formative cells. A group of 
formative cells, instead of first arranging themselves round a 
central space and then dividing, divide and the masses resulting 
from their division become aggregated together and subse- 
quently acquire a central cavity. Of the three methods the 
first is probably the most primitive, the other two having been 
derived from it. Viewed in this light, the flagellated chamber 
is formed in a manner essentially identical to that in which a 
canal is produced, and is, like the canal in its origin, an inter- 
cellular space. As may be seen in a later section of this paper, 
I am forced to believe that the formation of chambers and 
canals in the young Esperella as intercellular spaces, is best 
regarded as an instance of coenogeny. 
The chambers increase in number with the increase in 
extent of the canal system ; and the number of formative cells 
and indeed the quantity of mesoderm in general, decreases at a 
corresponding rate. The gradual manner in which the distribu- 
tion of the chambers, characteristic of the adult, is acquired, 
may be gathered from a comparison of the successive stages 
shown in Pls. XVII and XVIII, Figs. 44, 48, 50. In the later 
stages reared, the chambers are found to open into the canals, 
as is shown in Pl. XVII, Fig. 50, and Pl. XVIII, Fig. 48. 
Spicules. —The long spicules found at the posterior end of 
the swimming larva become distributed through the body of 
the sponge during the course of attachment, Pl. XVII, Fig. 36. 
In the young sponge, after attachment, they are found with 
their sharp ends projecting for a short distance all over the 
upper surface, Pl. XVIII, Figs. 55, 58. Sections show that the 
projecting spicules do not perforate the ectoderm, but that they 
lift the ectoderm up, supporting it like so many tent poles, 
Pl. XVII, Figs. 43, 45. The first indications of the spicular 
bundles which support the dermal membrane in the adult, are 
to be seen in such stages as Fig. 44, where a few spicules are 
