314 WILSON. [A'OL. 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 Pis. 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 PI. XVII, Fig. 50, and PI. 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 X\'II, 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 



