18 AETHUE DENDY. 



derm around the chamber has been pulled out into delicate 

 strands forming a kind of network, but mostly radially dis- 

 posed, and thus serving to suspend the dying or dead chamber 

 as shown in the figure. 



Fig. 28 {x) shows what is perhaps a later stage in events. 

 The chamber occupies a still smaller space, and the surrounding 

 mesoderm has become solid and homogeneous again. The 

 arrangement of the spicules and of the surrounding chambers 

 still indicates the space originally occupied by the dying 

 chamber. 



If the individual chambers die it is probable that they are 

 replaced by new chambers ; and, indeed, I shall give reasons 

 later on, in describing the exhalant canals, for supposing that 

 new chambers are actually interpolated between the old ones. 

 Thus the older parts of the sponge may be kept alive and 

 vigorous by the gradual replacement of the old flagellated 

 chambers, as they reach their limits of existence and die off, 

 by new ones. I have unfortunately found no evidence to show 

 how the new chambers originate, but since the older flagellated 

 chambers frequently branch it is not unlikely that they may 

 also bud, or the new chambers may be developed from amoeboid 

 cells as in the embryo ofStelospongos (2). The problem 

 is on much the same footing as the question, how are new 

 chambers constantly added around the margin of the growing 

 sponge-cup ? and, so far as I am aware, no one knows. All I 

 can say is that they commence life very small, and gradually 

 increase in size as they grow older (figs. 25, 27) ; they make 

 their first appearance in about the middle of the thickness of 

 the sponge-wall, and apparently do not originate as out- 

 growths of the gastral cavity. 



Another explanation of the unusual condition of the flagel- 

 lated chambers described above is suggested by some observa- 

 tions of SoUas, in his report on the "Challenger'^ Tetracti- 

 nellida (13), to the effect that the walls of the flagellated 

 chambers in this group sometimes appear contracted, under 

 which condition " fine filaments may be frequently observed 

 produced from the base of the choanocytes and extending 



