200 LIVERPOOL BIOLOGICAL SOCIETY. 
This species is a very interesting one, as from a super- 
ficial examination with a low power one might think it a 
ceratose sponge. Even with a high power the spicules 
are difficult to recognize in the thick ceratose fibres, whilst 
in other species of Chalina they are seen well with a low 
power. The thickness of the ceratose fibres is 0°02 to 
0075 mm. ‘The spicules are extremely thin oxea, 0:07 by 
0:°002 mm. ‘The width of the ceratose meshes varies from 
0°15 to 0°30 mm. 
If Lendenfeld* is right in his theory, as he most prob- 
ably is, that ‘‘ the skeleton of the Spongidze was developed 
from that of the Homorrhaphide by the entire replace- 
ment of the spicules by spongin,” then we must certainly 
think of forms lke Chalina gracilenta, which lost the small 
traces of spicules they still possessed, whilst simultane- 
ously essential changes in the canal system took place, and 
thus became changed into Ceratosa. In regard to the 
changes of the canal system, especially the change of the 
small flagellated chambers of the Monaxonida 1nto the large 
sac-shaped ones of the Ceratosa, we may perhaps accept 
the mechanical explanation which Keller + gives in a recent 
paper. As a ceratose skeleton has certainly less rigidity 
than a siliceous one, the flagellated chambers of the Cera- 
tosa are more hable to become compressed, and to be 
seriously affected in their function, than those of the silice- 
ous sponges. An increase in size of the flagellated chambers 
would therefore be of advantage, as even under pressure 
some parts of them would remain expanded and functional. 
Keller’s theory accounts well enough for the large flagel- 
lated chambers of the Ceratosa and Myxospongie, but 
scarcely for those of the Hexactinellida, which seem to 
* R. v. Lendenfeld, ‘‘ A Monograph of the Horny Sponges,” p. 770. 
+ Conrad Keller, ‘‘ Die Spongienfauna d. rothen Meeres.” 1, Halfte. 
Zeitschr. f. wissensch, Zoologie, 48, Band, 3. und 4. Heft, 
