BY R. VON LENDENFELD, PH.D. 311 



Vestibule. 



The great difference between the genera Aulena and Halme, lies 

 not only therein, that in the former the true body of the sponge is 

 formed by lamellfe, and in the latter by cylindrical threads ; but 

 particularly also in the great difference in the development of the 

 vestibule in these two. 



Whilst in Halme it is a simple empty space between the sponge 

 lamellte ; it is traversed by numerous fine membranes in Aulena. 

 (Plate XXXI., fig. 19, Plate XXXIL, fig. 21), which sub-divide 

 it into smaller, more or or less spherical compartments connected 

 with one another by large circular pores in these membranes. It 

 is apparent that by movements of these membranes the current of 

 water can be greatly influenced, and we find that there are nei'vous 

 and muscular elements contained in them by the united action of 

 which, no doubt the water current is regulated to the advantage 

 of the sponge. 



The meshes of the network formed by the true body of the 

 sponge measure, 7-2 mm,, and this is the extent of the vestibule 

 lacunae. 



The compartments into which the vestibule space is divided are 

 more or less spherical and measure 0-2-0"5 mm. in diameter. 



The whole structure has a froth-like appearance. (Plate XXXI., 

 fig. 19.) This tissue fills the whole of the vacant space between 

 the meshes of the sponge network. 



The thickness of the membranes is on an average 0-017 mm. 

 The circular pores (Plate XXXIL, fig. 21), are situated in the 

 middle of the fields limited by the lines where the membranes join. 



The membranes are supported by horny fibres, which pervade 

 the whole of the lacunose part of the sponge. 



The Significance of the Vestibule in Aulena. 



It is apparent that this structure although homologous to the 

 simpler one of Halme, being very diflerently developed, must 

 perform slightly different physiological function. 

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