BY R. VON LENDENFELD, PH.D. 299 



find in the lower proximal portion of this canal, sand grains, 

 which measure only 0-02 mm. or less on an average. (Plate 

 XXXVII., fig. 7 ; /.) The thickness of the layer they form is 

 only 0-5 — 7 mm. or less. A similar layer of similar sand grains 

 is found throughout the interior surfaces of the sponge. 



Growth of the Skeleton, 



Whether a selection in the sandgrains, which came in contact with 

 the sponge, by the latter, is executed in such a way as to retain 

 the large grains on the outer surface and the small ones in the 

 interior, cannot be ascertained, but there can be no doubt, that 

 also without such an act on the part of the sponge the disparity 

 in size of these elements of its cortex can be partly explained 

 without difficulty in the sense of F. E. Schulze. (1) The large 

 sand granules never enter the pores in the outer lamella because 

 there is not sufficient current in the interior of the sponge to 

 carry them along. The small ones are carried along also with the 

 weak current in the interior, and they are then retained on the 

 interior surfaces. 



The fact however that no small sand grains are found in the 

 outer surface cannot be accounted for in this way. There we must 

 assume that the sponge exerts some voluntary selection. 



As the sponge grows, the outer surface with the large grained 

 cortex is moved more and more from the basis of the sponge and 

 the main fibres grow after it always remaining in contact with it 

 they derive as it were the sand we afterwards find in them from 

 the cortex. 



We can conclude from this, that the region ivliere the sponge 

 grows is just below the surface. 



The connecting fibres grow out from the main fibres as room for 

 them occurs below the receding outer cortex. 



(1) i^. E. Schulze. Untersuchungen Ueber den Ban unci die Entwickehmg 

 der Spongien. Die Gattung Spongelia. Zeitschrift fiir wissenschaftliche 

 Zoologie. Band XXXIII., Seite 131. 



