S p o n g i e. 



12, Trichostemma hemisphiericum, M, Sars. 



nov. gen. et spec. 

 (PI. VI. fig. 1 15). 



Of this sponge, so peculiar in its appearance, I have seen a number of specimens of 

 various size; and all these shewed the same characteristic form, which must thus be consi- 

 dered as perfectly constant for the present species. 



The largest specimens have a transverse diameter of nearly 2" and a height of 1". 

 The form (see fig. 1 & 2) is more or less .high hemispherical, circular with the lower surface 

 plane, or in the middle a little concave; the upper surface evenly ^convex; the circular edge 

 rather sharp, and in its whole length closely bordered with a ring of numerous capillary 

 radiate spicula which together form a corona round the sponge like a nimbus, and in width 

 about V 5 V 7 of the sponge's transverse diameter. The plane or somewhat concave lower 

 surface of the sponge is sunk in the mud, and the circular rim of radiating spicula contri- 

 butes essentially to secure its position, by spreading out in the mud on all sides. Of the 

 whole sponge there is thus only the upper convex surface which projects above the mud. 

 From this upper surface arise a greater or less number of obtusely conical, fistular processes 

 curved in various manners, each having at its extremity a circular aperture, the so-called 

 out-flow aperture (osculum). These fistular oscula, of which the number may vary from a 

 very few up to 20, are always confined only to the highest part of the convex surface; be- 

 tween them and the circular rimmed border, there is always a tolerably wide field without 

 any oscula (see fig. 2) which is naturally accounted for by the sponge being probably sunk 

 in the mud considerably beyond the border-rim. The latter performs doubtless the same 

 service as the root-fibres in other sponges ; that is, it serves to fix the sponge in the soft 

 mud in which it is found. Only in a few specimens I have found that the lower concave 

 surface had attached itself to "a small stone or to a larger shell, round which it had grown 

 entirely; so that only a small part of the object was visible externally. The sponge must 

 therefore properly be said to be free, not attached, but only by help of its border-rim lying, 

 as it were, at anchor in the soft mud. 



The whole sponge is of rather firm consistency, and of a white grey color. The lower 

 surface is quite smooth, the upper convex part is on the contrary somewhat uneven or cor- 

 rugated with small roundish tolerably regular elevations and depressions. 



