W. LEUCKAETT. — GEN. CHAKACTERS. 281 



to that in tlie upper part of the sponge, though probably with 

 less energy. 



From near the center of the basal disc arises the stem, the 

 sponge-body proper, which is tubular. This is usually more or 

 less bent. In large specimens it may reach a height of one 

 meter. Its thickness at its base may about equal that of one's 

 finger, but is somewhat thinner in average specimens (11-12 mm. 

 dia.). As a rule it slightly narrows above for a short distance, 

 thence either to keep up a nearly uniform caliber or to apprecia- 

 bly thicken again (up to 17 mm.) towards the middle section of 

 the stem, where the lateral branches are in strongest develop- 

 ment. Further above, the stem shows a gradual tapering to the 

 apex. I find this end broken off in most cases. In the speci- 

 men of PI. XII, fig. 3, it is preserved, showing that the apex 

 tapers off to a point and is closed. The cross-section of the stem 

 is on the whole circular in outline ; sometimes rather polygonal. 



A well developed lumen, i. e., the gastral cavity, extends 

 through the stem from base to apex. Thickness of wall in the 

 middle of body 1.5-2.5 mm. Towards the base the wall becomes 

 much thicker at the expense of the caliber of the internal lumen. 



The moderately large oscula, to be seen here and there on 

 the wall, are sometimes round but more generally oval or elongate 

 oval apertures with the longer diameter disposed in the longi- 

 tudinal direction. They are by no means uniform in size ; the 

 largest may measure 10 mm. in the longer diameter. The oscular 

 margin is sharp-edged, scarcely thicker than a sheet of paper 

 and is usually raised into a low crateriform or lip-like rim. 



The distribution of oscula on the stem is quite irregular. 



