The Productivity of Fish Food in Oneida Lake 241 



diameter at the middle is .015-.02 mm. with the latter dimen- 

 sion the more common. The length of the gemmule amphi- 

 strongyli of the more abundant type is .04-.065 mm. and the 

 diameter .006-.007 mm. The length of gemmule amphi- 

 strongyli from the substratum layer is .07-. 10 mm. and the 

 diameter .004-.005 mm. The length of the amphioxi from 

 the same layer is .15-17 mm. and the diameter at the middle 

 .005 mm. 



The gemmules are somewhat flattened and the chitinous 

 layer which is of moderate thickness is prolonged into a 

 foraminal tubule which reaches slightly above the pneumatic 

 crust (fig. 6). The cellular structure of the pneumatic layer 

 (fig. 5) is less obvious than in S. fragilis, in part because of 

 the transparency of the walls of the spaces, and in part 

 because of the densely crowded mass of spicules with which 

 it is charged. The diameter of the gemmules in the plane of 

 the substratum shows considerable variability but averages 

 about .5 mm., and the height in the axis of the foramen and 

 perpendicular to the substratum is half or two-thirds as much. 

 Figures 7 and 8 show something of the marked difference 

 between the two surfaces af a cleared fragment of a gem- 

 mule layer. The actual difference is still greater since 

 figure 7 represents only a superficial layer of what is 

 actually a thick crust of spicules. 



Whether or not older colonies may form groups of gem- 

 mules in the sponge mass apart from the substratum, as do 

 those of S. fragilis, is not yet known. Colonies of the latter 

 species in the same collection have only the pavement layer 

 and are apparently rather young colonies, as are presumably 

 those of the new form. 



There are approximately 50 known species of Spongilla, 

 and of these the majority have smooth skeleton spicules. 

 Very few indeed combine the characters of spinous skeleton 

 spicules and a pavement layer of gemmules, and of these the 

 new species is the only one known from North America. 



