OF THE SPONGIAD^E. 59 



being evidently destined to sustain and protect extended 

 membranes, while the other is especially adapted for a 

 small curved surface by its form and small size ; each of 

 the figures being drawn with the same power, 660 

 linear. 



On the surface of the ovarium of Spongilla cinerea, 

 Carter, we find this description of spiculum still more 

 decidedly produced. It is of a cylindrical form and 

 entirely spined, and has just the amount of curvature that 

 is in unison with the surface on which it reposes. The 

 spines on the middle of the shaft are cylindrical, and 

 terminated bluntly so as to strengthen its hold on its 

 imbedment. Those of its apices, on the contrary, are 

 acutely conical and recurved, and are strongly produced so 

 as to form very efficient weapons of defence. This spiculum 

 is represented by Fig. 207, Plate IX. 



The birotulate and boletiform spicula of the second 

 group appear to be more purely structural, as regards 

 the skeleton of the ovarium. The rotulae are very closely 

 packed at both the external and internal surfaces of that 

 body, and the crenulation or dentation of each rotula is as 

 well produced on the internal as on the external ones, and 

 it appears to be very influential in maintaining each 

 spiculum in its proper position. In the natural condition 

 of the ovaria these spicula are entirely imbedded in its 

 walls, and other spicula of a truly defensive nature are 

 superimposed for its protection. The large spine in the 

 shafts of the birotulate spiculum from Spongilla plumosa, 

 Carter (Fig. 208, Plate IX), are also apparently subservient 

 to strengthening and maintaining the spiculum in its 

 proper situation, although they are acutely terminated, as 

 defensive spines usually are ; but in the same relative 

 position on the birotulate spicula of ftponffilla Meyeni, 

 Carter, we find the spines short, stout, and cylindrical, 

 spreading or budding at their apices, and evidently more 

 fitted for assisting to retain the spiculum in its proper 

 place than for defensive purposes. This spiculum is repre- 

 sented by Fig. 219, Plate IX. 



There is an apparent analogy between the expansions of 



