OF THE SPONGIAD^E. 



represents what are apparently the detached cilia and their 

 basal cells, and which were probably from Grantia compressa. 



If portions of a living sponge of this species be torn into 

 small pieces, and placed in a cell in sea-water under a 

 power of about 400 linear, groups of the detached cilia and 

 their basal cells will be readily seen at the margins of the 

 specimen ; they are usually thus clustered together, and 

 have a tremulous and indistinct motion. If a small speci- 

 men of the sponge be slit open and placed in a cell with 

 fresh sea-water, with the inner surface of the sponge towards 

 the eye so as to command a distinct view of the oscula, the 

 cilia will be seen in the area of that organ in rapid motion, 

 and the extraneous molecules attached to them exhibit the 

 extent and nature of their oscillations very distinctly (Fig. 

 313, Plate XXI). If the sponge be carefully torn asunder 

 in a line at right angles to its long axis, and the torn surface 

 be placed in a cell with a little fresh sea- water, we occa- 

 sionally obtain a favorable longitudinal section of some of 

 the large cells of the sponge, and we then see the cilia in 

 situ and in motion (Fig. 312, Plate XXI). 



The whole length of the cell, from the inner edge of the 

 diaphragm to its origin near the outer surface of the sponge, 

 is covered with tessellated nucleated cells, which have each 

 a long attenuated and very slender cilium at its outer 

 end. They are oval in form, and have a distinct nucleus. 

 When in vigorous condition their motions are rapid and 

 cannot readily be followed, but in some in which the action 

 was languid, the upper portion of the cilium was thrown 

 gently backward towards the surface of the sponge, and 

 then lashed briskly forward towards the osculum, and this 

 action was steadily and regularly repeated. Their motions 

 are not synchronous, each evidently acts independently of 

 the others (Figs. 314, a, b, Plate XXI). 



The numbers, situation, and peculiarities of their actions 

 fully account for the continuous and powerful stream that 

 issues from the great cloacal aperture of this and other 

 similarly constructed sponges. The natural rate of the 

 motions of these organs must not be estimated from the 

 sections last described, but the estimate must be made 



9 



