and Functions of' the Sponge. 1£9 



we observe many of the ova pass out spontaneously along with the 

 currents and feculent matter, (fig. SI. e.), and when they have 

 been discharged by the fecal orifices, or have fallen out from bro- 

 ken portions of the sponge, they do not sink to the bottom of 

 the water by their own gravity, like every other substance com- 

 posing the body of the animal, but continue floating and drifted 

 about by the currents. The most remarkable appearance exhibit- 

 ed by these ova, is their continuing to swim about by their own 

 spontaneous motions, for two or three days after their detach- 

 ment from the parent, when they are placed separately in ves- 

 sels of sea water, at perfect rest. During their progressive mo- 

 tions, they always carry their rounded broad extremity forward, 

 and when we examine them under a powerful microscope, we 

 perceive that these motions are produced by the rapid vibration 

 of ciliae, which completely cover the anterior two-thirds of their 

 surface, (fig. 28. a. to c). I have not perceived any ciliae on 

 the tapering posterior third of their body (fig. 28. c. to b), which 

 has a whiter and more pellucid appearance even to the naked 

 eye, than the ciliated anterior part. By examining the sponge 

 carefully with the microscope, we are surprised to find that many 

 of the mature ova are now hanging by their tapering extremity 

 from the parietes of the internal canals, (fig. 26. d, and fig. 21. g.), 

 either by having advanced themselves into the canals, or by 

 opening new passages for themselves, by the motions of their 

 ciliae. While in this fixed situation their ciliae are always in a 

 state of very rapid vibration, which has a tendency to tear them 

 from the sides of the canals, and when their connection is once 

 destroyed, they are driven headlong by the currents through the 

 fecal orifices (fig. 21.) The singular motions and structure of 

 the detached ova, are best observed by placing a few of them 

 together in a small drop of sea water, on a plate of glass under a 

 powerful microscope. They have all the same size, the same re- 

 gular ovate form (fig. 28), and the same bright yellow colour by 

 reflected light ; but by transmitted light they have an amber co- 

 lour, appear much less translucent in the central parts than to- 

 wards the sides, and have a rough granulated surface. Their 

 ciliae are longest, and exhibit the most distinct motions, on the 

 anterior part (fig. 28. «.), and become gradually shorter and 



OCTOBER DECEMBER 1826. ' T 



