138 



oval particles, in a state of independent motion, single and in masses. 

 The resemblance the movements of the single particles bore to the 

 peculiar motions of the more minute monads which I have been in 

 the habit of examining, at once convinced me that cilia were present, 

 if I could but see them. With a fresh supply of the granules, covered 

 with a piece of very thin glass ( T Tzyth of an inch), I proceeded to a more 

 strict scrutiny, using a very fine £lens (Smith and Beck), and with the 

 most careful adjustment regarding light &c. With this power I dis- 

 tinctly saw the individual cilia, slowly lashing and of extreme tenu- 

 ity. I repeated the examination several times, with the same result. 

 I should say that it is almost impossible to see the cilia with a lens 

 lower than £ in." 



" The appearance of the cilia was exactly similar to that repre- 

 sented in Plate III. fig. 19, b, of Dujardin's work on the Infusoria." 



From the description given by Dr. Dobie I have little doubt that 

 the species alluded to was Grantia compressa. 



I had previously to receiving this letter commenced, on the 7th of 

 September, my investigation, with a determination, in the first place, 

 to observe both the in- current and ex-current streams of water, and 

 to endeavour to trace the latter from the mouth of the sacculated 

 sponge inward, if possible, to the source of the motion of the fluid. I 

 selected specimens of the sponge not exceeding about a quarter of 

 an inch in length ; and on placing one of them beneath the micro- 

 scope, in a closed cell, after a short time the ex-current action com- 

 menced, and continued steadily flowing for a considerable period, 

 ejecting the fecal matter with much force. On examining the exte- 

 rior of the same specimen, the in-current action over the whole sur- 

 face of the sponge was equally well, though less forcibly, demonstrated. 

 The floating particles in the water, when within the action of the 

 in-current orifices, were at first slowly, but afterwards rapidly, drawn 

 towards the sponge, and the action was similar over the whole of its 

 surface, some descending on to the upper part of the surface of the 

 sponge, while others ascended to the lower part, with about an equal 

 degree of force. The fluid does not appear to enter the sponge by 

 well-defined or regular orifices, but to pass, by numerous irregular 

 pores, through the surface between the outer layer of the spicula. 



Having thus seen the continuous entrance and exit of the sur- 

 rounding fluid, I proceeded to an examination of the great saccular 

 cavity, by carefully opening the sponge from the mouth to the base, 

 through the compressed edges of the sac, with a fine pair of scissors, 

 and mounting the halves of the sponge, with the inner surface 



