130 Dr Grant's Observations on the Structure 



more imperceptible as we approach the tapering extremity, (fig. 

 28, c, 6.), which has a granulated translucent appearance^ but 

 exhibits no ciha?. The part of the surface on which the ciliae 

 immediately rest is more transparent than the other parts, and 

 appears like a thin gelatinous covering spread over the other 

 darker parts within. The ciliae are very minute transparent fila- 

 ments, broadest at their base, and tapering to invisible points at 

 their free extremities ; they have no perceptible order of succes- 

 sion in their motions, nor are they synchronous, but they strike 

 the water by constantly and rapidly extending and inflecting 

 themselves; and the result of these motions is, that the ova either 

 impel the water backwards from their anterior towards their taper- 

 ing end, or they advance through the fluid, carrying their broad 

 ciliated extremity forward. We sometimes observe them stand- 

 ing erect on their tapering extremity, and revolving quickly 

 round their long axis. This is particularly remarked, after they 

 have been swimming about for a day or two, and are about to fix 

 themselves on the surface of the glass. When viewed from a- 

 bove in this erect position, they appear perfectly circular, (fig. 27.) 

 with a translucent margin, and a complete circular zone of moving 

 cihae, (fig. 27. h.) ; and when they have continued to move their 

 cihse for some time in this erect position in a watch-glass, they 

 clear away all loose particles of matter from beneath their base, 

 and accumulate a perceptible zone of loose sediment (fig. 27. c.) 

 at a little distance from their circumference (fig. 27. h.). The in- 

 cessant slowgliding motions of these ova to and fro through the wa- 

 ter, are not like those of animalcules. They appear to have no defi- 

 nite object, and are not performed by starts, like the zig-zag mo- 

 tions of animalcules in search of prey ; yet the ova appear to have 

 a consciousness of impressions made on them. When they strike 

 against each other, or against any object in their course, they re- 

 tard a little the motions of their ciliae, glide for a few seconds round 

 the spot, and then renew the action of their ciliae, and proceed 

 in their smooth gliding course. They frequently collect in large 

 quantities at the surface of the water round the margin of the 

 vessel, in which the broken sponge is laid ; and I have observed 

 them particularly accumulate on that part of the glass jars 

 which was shaded most from the light, by the body of the pa- 

 rent. About a thousand ova are contained in every cubic inch 



