348 THE SPONGIDA OR PORIFERA. 



to be produced by cilia disposed round the inside of the pores." 



Grant correctly divined the cause of the circulatory currents in 

 the Spongida, but he never saw the cilia, though-rather curiously- 

 he saw and described the cilia of the free ciliated '"ova" (as he 

 named these reproductive bodies), which had been swept by the 

 water-currents from the interior of the Spongida out through the 

 oscula. In 1850, Dr. Dobie, at Berwick-on-Tweed, on examining 

 with a i/8th inch objective some gelatinous matter removed from 

 a specimen of Grantia (Sycaiidra) compressa (Fig. 4, Plate 

 XIV.), which he had previously slit up, "distinctly saw the 

 individual cilia slowly lashing and of extreme tenuity." In Sept, 

 1850, Bowerbank carefully examined, at Tenby, specimens of the 

 same sponge, about a quarter of an inch in length ; he saw parti- 

 cles of excrementitious matter ejected through the large osculum 

 (<?, Fig. 4, Plate XIV.) by a forcible and continuous current, and 

 " on examining the exterior of the same specimens the in-current 

 action over the whole surface 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 descend- 

 ing on 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," Bowerbank added, " 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." On making very rough transverse sections of this sponge, 

 Bowerbank at last, by careful microscopical examination, saw the 

 cilia lining the radial tubes or chambers (running from the pores 

 through the wall to the central cavity of the sponge) in action. 

 He figured the ciliated cells as shown in the illustration (Fig. 4, 

 Plate XVI.), and described them as " tesselated cells and cilia." 

 Like a good many other observers, Bowerbank failed to see the 

 characteristic part of the cell — the collar — as shown in the cells 

 (Fig. la and Fig. 2c, Plate XVI.). 



In 1857, Bowerbank showed how the action of the circulatory 

 system of the Spongida could be readily demonstrated. He took 

 a small but vigorous specimen of Spongilla fiuviatilis, and added 



