§ 7. 



CILIA, OR VIBRATILE ORGANS. 597 



of the present day refer them to the Vegetable kingdom ; 

 thus Professor Owen expresses his opinion, that if a line 

 could be drawn between animals and vegetables, the 

 Sponges should be placed among the latter. But these 

 bodies appear to me to hold an intermediate place — to 

 be on the boundary line, that intervenes between the 

 two grand* divisions of animated nature ; they are, in 

 short, true Zoophytes or Animal-plants ; it will, therefore, 

 be instructive to dwell a brief space on the investigation of 

 their structure, and economy. 



The living Sponge, when highly magnified, exhibits a 

 cellular tissue, permeated by pores, which unite into cells, 

 or tubes, that ramify through the mass in every direction, 

 and terminate in larger openings. In most Sponges the 

 tissue is strengthened and supported by spines or spicula 

 of various forms, and which, in some species, are siliceous, 

 and in others calcareous.* The minute pores through 

 which the water is imbibed, have a fine transverse gela- 

 tinous net- work, and projecting spicula, by which large 

 animalcules or noxious particles are excluded ; water inces- 

 santly enters into these pores, traverses the cells or tubes, 

 and is finally ejected from the large vents. But the pores 

 of the sponge have not the power of contracting and 

 expanding, as Ellis supposed ; the water is attracted to these 

 openings by the action of instruments of a very extraordi- 

 nary nature, by which currents are produced in the fluid, 

 and propelled in the direction required by the economy of 

 the animal. 



7. Cilia, f or vibratile orgaxs. — Although these 

 organs, which are termed Cilia, from their hair-like appear- 

 ance, are not confined to the class of animals which form the 

 subject of this inquiry, yet they play so important a part in 

 the economy of the Zoophytes and Crinoidea, that it will be 

 necessary to define their structure and functions. These 



* See Medals of Creation, p. 264. f From cilium, an eye-lash. 

 R r2 



