rS6 Dr Grant's Observations oti the Structure 



part we observe to advance first during the spreading of the 

 ovum, (fig. 29. h). It is a very remarkable circumstance, that 

 Aristotle is almost the only writer who has described this part of 

 the anatomy of the sponge. He observes, that they do not ad- 

 here by a continuous surface; that they have some intermediate 

 empty canals ; that they are fixed only at particular parts to 

 the rocks, and have a kind of membrane spread out under their 

 base (Lib. v. cap. 16.) He has accurately distinguished and 

 described the pores'and fecal orifices, and was as well acquaint- 

 ed with their functions as Ellis or Lamarck. He says, we ob- 

 serve on the upper surface of the sponge minute pores (tto^oi) 

 placed close to each other, and almost imperceptible, and a few, 

 about four or five, wide orifices (<pecn^oi), through which the ani- 

 mal is supposed to take in nourishment." — Ibid. 



The organs which we know to cause the currents in other 

 zoophytes, and in infusoria, are very small, short, almost im- 

 perceptible processes, termed cilise, disposed around the mouth, 

 or on the tentacula. They are kept in a state of very rapid 

 vibration during the expanded state of the animals, for the pur- 

 poses of nourishment, respiration, or progressive motion. The 

 highest orders of aquatic animals produce currents in the water, 

 by the contraction and relaxation of various muscular parts of 

 their bodies ; and the most perfect inhabitants of the dry land 

 produce similar currents in the air to oxidate their blood. We 

 are not yet acquainted with any zoophyte capable of producing 

 these currents, by contracting and dilating its axis ; and I have 

 already shewn, that the currents of the sponge are not pro- 

 duced by any contraction or dilatation of the mass of its body, 

 or of the pores, canals, or orifices. No naturalist has ever dis- 

 covered polypi in the sponge ; and, as I have used every effort 

 in vain to detect them with a microscope, magnifying nearly a 

 hundred times, it is very probable that no such organs exist. 

 If they be present and indistinguishable by such aid, they must 

 be at least a hundred times finer than a filament of silk, and the 

 ciliae of the tentacula of such polypi would bear no proportion 

 to the velocity and volume of the currents already described. 1 

 have stated above, that the currents can be distinguished by the 

 naked eye passing into the open pores of the S. panicea, and 

 they are readily seen through the microscope passing into the 



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