Natural- Historical Collections. 375 



have been observed by Dr. Grant " in many species, particularly of the SertU' 

 laria and Alct/onia.'" But, except in the observation contained in the above 

 extract, that " analogous hairs exist on many species of Medusa, Tritonia, &c." 

 which " may probably be considered as forming a part of the aerating organs," — 

 and in the fact mentioned in Dr. Fleming's Phil, of Zool. Vol. II. p. 470. that 

 " the branchiae in some species (of Tritonia) readily fall of, and, as if independent, 

 are capable of swimming about for a short time in the water, by means of minute 

 hairs with which their surface is covered, and which move rapidly, pushing for- 

 wards the distal extremity," — we have met with no approximation to Dr. Shar- 

 pey's discovery of cilia in the higher invertebrata. 



The process by which that gentlemen arrived at the discovery was original and 

 extremely beautiful, and the observations of Dr. Fleming serve to corroborate 

 what Dr. Sharpey has so widely extended. 



We think that two interesting deductions may be drawn from these observa- 

 tions, which the authors seem indisposed to recognize. 



In the 1st. place, we should be inclined to consider it as a fact resting on a 

 sound analogy, that the tentacula of polypi arp their respiratory organs, and that 

 the process is effected by successive portions of water being moved along by the 

 vibrations of the cilia on their surface. 



And, 2dly, we do not see occasion for the question put by Dr. Sharpey, whe- 

 ther the currents " are to be referred to any of the known properties of living 

 bodies, or or to a peculiar power residing in the parts V It is true that cilia 

 have not been observed in the tadpole, nor in the ascidia ; and Dr. Grant was un- 

 able to detect them in the sponge ; but Ellis could not even see them on the ova of 

 the campanularia dichotoma, probably, as Dr. Grant observes, from not having 

 employed a sufficiently high magnifying power ; and as the ascidia is the only 

 one of the mollusca examined by Dr. Sharpey in which he has not observed these 

 organs, we tliink the question should be, what magnifying power does it require 

 to see the cilia in those species in which they have not yet been discovered ? and 

 not whether there be a " peculiar power residing in the parts ?" Sir E. Home 

 was not even able to see the currents along the gills of the tadpole, though he 

 (or Mr. Bauer) had described this larva from its formation in the ovum through 

 the whole period of the existence of the external gills. But the fact is now placed 

 beyond doubt by Dr. Sharpey's observations. 



Dr. Grant, however, seems to have taken every means to discover cilia in the 

 sponge, in which he was led by analogy to expect them, — but without success. He 

 says, " 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 

 shown^ that the currents of the sponge are not produced by any contraction or 

 dilatation of the mass of its body, or of the pores, canals, or orifices. No na- 

 turalist has ever discovered 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 cilia of the tentacula of such polypi would bear no 

 proportion to the velocity and volume of the currents described. I have stat- 

 ed above, that the currents can be distinguished by the naked eye passing 

 into the open pores of the Spongia panicea, and they are readily seen through 

 the microscope passing into the pores in most of the other species. I was 

 therefore led to suspect that the currents are not caused by polypi on the sur- 

 face, but by cilia, or some similar apparatus, placed around the entrance of the 

 pores, or on the margins of the gelatinous net-works, or on the whole surface 

 of the internal canals. I first placed a thin layer from the surface of the S. 

 papillaris, in a watch-glass with sea water under the microscope, and, on look- 

 ing through its pores, I perceived the floating particles driven with impetuo- 

 sity through these openings ; they floated with a gentle motion to the margin 

 of the pores, rushed through with a greatly increased velocity, often striking 



