374 Natural- Historical Collections. 



On the Respiratory Currents of certain Aquatic Animals In a note to the 



important paper by Dr. Sharpey, " On the Mechanism of Respiration, &c." pab- 

 lished in this Number, (p. 334.) it is mentioned that we would, in another place, 

 exhibit the previous state of our knowledge on the subject. This we proceed to 

 do, as well for the purpose of giving its proper estimation to a discovery by our 

 friend Dr. Fleming, which appears to have been overlooked, as of bringing some 

 corroborative evidence to show the extensive distribution of the structure observed 

 by Dr. Sharpey. 



It has been for a considerable time ascertained, that the currents observed in 

 water in which zoophytes and infusoria are immersed, are owing to the rapid 

 motion of cilia disposed around the mouth, or on the tentacula of the animal ; 

 and similar bodies have been observed and descriljed by Dr. Grant in the ova of 

 zoophytes, whence their spontaneous motions are sustained. 



" The cilia on the surface of these ova," says Dr. Grant, " are minute filaments, 

 which may be compared to the small hairs covering the human body ; they do not 

 add to the internal organization of the ovum, nor render it as complex as that 

 of the adult animal which possesses highly organized polypi ; they are organs 

 which exist in the adult zoophyte, and in the simplest forms of animal matter, 

 the motions of the simplest gelatinous animalcules being performed by them ; 

 and they are necessary to prevent the ova from falling by their own gravity 

 like the seeds of plants, to be buried in the ever-moving sands." — Ed. Phil. 

 Journ, I. 152. 



Dr. Fleming, in a paper in the Wernerian Memoirs, Vol. IV. p. 488, was the 

 first to describe and figure the cilia by which currents are produced along the ten- 

 tacula of polypi, in the Sertularia cuscuta of Ellis, {Valkeria cuscuta, Flem. ;) 

 and to render the subject more clear to those who are less acquainted with the 

 nature of zoophytes, we have added Fig. 5. and 6. to the plate accompanying 

 Dr. Sharpey s paper, from the illustrations of Dr. Fleming's observations. 



" In some Sertularise which I have examined," says the author, " the anns 

 seemed furnished on all sides with suckers, analogous to those of the cuttle- 

 fish. On the arms of this species, however, I detected a very difl'erent ar- 

 rangeniient, and one which I suspect has not hitherto been noticed. Each arm 

 is furnished, laterally, with a row of short hairs or plates ; for the highest 

 magnifier which I could conveniently apply did not enlarge the object above a 

 hundred diameters, and was incapable of enabling me to determine their true 

 shape. The motions of these hairs, were, in conseqiience of the currents which 

 they produced in the water, sufficiently obvious. The hairs, on one side of the 

 arm, exercised a continued motion, so as to cause the water to flow from its 

 base to the extremity ; while those on the opposite side executed a motion the 

 very reverse of this, causing the water to descend from the extremity of the 

 arm towards its base. And again, if the hairs on the right side of one arm 

 were fitted to cause the water to ascend, the hairs on the left side of the con- 

 tiguous arm were found suited to produce a current in the opposite direction, 

 as exhibited (not from nature, but to render the description intelligible) at 

 Fig. 6. Plate VII. Analogous hairs exist on many species of Medusse, Trito- 

 niw, &c. in which they are obviously unconnected with the digestive system as 

 assisting prehensile organs, and may probably be considered as forming a part 

 of the aerating organs. In this Sertularia, their occurrence on the arms, which 

 are true prehensile organs, and belonging to the digestive system, may induce 

 a belief that they are merely parts of that system, and destined by the cur- 

 rents which they produce, to bring the small animals, their prey, more easily 

 within reach of seizure. The currents, however, which are produced by their 

 motion, seem better calculated for bringing fresh portions of water in contact 

 with the sides of the arms, than to bring animalcula; within the space which 

 they surround. They may possibly be destined to act as organs of touch, 

 though I am rather disposed to regard them as branchies, placed in the most 

 favourable position for receiving the influence of the oxygen of the water." 



Thus the existence of these little organs in zoophytes, and their probable use 

 in the functions of respiration, seems to have been clearly understood ; and they 



