1 0^ Natural History of Molluscous Animals ; — 



to supply the place of that regularly alternate and ceaseless 

 play of the resph-atory muscles of the vertebrates, Dr. 

 Sharpey has discovered that, in " the Mollusca, and other 

 inferior tribes of aquatic animals, the external covering of the 

 body generally, but especially of the respiratory organs, 

 possesses the power of impelhng the water contiguous to it 

 in a determinate direction along the surface, by which means 

 a constant current is kept up, and the blood exposed to the 

 influence of successive portions of the surrounding element : 

 this peculiar provision effecting, in those creatures, the same 

 purpose as the respiratory muscles in the more perfect 

 animals." These currents, in the Mollusca at least, and 

 probably in all the animals in which they have been detected, 

 are produced by the action of minute cilia, visible only with 

 a glass, which are in constant motion, and clothe all the 

 surfaces along which the currents are excited. Similar cilia 

 had been observed on the eggs and organs of many zoophytes 

 by previous naturalists, and in a few naked Mollusca, by 

 Dr. Fleming; but the merit of proving their existence in 

 all the great families of the Mollusca, with the exception of 

 the Cephalopoda and the Tunicata, and of pointing out 

 their use, is due to Dr. Sharpey. Carus came near the 

 discovery ; for he observed the currents in question, but left 

 uninvestigated their cause ; or, rather, he attributed the 

 phenomena to one which has probably little efficiency. His 

 words are : — " In a living bivalve, it is easy to observe that 

 the water gains access to the branchial laminae by the fissure 

 in the cloak, and escapes by the anal tube, which serves also 

 to evacuate excrement and ova. It has not, however, been 

 hitherto noticed, that this current is uninterrupted, and that 

 thus these animals, when not too deeply immersed, form an 

 eddy on the surface of the water. But as, in almost all other 

 animals, the influx of air or water to the respiratory organs 

 is intermittent, the simultaneous and continuous current into 

 the fissure of the cloak and out of its tube, of which I have 

 satisfied myself by numerous observations, must depend on a 

 very peculiar mechanism, which consists chiefly in the mus- 

 cularity of the cloak, but partly also in the mobility of the 

 gills themselves, and may be compared to the mechanism of 

 certain bellows, which produce an uninterrupted current of 

 air by means of double bags." [Camp. Anat., transl. vol. ii. 

 p. 148.) 



As this discovery appears to me the most important which 

 has been made of late years in the physiology of these 

 animals, you will permit me to transcribe, for your perusal, a 

 paragraph of considerable length from Dr. Sharpey's Essay, 

 with a view of giving some farther illustration of the process. 



