Scientific Intelligence. — Zoology. 383 



rum is effected, on the one hand, by the branchiae or gills, and, 

 on the other, by the air contained in the water, the necessity 

 of a constant contact of both is evident, and is shewn in a way 

 which is deserving of particular notice. If we place a living 

 mussel in a flat vessel filled with water, we will observe, after 

 every thing is at rest, that the mantle-slit provided with tenta- 

 cula, and the anus, will project ; and suck in water by the first, 

 and by the latter, after it plays round the floating gills, again 

 throw it out. If we strew the surface of the water with light 

 bodies, as dust, semen lycopodii, &c. we will observe this suck- 

 ing in and throwing out of the water, by the continual circular 

 motion of these floating particles. These circles vary much, 

 according to the size and the elasticity of the vessels wherein 

 the bodies are driven round by the currents of water, and suffer 

 numerous bendings and alternations, all of which, however, can 

 be referred back to the original current, as well represented in 

 figure first in the plate accompanying Unger''s memoir. It 

 consists of a double whirl, in opposite directions, of which the 

 diameter of each single whirl is several times that of the length 

 of the animal, hence shews the force with which the water is 

 thrown out or projected ; even how changes of place, as the 

 turning round of the animal, may be effected by the sudden and 

 violent emptying of the inspired water on the application of a 

 stimulus. The cause of this phenomenon is to be traced to the 

 very particular formation of the parts by which the process is 

 performed, viz. that the flowing water reaches its place of desti- 

 nation by one organ (the cleft in the mantle, the anterior tube 

 of the mantle), and is thrown out by another organ (posterior 

 mantle-tube or anus), which does not occur in those mollusca a 

 little higher in the scale, viz. univalve shells, as in them there is 

 but one passage for respiration. Secondly, to the alternate pres- 

 sure of the upper and inner free edge of both gills on the foot, 

 by which we can explain the continued stream of water from 

 the anus. This explanation agrees with that given by Cams 

 several years ago, in which he compares the operation here no- 

 ticed to ihe mechanism of a double bellows. A very easy and 

 simple experiment will convince any one, not only of the unifor- 

 mity, but also of the velocity with which the water is projected 

 by the above mentioned parts. If we drop a minute portion of 



