1888.] Mr D. M‘ Alpine on Bivalve Molluscs. 175- 
the progressive development of ideas. At first the motion was 
supposed to be that mechanically caused by a thin film of substance 
in an agitated liquid, the agitation being due to the act of mount- 
ing the specimen. jN'ext the movement was seen to be not tem- 
porary but persistent, and probably due to ciliary motion, but 
supposed to be imperceptible to the naked eye, and so compara- 
tively slow as only to be measured under the microscope. Then the 
microscopic slide was kept moist, and the moving mass seen with the 
naked eye to glide perceptibly along, and the climax in this direc- 
tion was reached, when the entire detached gill was transferred to a 
moistened plate where there was plenty of room to move, where it 
could either be completely immersed or simply kept moist, and 
where it travelled, not only horizontally, but vertically, and even 
when the plate was turned upside down. During the movement, 
when held fully in the light, the flicker of the lashing cilia could be 
plainly seen, and this is also beautifully visible when the gill is 
looked at in the opened shell. Last in the order of discovery was 
the movement of the entire animal, but it may appropriately take 
the first place in the order of description. 
Previous Observations. — Sharpey, under heading “ Cilia ” in Todd 
and Bowman’s Cydo'pcBdia of Anat. and Phys., 1835, and in Quain’s 
Anatomy, gives a resume of what has been done up to that date, and 
a full list of references. He there points out that in the detached 
arms of the plumed polyp of Trembley we have independent life 
and locomotion in the opposite direction to the currents set up by 
the cilia, and he notes that pieces of gills of the common sea-mussel, 
detached portions of the gills of the salamander, the external gills of 
the tadpole, the gills of the larvae of the newt or water-salamander, 
all when cut off “ moved through the water with the cut extremity 
forwards in a direction contrary to the currents.” Most of the ex- 
periments were made with the sea-mussel (Mytilus), in which the 
ciliary motion and locomotion are much more marked than in the 
fresh-water mussel and the oyster (Ostrea), with which, however, 
confirmatory experiments were made. 
Preliminaries . — It will be necessary to be agreed as to the posi- 
tion from which the moving are to be viewed, since it is 
impossible to have them detached and observed in motion in their 
natural position. If the valves of the shell are separated in the 
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