18 7 The American Naturalist. (January, 
driven out posteriorly; but instead, during further closure of 
the valves it is ejected from the lower border of one ear, where 
the mantle wall is low and thin, in the direction of the arrow å. 
The water expelled at this area is the most forcible current, and 
is probably of the greatest volume, as by its means the animal is 
impelled in the opposite direction, as indicated by 
the: arrow c. The valves open quickly and clap  } 
again. The second time as before the first water is 
driven out posteriorly; but when the mantle walls $ 
come in contact the direction of the excurrent is | 
again changed and it is forced out from the lower 
border of one ear in the direction of the arrow d. 
Being the strongest current, it impels the animal 
in the direction of the arrow e. At successive claps 
the water is driven out from alternate ears, as shown 
in the figure. The resultant action of the sev- 
eral currents and successive claps is to drive 
the animal in the direction of the free borders of 
the valves, or posteriorly. It is due to the alternate expulsion of 
water from either ear, as shown in the figure, that the animal pre- 
sents a series of zigzag jerks in swimming. The action of the 
first current expelled posteriorly, before the mantle walls come 
in contact, gives the animal an upward jerk, and it is in virtue of 
this jerk, combined with the momentum in a posterior direction, — 
that it maintains its position on the surface of the water, and also 
the high angle to the surface which it presents in swimming. 
This current is so powerful that by its action water may be | 
squirted by adults to the height of five inches or more from the — 
surface. In the shell a correlated feature of the swimming habit — 
is seen in the incomplete closure of the valves at the eared areas. 4 
Water may therefore pass out when the free borders of the a 
valves are in immediate contact, as they are at each clap, as indi- 
cated by the sharp clicking noise made in swimming. 
The tendency to equalize the form by growth in a horizontal 
plane in relation to the force of gravity acting in a perpendicular 
plane, or the geomalic tendency of Professor Hyatt,” is seen mark 
10 Trans A4 
ations of Planorbis at Steinheim, with Remarks on the Effects of Gravity — 
upon the Forms of Shells and Animals. Proc. Am. Ass, Adv. Sci., Vol. XXIX., 1880. 
eee 
PIET 
