872 THE AMERICAN NATURALIST. [VoL. XXXIII. 
part of the first cleavage plane, when viewed from the animal 
pole, bends at this cross furrow in the dextral forms, first to the 
right and then to the left ; in the sinistral forms the bend is first 
to the left and then to the right. When we know, therefore, 
between which cells the first cleavage plane runs, the four-cell 
stage, produced by reversed cleavage, can be distinguished at 
a glance from a four-cell stage of the normal type. The one 
is the mirrored image of the other. 
It is a remarkable fact that, not only in all dextrally coiled 
gasteropods whose cleavage has been carefully studied, but in 
many groups characterized by a typical bilateral symmetry, 
such as the lamellibranchs, Amphineura, annelids, and poly- 
clades, the cleavage is uniformly dexiotropic. The two sinis- 
tral gasteropods, Physa and Planorbis, form, I believe, the only 
instances in which, up to the present time, a reversed spiral 
cleavage is known to occur. 
It was with the desire of obtaining more data bearing upon 
this subject that I was led to study the cleavage of Ancylus, 
the common fresh-water limpet. In this genus the coil of the 
shell is almost entirely lost, and in some forms the shell appears 
to possess an almost perfect bilateral symmetry; but in many 
species of the genus the animal is truly sinistral, as the heart, 
lung, and anal and genital openings lie on the left side of the body. 
The genus contains both dextral and sinistral forms and has 
been divided into several subgenera, Ancylus proper containing 
the sinistral species, while the subgenus Acroloxus comprises 
only those that are dextral. The species studied, Ancylus rivu- 
laris Say, belongs to the sinistral group and is found in con- 
siderable abundance in a small lake near Ann Arbor, Mich. 
Specimens taken September 12, and kept in glass dishes in the 
laboratory, deposited eggs in the sides of the vessel, but not in 
any great number. The eggs are laid in minute, transparent, 
colorless capsules, each capsule, unlike that of the European 
species A. fluviatilis, containing but a single egg. The capsule 
is covered by a membrane and contains a semi-fluid, gelatinous 
substance in which the egg floats similar to that which sur- 
rounds the egg of Physa or Lymnea. It is a difficult matter, 
on account of its small size and the toughness of the membrane, 
. 
