BALANCE AND HEARING 35 
of the brain-case. And it is definitely known that in Molluscs 
of this kind maintenance of equilibrium and adjustment of the 
swimming movements are seriously interfered with if the oto- 
cysts are injured, which leaves little doubt as to the use of these 
organs. 
The otocysts of some of the free-swim- 
ming Sea-Snails (Heteropods) are particu- 
larly large and well-developed (fig. 1043), 
and are undoubtedly related to balance and 
steering. The majority of Snails and Slugs, 
however, are adapted to a creeping mode of 
life, the organ of locomotion being the mus- AS 
cular flat-soled foot, which is also concerned __ Fig. 1043.—Otocyst of a Hetero- 
with maintaining the balance of the body. faged. ‘The large ptoitn i seza 
Since the otocysts are presumably related ese rece nice is 
lined below by sensory cells, pro- 
to both these uses, it is not surprising to eee ae Farha el 
find them placed close to the upper surface _ * lined by cells beating long cilia 
of the foot, by the slightest movement of ~ — 
which they must therefore be affected, and H. J. Fleure has 
described an interesting arrangement in the Limpet and Sea- 
Ear which probably conduces to this. In the two forms men- 
tioned each otocyst is connected with the foot by a fibrous band, 
and there is a similar bond between the two otocysts (fig. 1044). 
These organs are thus kept “in touch” with 
the foot and with one another, and, being also 
moored by their nerves to the foot-ganglia, 
are kept steady, which seems desirable when 
their functions are considered. 
Orcans oF BaLancE AND HEARING IN 
Crustaceans (Crustacea). — Such higher 
forms as Lobsters, Prawns, Shrimps, and z asec aes 
Crabs are provided with otocysts lodged in cross section through Otocysts 
the bases of the small feelers or antennules, {0%)gn Poot of # Timpsh or 
These organs arise, as in cases already de- 
scribed, as pits in the ectoderm, and they usually, though not 
always, remain open through life. In a Lobster, for example, 
they are lined by a thin horny membrane continuous with the 
hard covering of the body, and studded with delicate bristles, 
at the bases of which are sense-cells (fig. 1045). The otoliths 
are sand grains which have been taken in from the exterior. 
