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- 

 cular flat-soled foot, which is also concerned Fi g . io 43 . otocyst of a 



i '.* .1 11 ri 11 Pd (Pterotrachea), in section, en- 



with maintaining the balance of the body. iar ge d. The iar ge otoihh is seen in 

 Since the otocysts are presumably related %*? 

 to both these uses, it is not surprising to vided with b " nches of short ' stiff 



A projections; the rest of the vesicle 



find them placed close to the upper surface ^ lined b y cells tearing long dik; 

 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. 



ORGANS OF BALANCE AND HEARING IN 

 CRUSTACEANS (CRUSTACEA). Such higher 

 forms as Lobsters, Prawns, Shrimps, and 



Ci i i i i i i Fig. 1044. Diagrammatic 



rabs are provided with otocysts lodged m cross section through otocysts 



the bases of the small feelers or antennules. gg- *** - 

 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. 



