BALANCE AND HEARING 33 
organs of hearing have been evolved. Indeed the auditory 
organs still retain, in ourselves for instance, the old function 
side by side with the new. : 
BaLancinc OrcGans or JELLY-Fisu (Hyprozoa).—Jelly-Fish 
are often provided with balancing organs placed at regular in- 
tervals round the edge of the umbrella. In the simplest case 
these are little pits lined by specialized sense-cells, from each of 
which a bristle projects. Within the pit one or more calcareous 
particles (otoliths) are found, and these also have been derived 
from the ectoderm. In many species the mouths of these pits 
close up, converting them into little sacs (otocysts) which lie 
close to the surface. Other 
kinds, again, possess short 
balancing-tentacles (tentaculo- 
cysts), evolved no doubt from 
some of the ordinary sort (fig. 
1041). In such instances the 
otoliths are derived from the 
entoderm cells which make up 
the inner part of the tentacle. «@ wx Pag 
Though these different or- Wig: doar Temnacaloeyaus off Jallprishy entarsea 
gans may be constructed in 4, Of Solmaris coronantha; 2, of Polyxenia cyanostylis, 
various ways they are affected 
by the same sort of stimulus. Their sensory cells are jolted by 
movements in the surrounding water and by the swimming move- 
ments of the animals themselves, and the otoliths appear to 
intensify the action, as it were. The sense-cells are closely con- 
nected with the nervous system, and this again with muscle- 
fibres. We have present, in fact, the necessary machinery for 
muscular reflex actions (see p. 9), under which may be included 
the checking or stopping of swimming movements actually in 
progress. 
One of the most obvious uses of the sense-organs described 
appears to be that of enabling their possessors to keep well 
below the surface of the water during rough weather, for crea- 
tures of such flabby and delicate structure are quite unfitted 
to withstand the buffets of the waves. Supposing that on a 
stormy day a jelly-fish is swimming obliquely upwards. When 
it comes sufficiently near the surface for the balancing organs 
to be stimulated with a certain degree of vigour by the swing 
VoL. IV. 97 
