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. 



BALANCING ORGANS OF JELLY-FISH (HYDROZOA). 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. 

 Though these different or- 

 gans may be constructed in 

 various ways they are affected 



by the same sort of stimulus. Their senspry 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 



Fig. 1041. Tentaculocysts of Jelly-Fish, enlarged 

 i, Of Solmaris coronantha; 2, of Polyxenia cyanostylis. 



VOL. IV. 



97 



