SENSORY ORGANS AND RECEPTION 



349 



when it is decreased, they become less active or inactive, and sink. The 

 ctenophores and fishes, alone among the animals listed, respond actively 

 to pressure decrease. These animals are buoyant, the fish possessing a 

 swim-bladder, and Pleurobrachia having a specific gravity nearly equivalent 

 to sea water (Table 9.4). 



The animals listed in Table 8.2 include some which have high specific 

 gravities (polychaetes, crustaceans) and which sink rapidly, and others 



TABLE 8.2 

 Barosensitivity in Marine Animals 



1 1 mb = 1 cm water. A bracketed figure indicates that this was the smallest pressure change tested. 

 (Data from Dijkgraaf (41); Hardy and Bainbridge (60); Knight-Jones and Qasim (88).) 



(medusae) which, although of low density, tend to be inactive for long 

 periods. A positive response to raised pressure has biological value to these 

 animals in that it tends to prevent them from changing level rapidly. Other 

 small planktonic animals which do not sink rapidly when resting are not 

 pressure sensitive (Calanus, Tomop ten's, Sagitta) (60, 88). 



Baroreceptors have not been identified in invertebrates. In bony fishes, 

 changes in pressure produce locomotory responses and gaseous secretion 

 or resorption within the air-bladder (p. 401). Suggestions have been made 

 that the air-bladder itself has a sensory function, being sensitive to pressure 

 changes through stimulation of receptors in its walls. Evidence for such 

 a function is inconclusive, and proprioceptors in the body wall or extero- 

 ceptors also may be involved (76). 



TASTE AND SMELL 



All organisms are sensitive in some degree to chemical changes in their 

 environment, and the senses of smell and taste are utilized in securing food, 



