348 



THE BIOLOGY OF MARINE ANIMALS 



gravity by climbing inclined or vertical surfaces. Asterina gibbosa climbs 

 up the wall of an aquarium, but when a large cork float is attached so as to 

 exert a strong upward pull, it moves down. Pull from the weight of the 

 animal on the tube feet appears to direct the movement. Geo-negative 

 responses on vertical surfaces are also shown by various gastropods (16, 

 49, 101, 110). 



The gravity and equilibrium receptors of vertebrates are the utriculus 

 and the semicircular canals (Fig. 8.26). The former responds to linear 



• Rotated Rostrum up 



— -o Rotated Rostrum down 



J I L_L 



j I 



24 48 12 96 120 144 168 192 

 Position in Degrees about Transverse Axis 



Fig. 8.30. Plot of Nervous Discharge from the Statocyst 



of the Lobster {Homarus americanus) 



Response of a single type I position receptor to continuous rotation in opposite 



directions about the transverse axis. Read solid curve from left to right; broken curve 



from right to left. Each point represents average frequency over a 12° interval. (From 



Cohen (27).) 



acceleration and tilting, the latter respond to angular acceleration. The 

 utriculus with its contained otolith resembles a statocyst. For accounts of 

 the functioning of these organs consult Lowenstein (97, 98). 



SENSITIVITY TO PRESSURE CHANGES 



Bony fishes which possess compressible air-bladders have long been 

 known to respond to pressure changes, but it is only recently that many 

 invertebrates, lacking gas organs, have been found to be pressure sensitive. 

 Some barosensitive species are listed in Table 8.2. When the water pressure 

 is increased, these animals generally become more active and swim upward; 



