320 THE BIOLOGY OF MARINE ANIMALS 



shorten and draw their sensitive portions away from the pigment towards 

 the exterior. 



When mechanical changes are performed extensively and expedi- 

 tiously, as described above, they are doubtless of great value in adjusting 

 the retina to changes of external illumination. In the light-adapted con- 

 dition the cones are exposed to stimulation, while the rods are partially 

 or entirely shielded from strong light. In the dark-adapted state the rods, 

 functional in dim light, are fully exposed, while the cones are displaced 

 out of the way. Examples of fishes whose retinae show photomechanical 

 changes are eels (Anguilla), scorpion-fish (Scorpaena), top-minnows 

 (Fundulus), sticklebacks (Gasterosteus), etc. 



Retinal pigment is abundant in many pelagic teleosts (mackerels, 

 tunnies, mormyrids, etc.), but it is uncertain to what extent the eye pig- 

 ment is migratory and whether the guanin forms a functional tapetum. 

 In bathypelagic species, such as Evermanella, the pigment epithelium con- 

 tains guanin but no dark pigments, and the tapetum is therefore non- 

 occlusible (16, 40, 118, 146). 



Kinds of Light Responses 



Since photic stimuli are utilized as sensory cues in so many forms of 

 behaviour it is possible to select only certain types of photic responses for 

 particular consideration. A well-marked category of photic responses 

 includes orientation reflexes controlled either by intensity differences or 

 the directional properties of light. Many animals show special behavioural 

 responses to sudden changes in light intensity. Tubicolous polychaetes 

 (sabellids, serpulids), gastropods (Onchidella, Chromodoris), cirripedes 

 and others respond by contraction to a sudden decrease of intensity. 

 Hagfishes (Myxine), some anemones (Cerianthus), Mya, Ciona and the 

 enteropneust Saccoglossus contract under sudden increase in intensity. 

 Still other species react to either decrease or increase of intensity, e.g. 

 sea-urchin Diadema. 



Allied to such responses are shadow reflexes and reactions to moving 

 objects. The former occurs even in the absence of image-forming eyes, and 

 then depends on successive temporal stimulation of photosensory cells, 

 either dispersed or aggregated into ocelli. Other responses, non-muscular, 

 evoked by photic stimuli are colour changes and luminescence (Pyrosoma) 

 (7, 16, 17, 32, 65, 66, 87, 107, 111, 114). 



Orientation to Light: Tropisms. The oriented responses to light of 

 lower animals are of two kinds : bending reactions and oriented locomotory 

 movements. Originally applied to all oriented responses, the term tropism 

 is now restricted in animal physiology to the bending reactions of sessile 

 animals. Examples are the heliotropic bending of hydroid polyps (Euden- 

 driurri) and sea anemones (Cerianthus), and the bending of sabellid tubes 

 towards the light. Moore (1 1 1), who made a particular study of Cerianthus, 

 found that the number of degrees through which the animal turned was 

 proportional to the logarithm of light intensity. 



