SENSORY ORGANS AND RECEPTION 355 



were detected; cod, whiting, plaice and flounder distinguished a salinity 

 difference of 0-2% o . The sensory avenues by which these changes are per- 

 ceived are unknown (14tf). 



Nervous discharges have been recorded from the olfactory stalks of 

 various fish (catfish, tench, etc.) Many different substances produce an 

 olfactory discharge, including meat extracts, proteins and starch. When a 

 drop of stimulation fluid is placed in the olfactory sac there is an appreci- 

 able latency before discharge begins (0-5-5 sec), after which the response 

 rises rapidly to a maximum and then declines more slowly (adaptation of 

 olfactory system). Following stimulation producing much discharge 

 there is a period of inactivity lasting 5-20 sec, during which period the 

 olfactory organ is insensitive to a second stimulus. The olfactory organs 

 respond to mechanical as well as chemical stimuli, and chemical stimula- 

 tion is most effective when the fluid contains solid matter in suspension (1). 



The entire integument plus mucous membranes possess chemical sen- 

 sitivity, mediated through the general chemical sense. The skin of the 

 dogfish Mustelus is sensitive to acids and alkalis, less so to salts and bitter 

 substances. Recent studies have been concerned with chemical irritants, 

 acting through the general chemical sense, capable of repelling fish. Copper 

 acetate is reported to be effective in repelling sharks. Some threshold 

 values (parts per million) for teleost irritants are: phenacyl bromide, 0-01 ; 

 potassium cyanide, 0T ; chlorine, 1-0. Substances repellent to sharks are 

 not necessarily effective against teleosts (71, 72, 117, 161). 



THERMAL SENSITIVITY 



Environmental temperature affects most body activities in poikilotherms. 

 Upper and lower limiting temperatures for activity have been established 

 for many species, and the effect determined of temperature on various 

 processes. Extremes of temperature have a general stimulatory action on 

 behaviour; very low temperatures produce anaesthesia, and high tempera- 

 tures abnormal muscular contractions. The present problem, however, 

 is one of thermoreception. In a temperature-gradient Paramecia tend to 

 aggregate at some optimal temperature by trial and error movements 

 (namely thermokinesis). Sudden thermal changes elicit either shock 

 reactions or modify the rate of movement. Zoea larvae of crustaceans, 

 for example, dart backwards when the water temperature is raised rapidly. 

 The capacity to discriminate thermal differences is low in most marine 

 invertebrates, and few instances of specialized thermoreceptors have been 

 recorded. 



Perception of Temperature Changes in Fishes. The thermal receptors of 

 elasmobranchs are the ampullae of Lorenzini, scattered over the head of 

 the fish. Each ampulla contains pyriform sensory cells and communicates 

 with the exterior by an ampullary tube filled with jelly. The sense cells are 

 innervated by the facial nerve. Sand (128) showed that the Lorenzinian 

 ampullae act as a thermometer and possess an autonomous rhythm which 

 alters with thermal changes. They respond to cooling by acceleration and 



