SENSORY ORGANS AND RECEPTION 



353 



and the sensory cells originate impulses which proceed along the olfactory 

 nerve fibres to the olfactory lobes of the brain. Free nerve endings of the 

 trigeminal nerve are also present in the olfactory epithelium and are 

 concerned with the general chemical sense. Allison (5) has presented an 

 extensive review of the olfactory system of vertebrates. 



Taste receptors occur in the mouth and elsewhere in fishes. In teleosts 

 they may be widely distributed, occurring in the buccal cavity, pharynx, 

 lips, fins, barbels or over much of the external surface. The taste organs 

 consist of groups of sensory cells bearing terminal sensory hairs (Fig. 

 8.1, end knobs). Proximally they make contact with sensory fibres derived 

 from the Vllth, IXth and Xth cranial nerves and the ramus lateralis 

 accessorius of the facialis. 



The senses of taste and smell in fishes provide cues for recognizing and 

 locating food, initiating avoiding reactions, for orientation, etc. Extra-oral 



Fig. 8.31. Olfactory Organs of Teleosts 



{Left) cod Gadus callarias; view of the interior, showing folds. (Right) tentacular 

 type of olfactory organ of Tetraodon pardalis. (After Bolk et al., 1934.) 



taste buds are used for detecting food material in close proximity. Tom 

 cod and hake have taste buds in the ventral fins, which are chemically 

 sensitive to animal food lying on the bottom. The rockling (Onos) has 

 taste buds associated with the dorsal fin as well, and when food material 

 is applied locally to that area, it turns around preparatory to seizing it. 

 Small action-potentials in facial fibres of the catfish have been detected 

 after chemical stimulation of taste buds in lips and barbel with acids and 

 meat juices (73). Gurnards (Prionotus, etc.) respond positively when meat 

 extracts are applied locally to the free ventral fin rays. Taste buds are 

 lacking here, and the chemical stimulus appears to be detected by nerve 

 fibres ending in peripheral knobs. The epidermis of fish also contains 

 elongate sensory cells, which may be concerned with the general chemical 

 sense (44, 117, 131, 132, 160). 



More distant sources of food may be detected by the odours they give 

 off. Dogfish locate food largely through the sense of smell. Blennies 

 {Blennius) have been conditioned to food stimuli and respond to meat 

 extracts (nereids, limpets, mussels) in concentrations of 0-01 %. The tuna 

 {Euthynnus) is attracted by extracts of tuna flesh, the attractive substance 



M.A.— 12 



