CHEMORECEPTION: LOCOMOTION AND ORIENTATION 



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Figure 1 Development of the head in Scyliorhinus: eye (op), forebrain (fb), midbrain 

 (md), auditory vesicle (au.v), and olfactory groove (ol). From Balfour (1876). 



in between filled with the water to be olfactorially "tested." The olfactory 

 epithelium proper is located mainly at the base of the troughs formed by the 

 septae or lamellae. The somata of the bipolar sensory cells are particularly 

 large in these fish (15 fim across and 20 nm deep). The olfactory sacs of 

 Mustelus were described by Sheldon (1909) as a pair of capsules, partly 

 divided by a flap of skin rostrally, and a fleshy ridge caudally. In this manner 

 two incompletely separated external apertures are formed. Like the barbs of 

 a feather, the olfactory lamellae bearing the receptors extend from a median 

 ridge inside each capsule. In addition to the many very thin axons of the 

 bipolar cells, the lamellae receive the endings of the terminal nerve. 



For efficient function of the olfactory system the water surrounding the 

 animals' head must be sampled continuously. This is achieved by respiratory 

 movements that draw water into the mouth; part of it enters the more 

 rostral nostril by suction and leaves through tKeTTrrore caudal nostril. Sheldon 

 demonstrated, with the use of a dye, that the water follows the medial ridge 

 in the capsule and is, in part, diverted into the interlamellar spaces. In 

 addition, in a swimming dogfish, water is forced through the nostrils by the 

 displacement of the animal. In fishes generally, similar mechanisms of water 



