270 CHEMICAL SENSES 



INTRODUCTION 



There is abundant anecdotal and experimental evidence for the importance 

 to elasmobranchs of the chemical senses, particularly olfaction. Although 

 these senses and their biological functions appear fundamentally similar to 

 those in other fish (Kleerekoper 1969, Hara 1971), we shall review here 

 some of the more significant literature on the chemosensory biology of 

 elasmobranchs. No attempt will be made to compile an exhaustive bibli- 

 ographic survey of the topic. Furthermore, the disparity in available informa- 

 tion on the two senses restricts the review almost exclusively to olfaction, 

 revealing a lamentable gap in our knowledge of the role of taste in these 

 animals. 



The scientific, as opposed to anecdotal, information on olfaction in elas- 

 mobranchs covers three fairly distinct areas: (a) the structure of the olfac- 

 tory organ and its neural pathways, (b) the role of olfaction in the procure- 

 ment of food, and (c) mechanisms of orientation through olfaction. 



THE OLFACTORY ORGAN AND ITS NEURAL PATHWAYS 



The literature on the structure of the organ in fishes in general (Kleerekoper 

 1969, Parsons 1971, Graziadei 1971, Hara 1971) and in sharks specifically 

 (Tester 1963) has been reviewed extensively in past years. A few of the main 

 aspects will be restated here and reference made to some more recent con- 

 tributions, particularly to new knowledge about the fine structure of the 

 olfactory epithelium and about the neural pathways. 



According to Balfour (1876, 1885), the olfactory organ develops rela- 

 tively late and becomes apparent later than the eyes and ears (Figure 1). 

 Immediately in front of the mouth, on the ventral side of the forebrain, a 

 pair of epiblastic thickenings forms the anlage for the organ, which does not 

 connect with the neuropore (Berliner 1902). Proliferation of the cells of the 

 anlage is followed by involution, the formation of a pit, and finally a blind 

 sac (Figure 2). Numerous folds are formed in the sac by the epithelium, 

 which is ectodermal in origin. 



Although the outer wall of the organ at first consists entirely of sensory 

 cells, these change progressively into indifferent ectoderm. Cell proliferation 

 in the remaining sensory epithelium leads to formation of the paired olfac- 

 tory nerve, which advances toward the brain of the embryo. In Scyliorhinus, 

 the olfactory grooves, at their edges, develop flap like extensions that project 

 toward each other and overlap, forming a tube with an incomplete roof and 

 an anterior and posterior opening (Biitschli 1921, Allis 1919) (Figure 3). 

 Unlike the condition in teleosts, the posterior opening connects with the 

 mouth in some species of elasmobranchs. For further detail, the reader is 

 referred to Bigelow and Schroeder (1948) and Tester (1963), who give an 

 extensive account of the position and shape of the nostrils in various shark 

 species. 



The olfactory sacs in elasmobranchs are located in the nasal capsules of 

 the skull. They are divided by several parallel septae, with the narrow spaces 



