VERTEBRATE RESPIRATION 13 



The function of the oesophageo-cutaneous duct seems to be for 

 the clearance of relatively large particles from the pharyngeal 

 cavity. In addition, clearing the respiratory chambers of even 

 larger particles can take place through the nostrils in a form of 

 'sneezing'. 



(b) BONY FISHES 



(i) Gill structure 



Although the gill system of cartilaginous fishes probably 

 represents a more primitive arrangement, from the functional 

 point of view it is better to consider a bony fish first of all and 

 then to discuss the differences shown by dogfish and sharks. 

 Teleosts have four branchial arches (fig. 2a) on each side of the 

 body which extend from the otic region of the neurocranium and 

 join ventrally near the basihyal. Each of these arches has along 

 its length a series of plates (the filaments) which form a double 

 row splayed out as a V from the branchial arch skeleton (fig. 

 2b), and each filament is supported by a gill ray. The filaments 

 are stacked one above the other and water passes in the spaces 

 which lie between the filaments attached to each side of an arch. 

 These spaces are not simple, however, for the upper and lower 

 surfaces of each filament are folded into a large number of 

 secondary folds or secondary lamellae. These folds are extremely 

 thin for they are the site of the gaseous exchange, and water 

 passes on both sides of them. A section across an individual 

 secondary fold (fig. 2c) shows that it is made up of two very 

 thin epithelial layers separated by a series of peculiar cells called 

 pillar (or pilaster) cells and supported by a basement membrane. 

 In life the gill filaments form a beautiful series of a delicate pink 

 colour because they are very profusely supplied with blood. This 

 supply comes from the ventral aorta and up each of the branchial 

 arches there passes an afferent branchial artery. From these 

 arteries a branch leads to each of the many filaments and in each 

 filament the small aff'erent blood vessel divides repeatedly to 

 supply each of the secondary lamellae. In the secondary lamellae 



