x RESPIRATORY ORGANS 287 



the conversion of the mandibular and hyoid arches into jaws, or 

 into skeletal supports for the jaws ; and posteriorly, in the 

 reduction which is evident when the generality of Fishes are 

 compared with such primitive Elasmobranchs as Chlamydoselachus 

 and Notidam/s. 



In most Fishes the concave pharyngeal margins of the branchial 

 arches are fringed with a double series of either cartilaginous or 

 bony tubercles or filaments, the "gill-rakers" (Figs. 161 and 

 164). The anterior row of gill -rakers on each arch usually 

 interdigitate with those of the posterior row on the preceding 

 arch, and in this way the two rows form a sieve-like mechanism 

 to prevent any solid particles, which may enter the pharynx 

 with the respiratory current of water, from passing into the gill 

 clefts and clogging or otherwise injuring the branchial filaments. 



In a few Fishes the gill-rakers are enormously developed, and 

 subserve a function similar to that of the baleen plates of the 

 Whalebone Whales in acting as a filter for straining from the 

 water the small pelagic organisms on which the Fish feeds. 

 This is notably the case in the great Basking Shark (Selaclie 

 maxima*) l in which the closely-set, flattened, tapering gill-rakers 

 may be so long as four or five inches, and, while somewhat 

 resembling " whalebone " in appearance, have the histological 

 structure of vascular dentine. The nature of the food, which in 

 the stomach of one specimen examined consisted solely of an 

 immense quantity of plankton, including Copepods and the larvae 

 of other Crustaceans, 2 affords clear evidence of the great value of 

 such a filtering mechanism to this Shark, and, at the same time 

 offers an explanation of the striking and significant reduction in 

 the size of the teeth, which, relatively to the dimensions of the 

 Fish, are so small as to be almost vestigial. A similar filter has 

 been observed in an extinct Selache (S. aurata) 3 from the 

 Antwerp Crag, and also in an existing South African Shark 

 (Rhinodon typicus) ; 4 and in the latter, as in the Basking Shark, 

 is associated with a marked reduction in the importance of the 

 dentition. The long slender gill -rakers of the Chondrostean 



1 Turner, Journ. Anat. and Phys. xir. 1879, p. 273. For references to other 

 writers see Turner, op. cit. 



2 For this information, which was based on an examination of a specimen, 

 parts of which are now in the Cambridge University Museum, I am indebted to 

 Dr. Harmer. 3 Van Beneden, quoted by Turner, op. cit. p. 282. 



4 Andrew Smith, also quoted by Turner, op. cit. p. 281. 



