CERATODUS. 23 



structure, providing for a more plentiful and rapid ingestion of food, and also for a 

 greater rush of water over the gills. If we compare the conditions of life we may get 

 fresh light upon the modification of the visceral arches which distinguishes the Teleostei 

 and most of the Ganoids and Elasmobranchs from the Dipnoi. Isolated river-fish, 

 feeding upon plants, small mollusks, and other stationary or slow-moving food, Ceratodm 

 and Lepidosiren retain a nearly primitive disposition of the visceral skeleton. 



This comparatively sluggish life also suffers the Dipnoi to retain a somewhat ineffec- 

 tive and not wholly aquatic respiration. Ceratodm has fixed gills with a single not very 

 extensive opening behind. Lepidosiren also has a decidedly small passage for the exit of 

 water. In Elasmobranchs we find fixed gills, and these more rigid than in Ceratodm ; 

 but a series of clefts unimpeded by an operculum l leads directly outwards from the 

 respiratory chamber, and these clefts the lateral undulations of the fish open and 

 close incessantly. 2 



AVhile the Plagiostomi have in this way become adapted to an active and predatory 

 life, other fishes, Teleostean or Ganoid, unrestricted in range and subject to fierce com- 

 petition, have in remote times assumed the form of mouth which permits the most 

 expeditious feeding. With greater activity there has simultaneously arisen a greater 

 demand upon the respiratory apparatus ; and in these fishes we find, provision for a 

 copious flow of water over the gills. The gape is usually large ; and the opercular slit 

 opens below as well as behind. With a mouth adapted for hasty and indiscriminate 

 feeding we find correlated, in most Teleostei, a special masticatory apparatus placed in 

 the throat upon the pharyngeal bones. Here again is an adaptive structure ; and its 

 small value for classificatory purposes has already appeared from attempts to found 

 orders of Teleosteans upon the presence and arrangement of this accessory apparatus. 3 



It may perhaps be supposed that the displacement of the suspensorium, though an 

 adaptation, is an adaptation of so highly special a kind, that it can hardly have arisen 

 independently in different animals, and that it thus implies a common origin, not shared 

 by others, for those species in which it occurs. This view would lead us to divide Pisces 

 into one primary group to include Ganoidei (exclusive of Ceratodm and Lepidosiren), 

 Teleostei and Plagiostomi (exclusive of Chimcera) ; and secondly, a group or groups 

 comprising all the rest. I think that the point will not be pressed so far. There are 



by a double condition of the quadrate, and, secondly, by the descent of the top of the pier away from the 

 side of the head and its attachment some distance down to the overgrown pier of the next arch. All 

 this modification shows that the Teleosteans are Vertebrates specialised to the uttermost for their own 

 kind of life, and that they are, indeed, one of the culminating branches of the vertebrate Life-tree?' 

 (Prof. W. K. Parker, " On the Development of the Skull in the Salmon," ' Phil. Trans.,' vol. clxiii, pt. 1, 

 p. 110 (1873). 



1 Chirncera and its ally Callorhynchus have a rudimentary operculum. 



2 This action may be studied wherever Dog-fish are kept in an aquarium. 



3 Giinther's ' Catalogue of Fishes,' preface to vol. iv. 



