Ixviii Introduction. 



and rudimentary lioraologue of the pulmonary organs of the 

 Amphibia, appears to put this order of Fishes into a position much 

 farther removed from the hig-her Vertebrata with which they were 

 thus classed, than that which the Dipnoi, and even the Ganoidei 

 occupy. 



Class, Pisces. 



Branchiate Vertebrata^, with motor organs in the shape of fins, 

 supported by numerous internal rays, and placed along the medio- 

 dorsal and medio-ventral lines, or along these lines and bilaterally 

 also. The endo-skeleton of Fishes takes far more various forms than 

 that of any other vertebrate Class ; and their exoskeleton is simi- 

 larly distinguished with reference to all other classes except the 

 Mammalia. 



The dermal exoskeleton may take the form of scales, as in the 

 great majority of Fishes ; of placoid or spiny dentinal formations, 

 as in Masmohranchii ; of enamelled scales or of bony plates, as in 

 Ganoidei ; and in Dipnoi, Ganoidei, and Teleostei, it extends into 

 the sub-cutaneous fibrous mesh, and along intermuscular aponeuroses 

 forming ' splint bones/ There are no scales in Marsipobranchii, 

 and the Spatidaridae, a genus of Ganoidei, have an almost entirely 

 naked skin. There are no splint bones in the ElasmohrancJiii. 

 The cutaneous system is further distinguished by the possession of 

 the system of the lateral line,'' which has not been detected else- 

 where, except in certain Amphibian larvae, and which is supposed 

 to be sensory in function. The epidermis is ordinarily jDrolonged as 

 a continuous, even if thin layer, superficially to the various dermal 

 formations, except in the cases of some of the outgrowths deve- 

 loped in the Elasniohranchii, and sometimes of the enamelled scales 

 of the Ganoidei. 



However various the endoskeletal structures of Fish may be, they 

 all agree in the non-possession of a sternum, the absence of which is 

 connected with the peculiarities of their reproductive processes ; 



* The low grade of organization to which the Pharyngohranchii , as represented by 

 the Lancelot, have attained, makes it convenient to omit this Order from consideration, 

 whilst detailing the characteristics more or less universally found in the other five 

 Piscine Orders, viz. Marsipobranchii, Teleostei, Ganoidei, E/asmohranchii, and 

 Dipnoi. 



