CJiaracteristics of the Vertebrata. Ixxxiii 



The Teleostei comprise an immense majority of existing Fishes. 

 They may be subdivided into two great sub-orders : the Pliysostomiy 

 which possess an air-bladder and an air-duct, and, with the ex- 

 ception of the Pike, bone corpuscles in their skeleton ; and the 

 Physoklisti (Haeckel) in which the air-duct is always absent, the 

 air-bladder sometimes, and in which, with the exception of the 

 Tunny, bone-corj5uscles are wanting. The vertebrae vary much in 

 number, and in the extent to which calcificatory deposit takes place 

 in them; but they are always individualized, though the most 

 anteriorly placed of them may be suturally united with each other 

 and with the basi-occipital bone. The gill-fringes may vary in 

 number accordingly, as upon each of four branchial arches a biserial 

 or uniserial gill is developed; but they never exceed the number 

 of four biserial gills, an opercular gill being never developed. 

 They have always more than a single bone in the opercular valve. 

 The cartilaginous cranium may either persist or disappear, but 

 cranial bones are always developed in addition to it. The scapular 

 arch has a clavicular element ; the anterior fins are i-arely absent ; 

 the position of the posterior, which are much more commonly 

 absent than the anterior, varies from the ' abdominal ' to the ' tho- 

 racic,' and from the 'thoracic' to the 'jugular' region. The aortic 

 bulb is not provided with more than two valves, nor has it a 

 covering of transversely striated muscular fibre. The optic nerves 

 decussate, but do not form a chiasma. 



The fifth order of Fishes, the MarsipohrancMi, consists of the 

 two families of Myxinoidei and Petromyzontidae. Their sac-like 

 gills are supported on a cartilaginous framework, which is more 

 superficially placed than the analogous visceral skeleton of hig-her 

 Fish. They have a single nasal opening, and have been hence 

 called ' Monorrhina,' in contradistinction to all higher Vertebrata. 

 They have no mandible, and higher Vertebrata have, in contra- 

 distinction to them, been on this account spoken of as ' Gnatho- 

 stoma.' They have no traces of air-bladder, of limbs, of limb- 

 girdles, of cranial bones, of scales, of spleen, or of pancreas. Their 

 tail retains the homocercal form characteristic of the early embryo 

 in other Fishes. They have no vertebral centra. The sympathetic 

 system is wanting, and the commencement of their aortic trimk has 

 neither striped nor smooth muscular fibres developed upon it. The 

 Myxinoids are less highly organized, being of parasitic habits, than 



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