Cha7^acteristics of the Vertehrata. Ixxi 



surface of the first vertebra ; and the biconical cavity thus formed 

 is filled with a structure formed by the development of the chorda 

 dorsalis, and of semi-g-elatiniform consistence. 



Fish are very rarely edentulous. Teeth are ordinarily present, 

 and are very variable in number, shape, and situation. Most of the 

 bones of the oral and pharyngeal cavities may be dentig-erous ; but 

 in Cyprinoids_, there may be only a sing-le tooth superiorly, carried 

 by the basi-occipital. The teeth are replaced as often as they are 

 shed, and in the family just mentioned, the inferior pharyngeal are 

 so shed and replaced periodically. It is only in Fish that the dental 

 series is continued in an unbroken row across the middle line, 

 without forming- a diastema corresponding- to either upper or lower 

 median raphe. Fish have no oral salivary glands, and the tongue 

 is only movable as a part of the hyoid apparatus upon which it is 

 carried. In the 3Iarsi]oohranchu , the branchial sacs open both in- 

 ternally and externally by the means of larger or smaller ducts, 

 which again may form a common duct before their inner or outer 

 termination respectively. In all other Fishes the branchial inlets 

 and outlets both have alike the form of fissures, the inlets leading 

 directly from the interior of the pharynx, and the outlets opening 

 either directly on to the external surface of the body, as in the 

 Sharks and Rays, or into a branchial cavity covered by the opercular 

 apparatus as in Ckimaerae, Dipnoi, Ganoidei, and Teleostei. The 

 oesophagus is ordinarily short ; and it is also, as the food is usually 

 swallowed with little or no comminution_, of considerable width. In 

 the Marsipohranchii, the digestive tract takes an antero-posterior 

 course, wdthoiit any external differentiation into stomach and in- 

 testine. In other Fishes the intestine is readily distinguishable from 

 the siphonal or coecal stomach ; and describes one or two convo- 

 lutions before terminating at the anus through the intermediation 

 of a short rectum, from which a colon can scarcely be said to be 

 differentiated. The length of the entire tract is shorter relatively 

 to that of the entire body than in the air-breathing Vertehrata gene- 

 rally ; it is however not inconsiderable in the species which support 

 themselves upon vegetable diet ; and the absorbing surface of the 

 canal is greatly increased in Dipnoi, Ganoidei, and Elasnwbranchii, 

 by the development of internal folds of the mucous membrane into 

 a spiral valve, which appears to be rudimentarily represented in the 

 Marsipohranchii by a longitudinal ridge running along the internal 



