VERTEBRATA PISCES. 49 



The gills remain throughout life, and the limbs take the 

 form of fins. 



59. Skeleton. The internal skeleton of some fishes 

 consists of cartilage or gristle ; in others it is partly 

 composed of cartilage and partly of bone. Most of the 

 extinct fishes, especially of those found in the primary 

 rocks, had cartilaginous skeletons ; but the skeletons of 

 the majority of modern fishes consist of true bone. 



In cartilaginous fishes the various parts of the skull 

 are joined together so as to form a single piece ; but in 

 the bony fishes the skull is very complex. 



The number of vertebrse is very various, ranging in 

 different fishes from seventeen to one hundred. In bony 

 fishes, the vertebrae are amphicoelus (Or. amphi, both; 

 koiloSj hollow) or concave at both ends. Their edges are 

 united together by ligaments, and the hollow space 

 between the vertebraB is filled with a gelatinous substance, 

 so that the spine is extremely flexible. The vertebral 

 column is divisible into the two regions caudal and 

 abdominal. The ribs are confined to the abdominal 

 region. Their lower ends are free, the sternum being 

 absent. 



60. The limbs, when present, take the form of " fins," 

 which are prolongations of the integument or skin, sup- 

 ported by bony or cartilaginous rays. The pair of fins 

 which correspond to the arms of a man are called pectoral 

 (Lat. pectus, the breast) fins. They are attached to bones 

 which bear some resemblance to the fore-arm in the 

 higher vertebrates. There is a pectoral arch in which 

 the scapula, clavicle, and coracoid bones are represented. 

 The humerus is wanting ; but there are usually bones to 

 represent the radius and ulna, and the carpus. To this 

 latter the fin rays are attached. 



61. The ventral (Lat. venter, the belly) fins correspond 

 to the legs of a man. They are united directly to a kind 

 of pelvic arch, the bones of the hind limb being unre- 

 presented. Besides these fins, which are called paired 

 fins, there are others which occupy the middle line of the 



15 E. D 



