PISCES, TRUE FISH 465 



The encloskeleton is principally bony and is composed of skull, 

 vertebrae, ribs, pectoral and pelvic girdles, and bony fin supports. 

 The main axis of the skeleton is the vertebral column and skull, 

 known as the axial portion. The first five vertebrae of the neck 

 or cervical region are fused together, but the remainder are sepa- 

 rate and are called amphicoelous because each end of the centrum 

 or body is concave. The parts of one of the simple vertebrae are 

 the body or centrum just mentioned, and the neural arch over the 

 spinal cord which lies in the neural canal. A neural spine extends 

 dorsally from the neural arch, and the parapophyses extend laterally 

 from the centrum and support the ribs. There are haemal arches 

 supporting haemal spines on the ventral side of the posterior ver- 

 tebrae. The adjacent vertebrae articulate at the centra and are held 

 in place by ligaments. Vertebrae six to fourteen bear ribs from the 

 transverse processes. 



The skull is very largely bone with some cartilage. The bones are 

 arranged bilaterally. The skull may be divided into cranium and 

 visceral skeleton. The cranium encloses the brain and is composed 

 of the frontal hones, postfrontals, parietals, supraoccipital, exoccipi- 

 tals, hasioccipitals, 'basisphenoids, alisphenoids, and parasphenoid. 

 The ethnoids, sphenoids, epiotic, quadrates, pterygoids and nasals pro- 

 tect and support the auditory and olfactory organs. 



The visceral skeleton supports the gills and includes the jaws. The 

 maxillary arch supplies both upper and lower jaws. The upper jaw 

 develops from a cartilaginous pterygoquadrate process into the pair 

 of premaxillae and pair of maxillae bones. The lower jaw is the 

 mandible. The premaxillae and mandible both have short spinelike 

 teeth. Just behind the mandible is found the hyoid arch, referred 

 to as number two. It supports the tongue, floor of the mouth, and 

 operele. The next five arches support the four gills and are known 

 as gill or hranchial arches. Each is composed of either four or five 

 parts and has spiny gill rakers at its anterior margin. They are 

 located in the lateral wall of the pharynx and are covered by the four 

 opercular bones of each side which compose the framework of the 

 operculum. 



The skeleton of the paired fins is known as the appendicular skele- 

 ton. The pectoral girdle, made up of scapula, coracoid, supraclavicle, 



