CHARACTERS OF FISHES. 



389 



Besides the fins which represent the limbs (pectoral and 

 ventral fins), fishes possess other fins placed in the middle line 

 of the body, and all of these alike are supported by bony spines 

 or " rays, 3 ' which are of two kinds, termed respectively " spi- 

 nous rays " and " soft rays." The " spinous rays " are simple 

 bony spines, apparently composed of a single piece each, but 

 really consisting of two halves firmly united along the middle 

 line. The " soft rays " are composed of several slender spines 

 proceeding from a common base, and all divided transversely 

 into numerous short pieces. The soft rays occur in many fishes 



Fig. 163. Skull of Cod (Morrhua -vulgaris) Cuvier. a Urohyal ; b Basihyal ; c 

 Ceratohyal ; d Branchiostegal rays ; / Prae-operculutn ; o Operculum proper ; j Sub- 

 operculum ; i Inter-operculum ; m Mandible ; n Inter-maxillary bone. 



in different fins, but they are invariably found in the caudal 

 fin or tail (fig. 162, c). The rays of the median fins, whatever 

 their character may be, always articulate by a hinge joint with 

 the heads of the interspinous bones. 



The skull of the bony fishes is an extremely complicated 

 structure, and it is impossible to enter into its composition 

 here. The only portions of the skull which require special 

 mention are the bones which form the gill-cover or operculum, 

 and the hyoid bone with its appendages. For reasons con- 

 nected with the respiratory process in fishes, as will be after- 



