STRUCTURE OF FISHES. 79 



ing gold or silver ; sometimes they present the richest tints of 

 green, blue, red, or black. The silvery matter which frequently 

 gives them such a beautiful metallic lustre, is secreted by the 

 skin, and is composed of a multitude of small polished plates. 



5. The skeleton of fishes is ordinarily bony ; but in many of 

 these animals, it always remains fibre-cartilaginous, or cartila- 

 ginous, and in some, this frame possesses even less solidity and 

 remains absolutely membranous. In this respect, they form the 

 connecting link or passage between the vertebrate and inverte- 

 brate animals. 



6. The bones never have a medullary canal, and the cartilage 

 which constitutes their basis is not like that of mammals and 

 birds ; for, when boiled in water, it does not yield gelatine. 



7. The skeleton is composed of a head, to which is joined a 

 highly developed hyoid apparatus, serving for respiration ; a 

 trunk and extremities. 



Fig. 42.* 



" K J l> / 



SKELETON OF A PERCH. 



8. The structure of the head is very complicated : we first 

 observe a middle portion, composed of a great number of bones 

 joined together by sutures, and forming a sort of immoveable keel 

 to which are suspended the bones of the jaws, cheeks, &c. This 



* Explanation of Fig. 42. Skeleton of a Perch: a. the skull, b. the 

 orbit, c. the nostrils, d. the intermaxillary bone, e. the maxillary bones, 

 -/. the lower jaw, g. the snb-orbital bone, h. the tympanic bone, and the 

 other bony pieces which separate the mouth from the cheeks, and support 

 the lower jaw, i. the operculum, j. the ante-operculum bone, /. the 

 scapula, or shoulder blade, m. the bones of the arm, n. the coracoid bone, 

 o. the pectoral fin, p. the pelvis, q. the ventral fin, r. the vertebrce, 

 *. the ribs, t. the interspinal bones, u. the bony spine of the first dorsal 

 fin, v, the cartilaginous spine of the second dorsal fiu, x. the anal fin, 

 y. the caudal fin. __^ 



5. What is the nature of the skeleton of fishes ? 



6. What is the character of the bones of fishes ? How does fish-bone 

 differ from that of mammals ? 



7. How is the skeleton of fishes divided ? 



8. What are the general characters of the head ? 



