CHAPTER VIII 



THE SKELETON 



All Fishes possess an internal skeleton which, in order that it 

 may be distinguished from the more superficial scaly exoskeleton 

 described in Chapter VII., is termed the endoskeleton. The 

 latter consists (i.) of an axial part, including the vertebral column 

 and the skull ; and (ii.) of an appendicular portion, consisting of 

 the skeleton of the limbs and their supporting pectoral and 

 pelvic girdles. 



The Vertebral Column.^ — The individual segments or vertebrae 

 which, arranged in a linear series, collectively form the vertebral 

 column, are highly complex structures, each being formed by a 

 number of vertebral elements, the sum total of which constitutes 

 a vertebra. Perhaps the best conception of the nature of vertebral 

 elements is to be gleaned from the study of such primitive Fishes 

 as the Elasmobranchs, in which not only are all the vertebral 

 components present, but they are less modified by suppression 

 and fusion than in most other Fishes, and on this account they 

 afford a convenient introduction to the study of the puzzling 

 eccentricities of vertebral structure in other groups. Selecting 

 any common Dog-Fish, such as Scyllium canicula, and starting 

 with an early embryonic stage, it may be stated that the first 

 indication of a vertebral column is the formation of the notochord, 

 which, invested by its chordal sheath, extends from the tip of 

 the tail to a point on the under surface of the brain just behind 

 the hypophysis or pituitary body. Subsequently, a number of 

 cartilaginous pieces are developed in connexion with the dorsal 



' This portion of the chapter is mainly based on the important researches of Dr. 

 Gadow and Miss Abbott. See Phil. Trans. 186, 1895, p. 163 et seq. where copious 

 references to the work of other writers are given. 



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