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 (1.) of an axial part, including the vertebral column 
and the skull; and Gi.) 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 seg. where copious 
references to the work of other writers are given. 
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