54 



The Skeleton of the Fish 



Most of the fishes with spines in the fins have thoracic ven- 

 trals. In the fishes with jugular ventrals these fins have begun 

 a process of degeneration by which the spines or soft rays or 

 both are lost or atrophied. 



Degeneration. By degeneration or degradation in biology 

 is meant merely a reduction to a lower degree of complexity 

 or specialization in structure. If in the process of development 



SOP 



FIG. 39. Lower jaw and palate of Sebastolobus cdascanus. (After Starks.) 



PA. Palatine. 



MSPT. Mesopterygoid. 



PT. Pterygoid. 



MPT. Metapterygoid. 



D. Dentary. 



AR. Articular. 



AN. Angular. 



Q. Quadrate. 



SY. Symplectic. 



HM. Hyomandibular. 



POP. Preopercle. 



IOP. Interopercle. 



SOP. Subopercle. 



OP. Opercle. 



of the individual some particular organ loses its complexity it 

 is said to be degenerate. If in the geological history of a type 

 the same change takes place the same term is used. Degenera- 

 tion in this sense is, like specialization, a phase of adaptation. 

 It does not imply disease, feebleness, or mutilation, or any ten- 

 dency toward extinction. It is also necessary to distinguish 

 clearly phases of primitive simplicity from the apparent sim- 

 plicity resulting from degeneration. 



The Skeleton in Primitive Fishes. To learn the names of bones 

 we can deal most satisfactorily with the higher fishes, those in 



