Anatomy of finfish and shellfish

Unit 10 - Skeletal system

10.2.1. The Axial Skeleton
Skull
The skull of the perch is actually a double structure consisting of two “boxes” of bone, one enclosed by the other. The outer skull is an armour of dermal bone. Primitive extinct bony fish had dermal bony armour covering most of their bodies. In the modern fish, the outer skull is virtually all that remains of this armour. Dermal bone forms, as its name implies, in the dermis of the skin and is not proceeded by a cartilage structure. The inner skull is composed of endochondral bone. Endochondral bone develops under the dermis and replaces existing cartilaginous structures. Hence the name “endochondral” denotes the bony tissue develops “within” existing cartilage structures. Elements of the inner skull form the cranium or brain case.
The perch skull consists of many small bones. You are not responsible for knowing the identity of these bones, but look closely at the skull to see the inner endochondral skull encased by the outer dermal skull.