SKELETAL SYSTEM

The skeletal system of vertebrates is composed of bone and/or cartilage.  Bone tissue is found only in the Subphylum Vertebrata. Some of the lower vertebrates do not possess bone, but all the higher vertebrates do.  As such, bone is often thought of as being typical of vertebrates.  In vertebrates, bone functions as a supporting tissue, a calcium reserve and as a hemopoietic (blood forming) tissue.

The skeleton is the basis of form and support of the vertebrate body.  Muscles attach to the skeleton and vital organs are surrounded and protected by skeletal elements.  As you examine the skeletons of the perch and the rat you should note a number of basic changes that have occurred in the evolution of the vertebrate skeleton.  Some of the changes involve a reduction in the number of bones in the skull and a reduction in the number of ribs.  Correlated with the move from an aquatic to a terrestrial environment are the increase in the complexity of the limbs and limb joints, the development of the pectoral and pelvic girdles and the strengthening of individual bones to support the weight of the organism on land.  The central structure of support in the lower vertebrates, the notochord, is progressively replaced functionally by elements of the vertebrae.  Although the notochord runs the length of the vertebral column in fish, in many it has been greatly restricted by the vertebrae.  In adult tetrapods, the only remnant of the notochord is the gelatinous material found in the intervertebral discs between successive vertebrae of the vertebral column.

In the study of the skeletal system in the perch and a more advanced vertebrate, the rat, you should try to determine which skeletal features are signs of typical evolutionary advancement and which may be specializations due to the animal’s way of life.

The skeleton of vertebrates is broadly divided into two parts:  the axial skeleton consisting of the skull, vertebrae and ribs; and the appendicular skeleton consisting of the pectoral and pelvic girdles and the bones of the appendages.

NOTE: The bones of the prepared skeletons are delicate and easily broken.  HANDLE THESE SKELETONS WITH EXTREME CARE!

 Perch Skeleton

 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.

The Vertebral Column

A series of endochondral bones called vertebrae form the vertebral column.  Vertebrae have several common features.  The large spool-shaped central portion of each is the centrum.  Extending through the middle of each centrum is a canal for the passage of the notochord.  As mentioned previously, many fish retain a notochord throughout life.

Above the centrum, an arch of bone surrounds and protects the spinal cord.  A dorsal projection, the neural spine, extends outward from the vertebral column (Figure 4 and 5).

The fish vertebral column is divided into two subdivisions:  the trunk and the tail (caudal).

Although fish do not have a neck, the first two trunk vertebrae are modified.  These vertebrae lack ribs.  The rest of the trunk vertebrae possess ribs.  Caudal (tail) vertebrae possess a ventral

portion which forms a hemal arch which surrounds blood vessels (Figure 4 and 5).

Figure 4: Representative Vertebrae of the Perch.

Figure 5: Skeleton of the Perch.

The Appendicular Skeleton and Fins

         Using the preserved perch and the skeleton,  locate the following structures:

The Median Fins

The dorsal fins of the perch have fin rays for support.  The anterior dorsal fin has ossified fin rays which provide stiff support, while the fin rays of the posterior dorsal fin are not ossified and are flexible.  Only the first two fin rays of the anal fin are ossified. The caudal fin is composed entirely of soft, unossified fin rays.

The Pectoral Girdle and Fins

The pectoral fins are attached to a bony girdle, the pectoral girdle. The pectoral girdle is composed of a number of fused elements. The girdle is also fused to the skull (the head and trunk of the perch move as a unit).  The fins are supported by soft fin rays.

The Pelvic Girdle and Fins

The pelvic fins are attached to the pelvic girdle, which is composed of two bony pelvic plates.  The plates may be fused along the midline.  The pelvic girdle is not attached to the vertebral column nor to the pectoral girdle, but is free-floating (embedded in muscle only).  Only the medial fin rays are bony, the rest are the typical soft unossified type.