Comparative Anatomy - ZOO 420 2(1+1) Zoology Department Dr. Elsayed Younis

The skeletal system (The skeleton) - The skeletal system arises from the mesoderm. - There are two different skeletal types: the exoskeleton ( which is the stable outer shell of an organism or hard skin derivatives of the vertebrates) and the endoskeleton which forms the support structure inside the body. - In general, the endoskeleton consists of: (A) The axial skeleton : comprises the parts which lie along the median axis of the body (skull, vertebral column and sternum)

(B) The appendicular skeleton : comprises the parts which lie on both sides of the axis (pectoral girdle, pelvic girdle, fore-limbs and hind-limbs) The general structure of the parts of the endoskeleton (A) The axial skeleton:- The skull:- is the skeleton of the head. It passes through three developmental stages ( membranocranium, chondro- cranium and osteocranium ) however, osteocranium is built up of bones and cartilages together. The skull is divided into two regions:- (1) The neurocranium: includes the cranium that surrounds

the brain and hence also called the brain-box, and the sense capsules which enclose the special sense organs. These are 3 pairs ( olfactory or nasal capsules , optic capsules and auditory or otic capsules ). (2) The splancocranium: consists of visceral arches (mandibular arch, hyoid arch, and branchial arches) The vertebral column: consists of segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs). The vertebrae are all built up on the same plan, and resemble each other very closely. ( vertebra usually consists of centrum, neural arch, neural foramen, neural spine and transverse processes)

The sternum: lies in the ventral wall of the thorax and attached to the ribs and pectoral girdles. (B) The appendicular skeleton :- pectoral girdle :- consists of two symmetrical halves, each half consists of coracoid, clavicle, scapula, and supra- scapula. pelvic girdle :- consists of two symmetrical halves, each half consists of ilium, ischium and pubis. fore-limbs and hind-limbs

The skeleton of Cartilaginous Fishes (Chondrichthes) The skeleton of the cartilaginous Fishes is entirely formed of cartilage and remains cartilaginous throughout life. The axial skeleton:- The skull :- is formed of neurocranium (cranium , sense capsules) and splancocranium (mandibular arch, hyoid arch, and branchial arches) The vertebral column: is formed of two distinct regions, the trunk and the tail. The vertebrae are amphicoelous, that is concave on both sides.

The sternum :- absent

The appendicular skeleton :- pectoral girdle :- consists of two dorsal scapular portions and a ventral coracoid portion pelvic girdle :- consists of transverse ischio-pubic bar fore-limbs and hind-limbs: replaced by the fins which is a characteristic feature of the fish There are two types of fins:- paired fins (pectoral fins and pelvic fins) unpaired fins (dorsal fin , anal fin and caudal fin )

The skeleton of Bony Fishes (Osteichthes) The skeleton of the bony Fishes is entirely formed of bones and is built up on the same plan like that of the cartilaginous fishes. The axial skeleton:- The skull :- is formed of a series of loosely connected bones; neurocranium (cranium , sense capsules) and splancocranium (mandibular arch, hyoid arch, and branchial arches). The vertebral column: is formed of two distinct regions, the trunk and the tail. The vertebrae are amphicoelous, that is concave on both sides. The sternum :- absent

The appendicular skeleton :- pectoral girdle :- consists of two dorsal scapular portions and a ventral coracoid portion pelvic girdle :- consists of transverse ischio-pubic bar fore-limbs and hind-limbs: replaced by the fins which is a characteristic feature of the fish There are two types of fins:- paired fins (pectoral fins and pelvic fins) unpaired fins (dorsal fin , anal fin and caudal fin )