1. SKELETAL SYSTEM
The hard framework of animal body forms the skeletal system.
OSTEOLOGY:- Study of bones and their structure and functioning is called osteology.

2. TYPES OF SKELETON It is of three main types:- Hydrostatic skeleton
Exoskeleton
Endoskeleton

3. HYDROSTATIC SKELETON:-
Fluid being incompressible act as a support system in many organisms. Eg-worms have hydrostatic skeleton.
EXOSKELETON:-
It consists of hard parts present on the surface of body. It is found in invertebrates as well as vertebrates. Eg-Shells of snails and chitinous plates of insects. Scales, feathers, hair, claws hoofs, nails and horns in vertebrates.
ENDOSKELETON:-
It includes hard parts present inside the body. Cartilages and bones are examples of endoskeleton in vertebrates. Some invertebrates such as corals and echinoderms, also have endoskeleton. It however consists of a non living horny or calcareous material.

4. DEVELOPMENT OF BONE TISSUE
Skeleton is formed of entirely of cartilage in an early embryo. Bone formation occurs later. The process of bone formation is called ossification. It involves deposition of calcium carbonate and calcium phosphate by bone cells in the matrix between them.

5. TYPES OF BONES
According to their source bones are of four types.
1. Replacing/Endochondral/Cartilage Bones:- These are formed due to the ossification of original cartilaginous endoskeleton Eg. Humerous and Femur.
2. Investing/membrane/ Dermal Bone:-These are formed by the direct ossification in the dermis of skin as thin plates and sink to get attached over the original cartilaginous endoskeleton. Eg. Parietals, Frontals, Nasals and Vomer of skull.
3.Sesamoid Bones:- (Having shape of seed). These are formed by the ossification in the tendons at the joints where additional strength is needed Eg.-Patella or knee cap, fabellae.

6. 4. Heterotopic or Visceral Bones:-
These are formed in organs quite dissociated from the rest of skeleton Eg. Os-cordis in Heart of ruminants; Os-penis (baculum) in copulatory organ of most bats, insectivores and some primates (not man); Os- clitoris in clitoris of many carnivores ; os-palpebrae in eyelids of crocodiles, bony plates in diaphragmatic muscles of camels, small bone in the crest of bird and snout of hog.

7. FUNCTIONS OF SKELETON
(1) Support (2) Shape (3) Protection (4)Movements (5) Blood corpuscles Formation (6) Hearing (7) Sound Production (8)Mineral storage (9) Fat Storage (10) Breathing

8. SKULL
Skull consists of three main parts:- Cranium which encloses brain, sense capsules that contains organs of special senses and visceral or pharyngeal arches which form the jaws and framework of pharyngeal wall.

9. DEVELOPMENT:- Three different embryonic components contribute to the skull in most vertebrates.
Chondrocranium:- It is composed of cartilage. It mainly forms the base of skull. It is replaced by bones in most cases.
Dermatocranium:- It is formed of dermal plates or membrane bones. These bones roof over the chondrocranium.
Splanchnocranium:- It consists of visceral arches. These are primarily cartilaginous but may get partly invested with or replaced by membrane bones.

10. VISCERAL ARCHES/SPLANCHNOCRANIUM
Basic Structure:- The visceral arches develop around the pharyngeal part of the embryonic gut from the mesenchyme surrounding it. There are typically 7 visceral arches.
Mandibular arch
Hyoid arch
5 Branchial arches.

12. Hyoid Arch:- It is second visceral arch. It consists of 3 segments.
Mandibular Arch:- It is largest and most modified of the visceral arches. It gets divided into dorsal and ventral halves. Each side of the dorsal half is called Palatopterygoquadrate Bar. The Palatoptergoquadrate Bars are applied to ventral surface of the chondrocranium. bear teeth and constitute the upper jaw. Each side of ventral half of mandibular arch is termed as Meckels contilage Meckels cartilage bear the teeth and form the lower jaw, wide gap between two jaws in mouth.
Hyoid Arch:- It is second visceral arch. It consists of 3 segments.
i) Ventral median Basihyal supporting buccal floor and tongue.
ii) Two lateral Ceratohyals also present on floor of buccal cavity
iii) Two dorsal Hyomandibulars. Lower end of Hyomandibular is attached to post. end of both jaws, the upper end articulates with cranium.
3. Branchial Arches: The remaining 5visceral arches are called branchial arches. Branchial arches support gills and wall of pharynx each branchial arch consists of 4- rod like segments on either side of mid-ventral dagger- shaped cartilage called basibranchials. The other 4 segments of each branchial arch are: a dorsal pharyngobranchial a ventral hypobranchial and lateral epibranchial and ceratobranchial.

13. EVOLUTION OF VISCERAL ARCHES
Mandibular Arch:
In bony fisches the Palatopterygoquadrate bar gets ossified and differentiates into primary and secondary structures. The primary structures are differentiated into palatine, pterygoid and quadrate. The secondary part consists of investing teeth bearing bones like maxilla and premaxilla.

14. AMPHIBIA, REPTILES & AVES MAMMALS
MANDIBULAR ARCH
CHONDRICHTHYES
OSTEICHTHYES
AMPHIBIA, REPTILES & AVES
MAMMALS
(i) Palatoptery -goquadrate
Palatopterygo-quadrate bar
Palatine, Pterygoid Qudrate. (Maxilla,Premaxilla) Investing Bones
Quadrate
Investing Bones
Incus
Fused with alisphenoid
(ii) Meckel’s Catilage
Meckel’s
Cartilage
Articular
Investing bones.
(dentary, angular, supra-angular, coronoid process)
Malleus
Meckel’s cartilage

15. The meckel’s cartilage gets ossified and forms lower jaw in bony fishes. It is formed of articular and investing bones like dentary angular, supra-angular and coronoid process.
In Amphibians, Reptiles and Birds the quadrate participates in jaw suspension and meckel’s cartilage changes to articular & investing bones.
In mammals quadrate is modified into incus. and articular bone of meckel’s cartilage forms malleus.

16. 2) Hyoid Arch- Hyoid Arch Chondrichthyes Osteichthyes Amphibia
Reptiles & Birds
Mammals
(i)Hyomandibular
(ii) Hyiod Proper
Hyomandibular
Hyoid Cornu
(a) Basihyal
(b) Ceratohyal
(c) Hypohyal
(d) Epihyal
Columella auris
Hypo – Branchial apparatus
Stapes
Styloid process
Stylohyoid ligaments

17. 3) Branchial Arches Branchial Arches Chondrichthyes & Osteichthyes
Amphibia, Reptiles & Aves
Mammals
1st
2nd
3rd
4th
5th
1st Branchial arch
2nd Branchial arch
3rd Branchial arch
4th Branchial arch
5th Branchial arch
Part of hyoid apparatus
Laryngotracheal Cartilages
Post. Horn of Hyoid
Thyroid cartilage
Cricoid Cartilage
Arytenoid & Tracheal cartilage.

18. ASSISTANT PROFESSOR Deptt. of ZOOLOGY
KAVITA DHIMAN
ASSISTANT PROFESSOR Deptt. of ZOOLOGY