1. Getting to know a fish - Internal Structures
    2.
    1. Skeleton

The term "skeleton" is often used to describe the compression-resistance elements that give much of the body support. In practice, the skeleton is a kinetic system of compression- resistant and tension-resistant elements, with the various activating muscles.

      1. Cartilage and Bone – Compression Elements

Fishes have many bones. Compression elements are:

The bones are organized into an axial and appendicular skeleton.

Axial Skeleton

Appendicular Skeleton

Skull (cranium)

Postcranial skeleton

  

Neurocranium

Branchiocranium (visceral cranium or splanchnocranium derived from splachnic mesoderm)

Notochord.

Vertebral column.

Pectoral girdle.

Pelvic girdle.

Median fins.

Chondro-cranium

Dermato-cranium

      
  • Ethmoid.
  • Orbital.
  • Otic.
  • Basicranial
  • Mandibular – upper and lower jaws.
  • Palatine.
  • Hyoid or suspensorium.
  • Opercular.
  • Branchial
  • Precaudal.
  • Caudal.
  •  
  
   

Caudal complex.

  • Protocercal.
  • Heterocercal.
  • Hypocercal. Found in cephalaspids (extinct).
  • Homocercal.
  • Leptocercal (Diphycercal).
  • Isocercal.
  • Gephyrocercal.
  

 

      1. Ligaments
        2.

Ligaments are non-elastic fibrous strands of connective tissue that attach bones and/or cartilage to one another. The skin contains a matrix of connective tissue that resists extension along the axis of the spirally-wound fibers. Some think this acts as an external ligament involved in transmission of myotomal locomotor muscle force to the body and caudal fin.

      1. Muscle

There are three types of muscle: skeletal, smooth, and cardiac.

Fish have three major types of skeletal muscle fibers: fast glycolytic (FG – "white"), fast oxidative (FO – "pink"), and slow oxidative (SO – "red").

    1. The Abdominal Cavity
      2.

      The abdomen contains the internal organs associated with most functions of the animal; liver, spleen, stomach, intestines, and gonads. The urinary and gonadal tracts exit at the cloaca. The abdomen is lined by the peritoneum. The gut length and components vary associated with general food habits (herbivory, degree of detrivory, omnivory, carnivory).

      The swimbladder is one mechanism used to regulate the density of a fish, and its location in the water column.

    2. Cardiovascular System
      3.

      Fish have four-chambered hearts located in the pericardial cavity, but unlike mammals, the chambers are all in series. There is a single circulation in most fishes – heart Þ ventral aorta Þ gills Þ dorsal aorta Þ systemic circulation Þ large veins Þ heart.

    3. The Central Nervous System

Fish brains are quite small compared to other vertebrates, approximately 1/15 the size of a bird or mammal brain in animals of equal