The Fishes

Phylum Chordata

Subphylum Vertebrata

Cartilaginous fishes: Class Chondricthyes

Sharks, skates, and rays.


1. Fins:

a. Caudal fin - tail or posterior fin.
b. Paired pectoral fins - ventral and posterior to external gill openings.
c. Paired pelvic fins - ventral and posterior to pectoral fins.
d. 2 dorsal median fins

2. Mouth: ventral with jaws; olfactory sacs do not open into the oral cavity.

3. Placoid scales on skin - scales that are composed of dentin and are covered with enamel-like substance (denticles sp.); modified placoid scales serve as teeth.

4. Endoskeleton entirely cartilaginous; notochord persists in adult; vertebral column is complete and separate from the notochord.

5. Digestive system of a "J"-shaped stomach, intestines, liver, gallbladder, and pancreas.

6. Circulatory system of several pairs of aortic arches; dorsal and ventral aorta, capillary and venous systems, hepatic (liver) portal and renal (kidneys) portal systems; 2-chambered heart; high []'s of urea and trimethylamine oxide in blood (nitrogenous waste products typical of carnivorous animals which take in more nitrogen than needed): protein (polymer of amino acids) -----> Single amino acids.

7. Gaseous respiration by means of 5 to 7 pairs of gills with separate, external gill slits.


9. Brain of 2 olfactory lobes, 2 cerebral hemispheres, 2 optic lobes, a cerebellum, and a medulla oblongata; 10 pairs of cranial (skull) nerves. (page 644)
Cerebellum - controls balance and coordination - small region that lies just above the medulla.
medulla - most posterior region of the brain that is really a conical continuation of the spinal cord; together with the midbrain, makes up the "brain stem" or medulla oblongata. Regulates subconscious activities such as heartbeat, respiration, etc.

10. Sexes are separate; paired gonads (sex organs: ovaries in females and testes in males); reproductive ducts open into the cloaca (excretory structure); sex organs and excretory organs.





Bony fishes: Class Osteichthyes

General Characteristics:

1. Bony skeleton (mostly); most primitive or simple skeleton in the vertebrate animals; numerous vertebra; notochord may persist; post anal tail present in adults.

2. 3 types of dermal scales:

a. Ganoid scales - diamond-shaped; simple scales with layers of silvery enamel on upper surface and bone on the lower surface.
b. Cycloid scales - thin, flexible scales of dermal material arranged in roughly, concentric circles with smooth margins.
c. Ctenoid scales thin, overlapping dermal scales of advanced bony fishes; exposed posterior margins are rough with fine, toothlike spines.
d. some without scales.

No placoid scales on any bony fish.

3. Fins both singly and a median; also paired lateral fins. Fins have rays of cartilage or bone.

4. Mouth with many teeth and movable jaws (some toothless fish do exist.). Olfactory sacs paired and may or may not open into the oral cavity.

5. Respiration by gills supported by bony gill arches and covered by a common operculum - opening for water passage to outside from internal gill cavity.

6. Swim bladder usually present with or without a duct connected to the pharynx.

7. Circulation system of a 2-chambered heart, arterial and venous vessels, and usually four pairs of aortic arches; blood contains nucleated red cells.

8. Nervous system of a brain with small olfactory lobes, cerebrum. Large optic lobes and cerebellum; 10 pairs of cranial nerves.

9. Sexes separate (sex reversal in some spp.); gonads are paired; fertilization usually external; larval forms may differ greatly from adults.


Three distinct groups:

1. Lobe-finned fishes:

2. Lungfishes:

3. Ray-finned fishes:

Structural and Functional adaptations:

1) Locomotion in water - driven by powerful trunk and tail muscle that flex and extend in a "zigzag" pattern.

2) Buoyancy and the swim bladder:



3)Fish Respiration:

A)Gills - respiratory organs which are composed of their filaments covered with a thin epidermal membrane that is folded repeatedly into platelike gill lamellae - tissue richly supplied with blood vessels for gas exchange. The gills are located inside the pharyngeal cavity and are covered by the operculum. Flap of tissue which covers the gill - protects the gills.

B)Lungs - allow some fish to breathe air, but are not developed enough to provide 100% of the fish's respiratory needs. Most of the oxygen that fish breathe is provided by the dissolved oxygen in the water and therefore must be respired by gills.

4)Osmotic regulation: Fresh water is very dilute with respect to [salt] (salinity). The fresh water salinity is much lower than that of the blood of freshwater fishes.
Water: tends to enter the cells osmotically and salt is lost due to diffusion. Most of the water intake and salt loss occurs across the gills.

Classification Osteichthyes

Subclass Actinopterygii, ray-finned fishes

Subclass Sarcopterygii, fleshy-finned (lobe-finned) fishes, e.g. coelacanth