Habitat/Lifestyle

Most live in marine habitats all their lives, but a few species of sharks and rays live in freshwater during all or part of their lives. They are found in nearly all aquatic ecosystems and depths, except the most extreme conditions. Cartilaginous fishes are carnivorous and most species feed on live prey. There are some that feed on the remains of dead animals and still others that are filter feeders.

Feeding/Digestion

They are scavengers and carnivores, while some large species are filter feeders. There is typically a pharynx, esophagus, stomach, and intestine emptying out through cloaca. A pancreas and liver secrete digestive juices and other materials into the intestine. The intestine has a spiral valve to slow the passage of food and allow it extra time to digest.

Excretion/ Osmoregulation

Sharks excrete through the same place where they have sexual intercourse through. They do not have separate opening from excreting and reproduction. The urogenital system is made up of the urinary and reproductive systems. Both the male and female urinary system work similarly. The blood of the shark usually has unequal concentration of solutes within their body as there are in the ocean which they live. So, they maintain osmotic balance with the seawater by holding onto urea.

Life History/Reprodution

They give birth to only a few young after a long gestation period, which grow slowly and reach sexual maturity late. Although their rate of survival is usually high, if populations become depleted rapidly, recovery can be difficult and lengthy.

There are three types of reproduction:

• oviparity - the female lays eggs that develop over a period of a few months
• oviviparity - the eggs are hatched inside the oviduct of the female and develop in the uterus until fully grown
• viviparity - embryo is nourished inside of the female by the placenta and the gestation period lasts a year before the newborn is released

Gas Exchange/Respiration

Chondrichthyes possess 5-7 pairs of gill slits. The O₂ diffuses into the gills as water passes over them on their way out of the organism's body. 
They bring water into their bodies using one of two methods:

• Ram ventilation - requires forward movement by the organism. They collect water in the mouths which is then passed through the gills. As the water passes over the gills and out of the body, oxygen diffuses into the gills allowing the organism to breath. This method is typically used by pelagic members (ones that feed in open water) of this class.
• Spiracle pumps - demersal species of this class rely mostly on the use of spiracles, tiny holes behind the eyes, to pump water into their bodies. At this point, the water follows a similar path, moving across the gills to allow it to diffuse into them as the water leaves the body.

Support/Skeleton

The skeleton of these animals are cartilaginous. The notochord, which is present in the young, is then replaced by cartilage later. Many possess two dorsal fins, one anterior (just behind the head) and another posterior (in front of the tail).

Transport/Circulation

They have a closed circulatory system with a two chambered heart. Blood flows into the heart sand then out to the gills. In the gills, blood picks up oxygen from the water and circulates throughout the body.

Sensory/Nervous System

The eye is the most complex organ of the head and is a lateral out pocketing of the anterior end of the brain tube. It later acquires a lens of epidermal origin. They have a later line system that enables them to sense objects and changes in their environment.
​A line of fluid-filled canals running along sides of a fish that enable it to detect movement and vibration in the water.
Chondrichthyes can sense electric currents in the water using Ampullae of Lorenzini. 
Olfactory pits - blind sacs opening to the external environment that contain olfactory receptors. The size varies with fish.

Distinguishing Characteristics

• They exhibit
  
  • internal fertilization by line use of pelvic claspers
    
  • bilateral symmetry
• Have
  
  • heterocercal tails
  • placoid scales
    
  • cartilage skeletons
    
  • teeth unattached to the jaws
    
  • the absence of swim bladders

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Sharks, Rays, Skates, Sawfish

• no swim bladder, 5-7 pairs of gills, rigid dorsal fins, small placoid scales on skin

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Chimeras

• long, thin tail, spine in front of dorsal fin, no stomach, small mouth