Sharks, skates, rays, and chimaeras are all members of the Class Chondrichthyes. Their endoskeleton is entirely cartilaginous (Chondros = "cartilage"; "icthys" = "fish") and all are carnivorous (as exemplified by the great white shark (Carcharodon), the principle character in the "Jaws" movie serials).
As outlined in the previous section, fishes are divided into two broad groups based on the presence or absence of jaws. Those without jaws belong to the Superclass Agnatha. Agnathans are mostly represented as heavily armored fossil forms that reached their height during the Upper Silurian and Lower Devonian (more than 350 million years ago). Living species are smooth-skinned, completely unprotected by armor, and represented by the lampreys (Class Cephalaspidomorphi) and hagfish (Class Myxini). The agnaths are considered to be an evolutionary dead end, a side branch in the phyletic tree that did not lead to the more advanced fish.
Most of today's fishes have jaws and therefore belong to the Superclass Gnathostomata. Jaws probably developed from the anterior visceral arch that first served to support the gills. The presence of jaws and development of paired appendages resulted in efficient eating and locomotion that made the fish the evolutionary success they are today. With most of the earth's surface covered by water, fish have had the opportunity to colonize a variety of habitats. In the oceans they can be found from the intertidal zone to the deepest of the trenches. As to the freshwater habitats, they are found in any sizable body of water. The only habitats fish have not been able to colonize are extreme: the Dead Sea, the bottom of the Black Sea, and areas heavily polluted by humans. Given the diversity of habitats within which fish are found, you're probably not surprised that there are thousands of known species. The Superclass Gnathostomata is divided into two classes: Class Chondrichthyes (cartilaginous fishes) and Class Osteichthyes (bony fishes).
Members of the Class Chondrichthyes include the sharks, rays, skates, and sawfishes (Subclass Elasmobranchii) and the chimaeras or ratfishes (Subclass Holocephali). The skeleton is cartilaginous, and there is no bone (except for the teeth and scales, although some portions of the skeleton may have calcium deposits). The notocord persists into the adult, and they have both median and paired fins (on the pectoralpelvic girdles). They have two-chambered hearts (one auricle, one ventricle) with a sinus venosus feeding blood into the auricle and a conus arteriosus receiving blood from the ventricle. Only deoxygenated venous blood flows through the heart (which is then pumped through gills before going to the rest of the body). Red blood cells are present, but they're nucleated and oval. Respiration is by five to seven pairs of gills, each located in a cleft and supported by cartilaginous visceral arches. Sexes are separate (dioecious), and the gonads are usually paired. Gametes are discharged into a cloaca (along with wastes from the intestine and kidney). Depending on the species, they may be oviparous (eggs develop and hatch outside the mother's body) or ovoviviparous (internal fertilization with eggs developing within the female's body, but without sharing the maternal blood flow). The eggs are large with lots of yolk (since they're on their own after fertilization). The outer covering is usually thick and leathery, often with tendrils that can attach and hold on to vegetation during development.
The elasmobranchs include two orders of Paleozoic sharks, an order of extinct and modern sharks (Order Squaliformes), and an order of skates, rays, and sawfishes (Order Rajiformes). The caudal fin of sharks differs from that of bony fishes because it is asymmetrical. This heterocercal fin provides both lift and forward thrust for the animal when swimming (an important consideration since sharks are heavier than water and have no swim bladder to keep them buoyant). Squalus are common in the North Atlantic where they may grow to over a meter in length. Dogfish sharks give birth to live young (they're ovoviviparous). Although there are many rows of teeth, only the outer row or two is functional. The inner rows are replacement teeth, which move forward when outer teeth break off. Unlike ours, the upper jaw is not rigidly connected to the braincase and can be moved to open the jaws. Although most sharks are predatory or scavengers, the largest sharks (whale sharks and basking sharks) feed on planktonic organisms they filter from the sea.
The outer skin layer is the epidermis, beneath which a thick, fibrous dermis can be seen. The outer, hard portion of the scale is enameloid, one of the toughest materials produced in the animal kingdom. Beneath this, a layer of dentine can be seen surrounding a vascular pulp.
There is no biological difference between skates and rays (skates are just the smaller ray species). Rays are dorsoventrally compressed and have anterior fins attached along the sides of the head and trunk. These winglike fins undulate to provide lift and forward thrust while swimming. Most rajiforms are bottom-dwelling beasts that feed on clams and other goodies they find on the seabed. The manta rays, on the other hand, have developed a surface-dwelling, pelagic mode of life. Like the whale sharks, manta rays filter planktonic organisms from the sea with a sieve formed from gill rakers in the pharynx. Although mantas can be very large (some with a "wingspan" of nearly 6 meters and weighing nearly 3000 pounds), they are not dangerous. All rays have electric organs in their tails that usually produce weak currents useful only for communication (although electric rays can give a surprising jolt of over 200 volts). The tails of many rajiforms can be used for offense or defense. Stingrays, for example, have whiplike tails with serrated poison spines on the dorsal surface.
Ratfishes or chimaeras superficially appear to be intermediate in body plan between the sharks and the more advanced fishes (although they're actually an offshoot of the sharks). They have a few scales and an operculum(flap) covering their gill slits. Unlike sharks, the upper jaw is fused to the skull and the teeth are not replaced, but continuously grow to compensate for wear.
The shark skeleton is divided into two main portions: an axial skeleton and the appendicular skeleton (supporting the fins). The skull consists of the chondrocranium neurocranium) and splanchnocranium (visceral skeleton). The chondrocranium encloses the brain, while the visceral skeleton supports the jaws and pharynx.
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 Squalus acanthias, Lateral view of the shark chnodrocranium |
 Ventral view of the chondrocranium and the visceral skeleton of a shark |
  Dorsal (left) and ventral (right) views of the shark chondrocranium. |
 Dorsal view of the brain of a shark. |
 Caudal fin of a shark. |
