|
Fishes are aquatic vertebrates that have fins and gills throughout their
adult lives. They are a very heterogeneous assemblage of 4 distinct classes:
(1) jawless fishes (Agnatha); (2) extinct, armored Placodermi;
(3) shark-like or cartilaginous fishes (Chondrichthyes); and
(4) bony fishes (Osteichthyes). Amphibians are land vertebrates
(tetrapods) with aquatic, gill-breathing larvae and lung-breathing
adults that lay their eggs in water.
General characteristics of fishes:
Class Placodermi: An extinct group in which jaws first evolved. Paired fins also evolved in this group and are retained in all further vertebrate classes. Many placoderms were predators from 6 inches up to 50 feet long. Class Chondrichthyes: Cartilaginous fishes, including sharks, skates, and rays. Bone is reduced to a series of tooth-like denticles embedded in the skin. The rest of the skeleton is made of cartilage only. Class Osteichthyes: Bony fishes, including the vast majority of fishes. Scales and internal skeleton are both usually bony. A wide variety of sizes, shapes, and habits occurs in this group. One great subgroup has fins with ray-like supports but no internal muscles; a much smaller subgroup has fleshy, lobe-like fins with internal muscles.
Origin of land vertebrates (tetrapods): The first tetrapods (amphibians) evolved from a group of bony fishes called Crossopterygians, who already had lungs and internal nostrils. The critical change transformed the fleshy fins into walking legs. Class Amphibia: Eggs are laid in contact with fresh water, then fertilized externally. Larvae ("tadpoles") breathe with gills, then undergo metamorphosis into an adult, usually with lungs and legs. Living species always have slippery, moist skin. Examples: salamanders, newts, frogs, toads, and extinct labyrinthodonts. |
| Prev | rev. Nov. 2019 | Next |