What makes a jawless fish unique?
Fish without jaws were the first vertebrates. Just like the molluscs and arthropods, the early fish had a hard outer covering. This armor plating around their front ends was made of bone. Perhaps it was a defense against sea scorpions. They first appeared around 500 million years ago, during the Ordovician period as small fish-like animals called ostracoderms. They had probably ate by sucking in mud through their mouths. They filtered out particles of food as the water left through their gills. Jaws appeared later, and so did paired fins which were used for swimming. Some had two fins, some four or more. Many of them had flattened bodies, and were probably not very agile. Early fish contained a swim bladder — a bag of air to help them float. The strong bone casing around the head allowed the brain, eyes and other senses to develop. Some jawless fish still survive today. They are the lampreys and hagfish. Sources:
“Agnatha (Greek, “no jaws”) is a class or superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. There is a light sensitive pineal eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not have an identifiable stomach or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are ectothermic, with a cartilaginous skeleton, and the heart contains 2 chambers.” Read more about them here: http://en.wikipedia.org/wiki/Agnatha Sources: http://en.wikipedia.
Agnatha (Greek, “no jaws”) or jawless fish is a class or super class of jawless fish in the phylum Chordata, subphylum Vertebrata. Many recent textbooks regard the group as paraphyletic but recent molecular data, both from rRNA and from mtDNA strongly supports living agnathan’ s as monophyletic. It has existed since the Cambrian, and continues to live now. There are two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish, with about 100 species in total. Although they are in the subphylum Vertebrata, hagfish technically do not have vertebrae; they are sometimes classified in Craniata. . There is a light sensitive pineal eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not have an identifiable stomach or any appendages. Fertilization and development are both external. Sources: http://en.wikipedia.
