The vertebrates (from Latin vertebratus, with vertebrae) constitute a subphylum of chordate animals, comprising agates, fish, amphibians, reptiles, birds and mammals. They are characterized by the presence of spine targeted and skull that protects their brain.
Other additional features are the presence of a generally symmetrical muscular system - the bilateral symmetry is also a feature of vertebrates - and of a central nervous system, formed by the brain and spinal cord located within the central part of the skeleton (skull and spine).
The earliest traces of vertebrates were found in the Silurian period (444 to 409 million years ago).
Chord Characteristics and Classification
Every chord presents, at least at some stage of its existence:
- notochord, situated along the dorsal median axis of the animal;
- one nervous tube dorsally located above the notochord;
- cracks located bilaterally in the pharynx;
- post anal tail, primarily important for propulsion in the aquatic environment. Of it, only a trace - the coccyx, formed from a set of small vertebrae at the end of the spine - remained in humans.
In invertebrate groups, morphological characteristics have always been defined from the study of adult animals. In chordates, however, the characterization of the group should be sought in the embryonic phase. It is at this stage that the entire chordate presents the four typical characteristics of the group: notochord, dorsal nerve tube, pharynx clefts, and post-anal tail.
In adulthood of the most complex vertebrates, these structures either disappear, such as the notochord and pharynx clefts, or undergo considerable modification, such as the vastly expanding nervous tube, leading to brain differentiation. and the spinal cord.
A satisfactory chordate classification is to group them into three subphylies: Urochordata, Cephalochordata and Vertebrata (or Craniata). The urocordados and cephalocordados are also known as protocordados. The protocordates have no skull, no cartilage, no bones.
Among vertebrates, the most primitive are those with a circular mouth, not endowed with jaws. These make up the groups of mandibular vertebrates or agates (from Greek, The = no + gnathos = maxilla).
Because they have a circular mouth, they are also known as cyclostomados (from Greek, kuklos = circle + stoma = mouth). The best known specimens today are the lamprey.
In more complex vertebrates, the mouth has jaws. They are the gnatostomados, which include two groups: fish - which in turn contains the class of cartilaginous and bony fish - and tetrapods (Greek, tetra = four + pods = feet), so-called because they have even locomotor appendages (including amphibians, reptiles, birds and mammals).
Also known as tunicates, a name given to the body's wrap, a thick tunic whose chemical composition is tunicine, a substance similar to cellulose.
The best known representatives of this group are the ascidians, marine chordates that can live in isolation or forming colonies. One of the isolated forms very found in Brazilian beaches resembles, in adults, a piece of tar approximately 8 cm high, attached at one end to the substrate (rocks, ship hulls, etc.).
Notice in the picture above the two openings. The largest is the inhalant siphon, allowing the ingress of water bringing oxygen and food particles that are trapped in the crack-pierced pharynx. By ciliary beating, food is taken from the pharynx to the stomach. Water entering the animal exits through the second siphon, the exhaling siphon, carrying the excretion products. They are therefore filtering animals.
The ascidians are hermaphrodites. Fertilization is external. Gametes are carried by water through the exhaling siphon. Fertilized eggs generate small larvae. The larva, shown above, looks a lot like the frog larva (tadpole) suggesting strong kinship with vertebrates.
The larva of the ascidians is free swimming. The adults are fixed. In the tunicate larva, the notochord is restricted to the tail.