Protochords are invertebrate chordate animals. The group has few representatives and all are small marine animals.
Protochords represent the most primitive chords. The term protochord comes from the Greek, protos "first, primitive".
Examples of protochordates are sea squirts, salt pans and amphioxus.
The chordate group includes protochordates and all vertebrate animals. During the embryonic stage, all chordates have a dorsal nerve tube, notochord, pharyngeal clefts, and a post-anal tail.
General features
The main characteristic of protochords is the absence of skull and spine, that is, they are invertebrates.
Protochords do not have a differentiated head. Some, like amphioxus, have a body shape reminiscent of fish.
The protochords usually live on the coast, buried in the sand or attached to rocks and algae. They can be found free-living or forming colonies.
The protochords perform sexual reproduction with external fertilization. In some cases, as in ascidians, they may present asexual reproduction by budding.
Urochordates and Cephalochords
The protochords are divided into two sub-phyla: Urochordata and Cephalocordata. See the main characteristics of these groups:
Urochordata (Urochords or Tunicates)
- Represented by sea squirts and salt marshes. As adults, they look little like the chordates;
- They present notochord in the caudal region during the larval stage;
- They are sessile marine animals that can live in isolation or in colonies;
- They are covered with a protective tunic, formed from tunicin. Hence the name of the group;
- They are fed by filtration;
- Partially open circulatory system;
- The nervous system differs during the stages of life. When larvae, it consists of a nerve tube. In adults, it reduces to a nerve gland under the pharynx.
Cephalocordata (Cephalochordates)
- Represented by the amphioxus, a small transparent animal with a spear-shaped body;
- They have a lifelong notochord, which serves as a flexible skeleton;
- The mouth is surrounded by filaments, called buccal cirrus;
- They are fed by filtration;
- Closed circulatory system;
- The nervous system consists of a dorsal nerve tube;
- They are dioecious animals.
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