Difference Between Chordates and Protochordates

The main difference between chordates and protochordates is that the chordates are the animals that have distinct characteristics such as a notochord, dorsal nerve chord, pharyngeal slits, and a muscular tail whereas the protochordates are an informal group of invertebrates within chordates. Furthermore, the three subphyla of chordates are Vertebrata, Urochordata, and Cephalochordata while protochordates consist of both Urochordata and Cephalochordata.

Chordates and protochordates represent animals with a higher organization within the kingdom Animalia.

Key Areas Covered

1. What are Chordates
     – Definition, Characteristics, Classification
2. What are Protochordates
     – Definition, Characteristics, Classification
3. What are the Similarities Between Chordates and Protochordates
     – Outline of Common Features
4. What is the Difference Between Chordates and Protochordates
     – Comparison of Key Differences

Key Terms

Chordates, Lancelets, Protochordates, Tunicates, Vertebral Column, Vertebrates

What are Chordates

Chordates are animals that belong to the phylum Chordata with the highest organization in the animal kingdom. The four main characteristic features of the chordates are

1. Notochord – A rod-shaped, flexible structure present at the embryonic stage of chordates. It provides structural support.
2. Dorsal, hollow nerve cord – A bundle of nerves, which runs dorsally to the notochord. Non-chordates may have a dorsal nerve cord that is solid.
3. Pharyngeal slits – The openings in the pharynx, which allow the removal of water that enters the mouth.
4. Post-anal tail – A posterior elongation made up of bones and muscles, which helps with balance or locomotion.
   

   Figure 1: Chordate Anatomy
   1. brain like blister, 2. notochord, 3. dorsal nerve cord, 4. post-anal tail, 5. anus, 6. food canal, 7. blood system, 8. abdominal porus, 9. overpharynx lacuna, 10. gill’s slit, 11. pharynx, 12. mouth lacuna, 13. mimosa, 14. mouth gap, 15. gonads (ovary/testicle), 16. light sensor, 17. nerves, 18. abdominal ply, 19. liver like sack

Chordates can be subdivided into two groups as vertebrates and invertebrates chordates or protochordates.

Vertebrates

Vertebrates evolved from craniates that evolved from protochordates. The clade Craniata possesses a cranium, a bony structure that surrounds the brain. The two members of craniates are vertebrates and hagfish. Though all vertebrates possess the characteristics of a chordate, the notochord of adult vertebrates is replaced by the vertebral column or the spine. The vertebral column comprises a series of vertebrae, each separated by mobile disks.

Figure 2: Skeleton of a Blue Whale

Moreover, vertebrates are the only chordates with a brain as a part of a central nervous system. Vertebrates include mammals, birds, reptiles, amphibians, bony fish, cartilaginous fish, and jawless fish.

What are Protochordates

Protochordates represent the invertebrate chordates that belong to the two subphyla of chordates called Urochordata (tunicates) and Cephalochordata (lancelets). Cephalochordates evolved from urochordates, who in turn,  developed into vertebrates. Though protochordates have all the main characteristics of a typical chordate, they never develop a bony notochord like vertebrates.

Urochordates

Urochordates have a tunic made up of a cellulose-like carbohydrate material, covering the outer body. Only the larva form of urochordates have all the four characteristics of a typical chordate. Adult urochordates only have pharyngeal slits. Most urochordates are sessile and they are filter-feeders. 

Figure 3: Tunicates

Cephalochordates

Cephalochordates have all the four characteristics of a typical chordate in their adult stage. Their notochord extends through the whole body and runs into the head. But, they do not have a ‘true’ brain. Cephalochordates do not have an exoskeleton like urochordates and their body is covered with an epidermis and dermis.

Similarities Between Chordates and Protochordates

• Both chordates and protochordates have a notochord, dorsal, hollow nerve cord, pharyngeal slits, and a post-anal tail during any of their life stages.
• Both include urochordates and cephalochordates.
• They are animals with the highest organization on earth.

Difference Between Chordates and Protochordates

Definition

Chordates refer to any animal having, at least at some stage of development, a notochord, dorsally situated central nervous system, gill slits, and a post-anal tail while protochordates refer to a major division of Chordata usually comprising Urochordata and Cephalochordata.

Subphyla

The three subphyla of Chordates are Vertebrata, Urochordata, and Cephalochordata while Urochordata and Cephalochordata collectively are the protochordates.

Vertebral Column

Only chordates with a vertebral column are vertebrates while protochordates lack a vertebral column. This is one main main difference between chordates and protochordates.

Brain

Vertebrates are the only chordates with a brain while protochordates lack a brain.

Cranium

Vertebrates have a cranium while protochordates lack a cranium.

Endostyle

On the other hand, vertebrates lack an endostyle while protochordates have an endostyle, which aids in filter-feeding.

Chambered and Muscular Heart

Also, vertebrates have a chambered and muscular heart while protochordates have a heart with no chambers.

Red Blood Cells

Furthermore, vertebrates have blood cells in their blood while protochordates do not have blood corpuscles.

Kidneys

Moreover, vertebrates have kidneys while protochordates do not have kidneys.

Conclusion

Chordates are the animals with a notochord, dorsal, hollow, nerve cord, pharyngeal gill slits, and a post-anal tail. They include vertebrates, cephalochordates, and urochordates. Protochordates are an informal group made up of cephalochordates and urochordates. They do not have a vertebral column like vertebrates. Thus, the main difference between chordates and protochordates is the presence of a vertebral column.

Reference:

1. “Chordates.” Boundless Biology | Lumen, Open SUNY Textbooks, Available Here

Image Courtesy:

1. “BranchiostomaLanceolatum PioM” By Piotr Michał Jaworski; PioM EN DE PL – Own work (CC BY-SA 3.0) via Commons Wikimedia       
2. “BlueWhaleSkeleton” By Bronwen Lea – Cropped and reduced from a photograph taken by Bronwen Lea, 14th May 2004.Uploaded initially to WP:EN by Stephen Lea at en:Image:BlueWhaleSkeleton.jpg 07:22, 20 May 2004 (CC BY-SA 3.0) via Commons Wikimedia     
3. “Tunicate medley komodo” By Nhobgood Nick Hobgood – Own work (CC BY-SA 3.0) via Commons Wikimedia