Both Acrania and Craniata belong to the phylum Chordata. Acrania are primitive chordates that lack a cranium and belong to the phylum Chordata. Craniata are higher chordates that possess a cranium and belong to the phylum Chordata. Let’s talk in details about the two.
What is Acrania?
Acrania is refer to a group of primitive, jawless fish that lack a cranium or skull. They are considered to be the most primitive vertebrates. They are characterized by their lack of a protective bony covering for the brain. Instead of a true skull, they possess a cartilaginous structure.
Acrania includes organisms like hagfish, lancelets, tunicates and lampreys. These animals have elongated, eel-like bodies and are commonly found in marine environments. They have relatively simple sensory organs and lack jaws. However, they do possess a notochord, which is a flexible rod-like structure that runs along the length of their bodies and provides support.
What is Craniata?
Craniata is a subphylum of the phylum Chordata, which includes animals that possess a cranium, or a bony, cartilaginous, or fibrous structure that surrounds the brain, jaw, and facial bones. This subphylum includes vertebrates and some invertebrate chordates, such as hagfish and lampreys.
In other words, craniata includes all animals having a notochord at some stage of their development. Craniata is defined by the presence of a cranium, or skull, which encloses and protects the brain. Craniata have pharyngeal slits, which are openings in the pharynx that allow water to pass through and are used for feeding and respiration in many species.
The Craniata group includes all vertebrates, which are animals with a backbone or spinal column. They have adaptations and lifestyles which allow them to inhabit different environments around the world, from marine and freshwater habitats to terrestrial ecosystems.
Examples of Craniata include:
- Vertebrates, such as fish, amphibians, reptiles, birds, and mammals, which possess a backbone and a cranium.
- Hagfish, which lack a backbone but have a cranium and are considered the most primitive living Craniata.
- Lampreys, which have a cranium and a notochord but lack a backbone.
Acrania vs Craniata: Key Differences
| Basis | Acrania | Craniata |
| Skull | No skull or incomplete skull. | Well-developed skull. |
| Brain | Lack of a protective bony covering for the brain. Limited brain development. | Brain protected by a bony or cartilaginous skull. More complex brain development. |
| Jaw Structure | Lack jaws. | Possess jaws. |
| Vertebral Column | Lack a true vertebral column. | Possess a vertebral column. |
| Sensory Organs | Relatively simple sensory organs. | Well-developed sensory organs, including eyes, ears, and olfactory organs. |
| Evolutionary Stage | Considered more primitive in evolution. | Higher on the evolutionary scale. |
| Examples | Hagfish, lampreys, Lancelets. | Fish, amphibians, reptiles, birds, mammals. |
| Circulatory System | Possess a single-chambered heart. | Possess more complex circulatory systems, including multi-chambered hearts. |
| Habitat | Found be in marine environments. | Can inhabit habitats like marine, freshwater, and terrestrial environments. |
| Reproduction | Lay eggs without a larval stage. | Reproductive strategies, include internal fertilization and larval stages. |
| Respiratory System | Use gills for respiration. | Use respiratory structures like gills, lungs, or a combination of both. |
| Body Form | Elongated and eel-like. | Exhibits different body forms. |