Difference between Acrania and Craniata

Acrania and Craniata are two groups under the phylum Chordata that represent lower chordata and comparatively higher chordata, respectively. This article focuses on the salient features and differences between the two groups.

What is Acrania?

Acrania, better known as Cephalochordate, possesses no fetal skull (cranium), hence the name Acrania. They are also commonly known as lancelets. It comprises any of more than two dozen species in the subphylum Cephalochordata.

They are exclusively marine small-sized chordates. The whole body is segmented; the notochord lengthens from near the terminal of tail to the anterior section of the body. They have excretory system, sexual glands but no heart. They have a notochord and gill slits are persistent.

They exist as separate sexes and the eggs and sperm are shed directly in water where fertilisation occurs. They can be seen inhabiting temperate and tropical regions in soft bottoms, such as sand or gravels in shallow coastal waters. The segmentation in their bodies helps to coordinate their locomotion. They also possess a dorsal fin.

The notochord extends through the entire length of the body and provides support. The lancelets feed on the small particles suspended in the water. The blood circulation takes place by vessels as in vertebrates but there are no corpuscles in the blood.

The cephalochordates share a number of features with the vertebrates. This concludes that they have definitely evolved from a common ancestor and are sister groups. The taxonomic classification is as follows:

Kingdom: Animalia

Phylum: Chordata

Subphylum: Cephalochordata/Acrania

What is Craniata?

Craniata (vertebrates), belong to the subphylum Vertebrata of the phylum Chordata. Craniata includes all the major classes such as Agnatha, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and Mammalia. They are referred to as Craniata because unlike acrania, these organisms do possess a skull.

The group includes organisms that are either aquatic or terrestrial, have a body that is divided into head, neck, trunk and tail, with a vertebral column and a brain protected by cranium. The aquatic vertebrates have persistent pharyngeal gill slits that help in respiration. Generally, they have a distinct heart that helps in blood circulation.

The vertebrates mainly feed on plants, invertebrates and other vertebrates. The classification is as follows:

Kingdom: Animalia

Phylum: Chordata

Subphylum: Craniata/Vertebrata

Acrania vs. Craniata

Acrania

Craniata

Description

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.

Alternative Name

Cephalochordates

Vertebrates

Vertebral Column

They possess a notochord.

They possess a vertebral column.

Body Structure

The whole body is segmented, head section is not separate.

They have a well developed body with separate head, neck, trunk and tail.

Presence of Cranium, Skull, Brain and Jaw

Absent

Present

Presence of Heart

They do not have a heart and the blood does not contain corpuscles.

They have chambered hearts and the blood consists of corpuscles.

Feeding Habits

They feed on suspended particles in the water.

They feed on plants, invertebrates and other vertebrates.

Development

Usually, their development is indirect, via a larval stage.

The development can be direct or indirect.

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Frequently Asked Questions on Difference between Acrania and Craniata

Is Craniata the same as Vertebrata?

Yes, the Craniata group is interchangeably known as Vertebrata.

Do Cephalochordates have a skeleton?

No, cephalochordates do not have a mineralised skeleton.

Are cephalochordates hermaphrodite?

No, a majority of cephalochordates exist as separate sexes.

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