Diagram of Cephalochordate

Cephalochordata is a subphylum under the chordates. It includes the following five synapomorphies –

• Pharyngeal gill-slits: It is a series of openings that connect the inside of the throat to the outside of the neck. These slits are often used as gills.
• Endostyle: It is an organ found in the pharynx and helps in the filter-feeding process.
• Notochord: It is a cartilaginous rod-like structure that runs underneath and supports the nerve cord.
• Dorsal tubular nerve cord: It is a bundle of nerve fibres that runs down and connects the brain with the lateral muscles and the other organs.
• Post anal tail: It is an extension of the body post the anal openings.

Cephalochordates are segmented animals with elongated bodies that have a primitive fish-like appearance. These are typical chordates that have chordate characteristics in the larval and adult stages. Example – Amphioxus (Lancelets).

Classification

Labelled Diagram of Cephalochordate

Amphioxus – Cephalochordate

The cephalochordata is a subphylum with only two recognized genera – the Branchiostoma and the Asymmetron. The Branchiostoma (previously known as Amphioxus) has around 23 species, and the Asymmetron has around 6 species. Usually, the common name amphioxus or lancelets is used for all cephalochordates.

Amphioxus or Lancelets are fish-like filter-feeding chordates that belong to the order Amphioxiformes. They are common in shallow and sandy habitats. Also, they are mostly found in warmer coastal waters than temperate waters. They are eaten as food by animals as well as humans in some parts of the world.

Chordate Features

• They have a hollow dorsal tubular nerve cord.
• Their long notochord extends from the anterior to the posterior end on the dorsal side. It extends up to the cephalic region and hence the name cephalochordates.
• Gill slits are seen in the pharynx region. There are around 100 pharyngeal gill slits that are used to strain food particles out of the water.
• Post anal tail is present.
• They also have liver diverticulum.
• Development of the hepatic portal system is seen.
• The body musculature is distinguished into V-shaped blocks called the myotomes. The presence of myotomes or muscle segments that are useful in locomotion.
• Presence of fins – caudal, dorsal and ventral.

Primitive Features

• Their excretory system comprises the protonephridia.
• The presence of solenocytes is not reported in the chordates. But, the cephalochordate (Amphioxus) has solenocytes associated with the nephridium.
• Absence of kidney, heart, paired limbs and a distinct head.
• They also do not have distinct sense organs and a true brain.
• Their gonads are without gonoducts.

Special Features

• They have a ciliary mode of feeding, and hence the pharynx is intensified with many gill slits. The oral hood is also well-developed with buccal cirri. These are slender projections around the opening of the mouth.
• The gill slits are encircled by body wall folds (metapleural folds) to establish a body cavity called the atrium. This atrium is also well-developed because of the ciliary mode of feeding.
• The food particles are trapped in the mucus, and the water moves through the gill slits and out to the atrium via an opening called the atriopore.

The sexes are separate in cephalochordates, and fertilisation is external. They have 26 gonads present from the 25th to the 51st myotomal segments. Asexual reproduction never happens.

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