Protochordata: Origin of Chordates | Zoology Optional Notes for UPSC PDF Download

History and Taxonomy of Branchiostoma

• In 1974, the German zoologist P. S. Pallas attempted to classify these animals, naming it "Limax lanceolatus" and describing it as a slug, but his work was based on ill-preserved specimens and this name was subsequently rejected.
• In 1836, W. Yarrell recognized the uniqueness of these animals and named them "Amphioxus lanceolatus."
• In 1834, O. G. Costa christened them as "Branchiostoma."
• By the rules of taxonomic priority, the name "Branchiostoma" is accepted as the generic name, while "Amphioxus" is used as the common name.

Geographical Distribution of Branchiostoma

• Branchiostoma has a cosmopolitan distribution, found on sandy shores in tidal areas to considerable depths.
• It inhabits tropical and temperate seas.
• The common lancelet, Branchiostoma lanceolatus, has been recorded from the west and southern European coasts, East African coasts, and western and southeastern Indian coasts.

Habit and Habitat of Branchiostoma

• Branchiostoma lives in both marine and estuarine habitats.
• It is commonly found in sandy shores.
• It resides in dilute sea water within the salinity range of 15.4% to 33.1%.
• Branchiostoma can swim actively in water and burrows in the sand but remains near the surface, maintaining a constant water current.

Feeding and Swimming

• Branchiostoma can swim vertically in water without using its fins.
• While swimming, the fins do not play a role in propulsion.
• It feeds on microorganisms brought into its pharyngeal cavity by the respiratory water current, utilizing a ciliary mode of feeding.

External Structure of Branchiostoma

• Branchiostoma is a small, lancelet-shaped creature with pointed anterior and posterior ends.
• Its body measures 5 to 8 cm in length and is elongated and flattened from side to side.
• The anterior end projects forward as a snout or rostrum.
• Below the snout is the oral hood with over twenty stiff buccal or oral cirri.
• The mouth is hidden within the oral hood, and the anus is situated on the left side of the ventral fin.
• The atrium opens through a round atriopore located close to the anterior end of the ventral fin.
• The body has gill slits, usually not visible from outside.
• It features dorsal and ventral fins supported by connective tissues called fin-ray boxes.
• Two longitudinal meta-pleural folds extend along the ventro-lateral sides of the anterior two-thirds of the body.
• These folds help during burrowing in the sand.

Body Wall of Branchiostoma

• The epidermis consists of a single layer of columnar cells.
• Epidermal cells contain sensory hair (receptor cells) but lack gland and pigment cells.
• Beneath the epidermis are the cutis and sub-cutis layers, the latter having a gelatinous matrix containing fibers.
• Canals traverse through both cutis and sub-cutis layers.
• The muscle layer, composed of myotomes, is attached to the gut and undergoes thickening near the notochord to form myotomes.
• About 60 pairs of myotomes run through the entire length of the body, separated by dense connective tissue septa called myocommas.
• The myotomes allow the body to twist sideways during locomotion.

Supporting Structure of Branchiostoma

• The notochord extends from the head to the tail end and is situated above the alimentary canal.
• Other supporting structures include gill rods supporting the gill bars, the skeletal oral ring supporting the oral hood and cirri, and the fin-ray boxes supporting the fins.
• There are two pairs of cardinal veins collecting blood from the ventrolateral sides of the body wall.

Excretory Organs of Branchiostoma

Nephridia

• Branchiostoma has about 90 to 100 paired segmental nephridia, called protonephridia, situated on the dorsolateral wall of the pharynx.
• Each nephridium is a bent vesicular sac with horizontal and vertical limbs, corresponding to each primary gill-bar.
• Elongated tubular flame cells or solenocytes open into the vesicle and have a long tubular stalk with a balloon-like cell body at the terminal end.
• The solenocytes are associated with nephric glomerular sinus.
• Excretion occurs through the solenocytes via diffusion, with waste products passing into the vesicle through the tubular part.

Nephridium of Hatschek

• A tube called the nephridium of Hatschek is also considered to have an excretory function, extending from the mouth to the right side of the notochord.

Brown Funnels and Atrial Wall

• A pair of brown funnels in the atrium is thought to play an excretory role, although they may serve as receptor organs.
• Groups of cells in the atrial wall are involved in excretion.

Gonads

• The gonads, particularly in the testes, contain yellow masses with uric acid, expelled with the gametes.

Nervous System of Branchiostoma

• Branchiostoma has a hollow dorsal nerve cord located just above the notochord.
• The anterior part forms the brain, while the posterior part remains as the spinal cord.
• Paired nerves emerge from the dorsal and ventral nerve roots in each segment of the body.
• The nervous system includes a ventricle at the anterior end and the infundibular organ, which contains tall cells with cilia and produces Reissner's fiber.
• Photo-sensory cells are present on the spinal cord.
• The spinal cord is characterized by a narrow central lumen and the orientation of grey matter and white matter is similar to that of vertebrates.
• There are sensory cells on the surface of the body, especially on the dorsal side.

Receptor Organs in Branchiostoma

• Branchiostoma features various receptor organs, including pigment spots on the anterior wall of the brain, eye-spots on the spinal cord, Kolliker's pit, sensory papillae on the oral cirri and velar tentacles, infundibular organs, and epidermal sensory cells on the body's surface.

Reproductive System of Branchiostoma

• Branchiostoma exhibits separate sexes with simple pouch-like gonads located in the ventro-lateral sides of the pharyngeal region between the 10th and 30th segments.
• Gametes are discharged into the atrium and released into the water through the atriopore for fertilization and development.

Development of Branchiostoma

Primitive, Degenerated, and Specialized Features

• Branchiostoma displays a combination of primitive, degenerative, and specialized features.
• Primitive characteristics include a persistent notochord, single-layered epidermis, myotomic segmentation, and ciliary mode of feeding.
• Degenerative features include a sedentary lifestyle, reduced brain and sense organs, and retrograde development in adults.
• Specialized features include a spacious pharynx, a large number of gill-slits, and specialized feeding structures.

Affinities and Systematic Position of Branchiostoma

• Branchiostoma is classified within the phylum Chordata due to its possession of key chordate features, including the notochord, dorsal tubular nerve cord, and gill-slits.
• Its precise position within the phylum is still uncertain, and it has been proposed to be related to various groups, including annelids, mollusks, echinoderms, vertebrates, hemichordates, and urochordates.
• Its close relationship with urochordates and similarities in larval development suggest a connection between Branchiostoma and urochordates.
• While comparisons have been made with various animal groups, the exact systematic position of Branchiostoma remains a subject of debate in evolutionary biology.