Notes on Branchiostoma (Amphioxus)-lancelet

Branchiostoma, formerly called Amphioxus, is popularly known as lancelet (from lancet). It was first discovered and described by a German taxonomist, Pallas (1778). The name Amphioxus, which is still used widely, was proposed by Yarrel in 1836.

Branchiostoma is a marine animal commonly inhabiting shallow offshore sands in temperate oceans. Mostly, it remains buried in sand in an upright condition with only the anterior region protruding above the sand surface. However, at night they leave the sand and swim actively by lateral undulations of the body caused by contractions of muscles.

Branchiostoma is a typical ciliary feeder and feeds on planktonic microorganisms present in the seawater. Branchiostoma is a small fish-like animal, about 5 to 7 cm long. Its body is slender, laterally compressed and pointed at both ends,hence the common name ‘lancelet’, which means ‘little lance’. The posterior end is more tapering than the anterior end. The streamlined body is well adapted for burrowing as well as swimming. The body of the animal is somewhat translucent and displays a faint flesh colour.

The body of Branchiostoma is divisible into two regions, namely agreater anterior trunk and a short posterior tail. A distinct head is absent and the anterior end of the trunk projects in front as a pointed snout or rostrum.

The trunk bears three openings, namely, mouth, atriopore and anus. A tentaculated structure called oral hood is present at the anterior end of the trunk below the rostrum . lt is part of the digestive system. The oral hood is a frill-like membranous structure located antero-ventrally below the snout. It is formed by the outward extension of the integument. The oral hood opens out by a wide, oval aperture, the mouth. The free margins of the oral hood surrounding the mouth is beset with 10 to 11 pairs of slender, flexible, ciliated, tentacle~like processes bearing sensory papillae called oral cirri or buccal cirri or tentacles.  The oral hood encloses a spacious funnel-shaped cavity called buccal cavity or vestibule into which the mouth opens. Since this cavity is lined with ectoderm, it is regarded as stomodaeum and its external opening as the true mouth. The ectodermal lining of the vestibule is folded to form a complex system of 6 to 8 pairs of ciliated and grooved ridges or patches, collectively referred to as wheel organ or Muller's organ . The coordinated movement of the cilia of the wheel organ set up whirling water current that sweeps microscopic food organisms into mouth. The mid-dorsal tract of the wheel organ is the largest and bears a ciliated glandular groove called Hatschek's groove, which terminates in a small depression, the Hatschek's pit. Both the groove and the pit secrete mucus that helps in trapping food particles. Atrium is a large and spacious cavity surrounding the pharynx. Itopens out by a small rounded atriopore located mid-ventrally just in front of the ventral fin. The third opening, anus is a small, circular aperture lying at the base of the caudal fin slightly to the left of the mid-ventral line. Paired fins are absent in Branchiostoma. But three median or unpaired fins -dorsal fin, caudal fin and ventral fin - can be recognised. Dorsal fin is a membranous fold extending along the whole length of the trunk mid-dorsally. It is supported by about 250 to 300 rectangular fin ray boxes formed of stiff connective tissue arranged in a single row. The dorsal fin continues behind and turns around the tail as caudal fin. It then runs forward along the mid ventral line of the posterior trunk region as them ventral fin and terminates near the atriopore. Ventral fin has two rows of fin ray boxes where as the caudal fin lacks fin rays.

The flat and longitudinally ridged ventral surface ofthe trunk is called epipleur. lt forms the floor of the atrium. The lateral edges of the epipleur are produced into two thin, hollow folds known as metapleural folds or atrial folds. The metapleural folds are continuous in front with the oral hood. They run posteriorly and meet just around the atrium. Branchiostoma has no exoskeleton. The endoskeleton consists of the notochord, gelatinous rods and plates, fin ray boxes and tough fibrous connective tissue. The notochord extends throughout the length of the body and lies mid-dorsally beneath the nerve cord and above the gut. The body cavity of Branchiostoma is a true coelom lined with mesodermal epithelium. lt is enterocoelic in origin (derived from archenteron) and filled with coelomic fluid. Coelom is spacious in embryos. But, in adults it becomes greatly reduced owing to the development of an ectoderm—lined cavity known as atrium. Atrium is spacious, ectoderm—lined cavity surrounding the pharynx and intestine laterally as well as ventrally.it is closed in front and opens outside through atriopore located anterior to the anus.

Branchiostoma is a microphagus animal feeding on diatoms,protozoans and other microscopic organisms and organic particles suspended in seawater. It is a ciliary or filter feeder. The excretory organs are segmentally arranged protonephridia instead of nephrons.

The nervous system of Branchiostoma is very simple but it resembles that of higher vertebrates in general plan.

The sexes are separate (gonochoric) in Branchiostoma but there is no sexual dimorphism as male and female individuals look alike. Fertilization is external and the development is indirect involving larval stage.

Evolutionary Significance of Branchiostoma

Though a primitive chordate, the general organisation of Branchiostoma reveals a curious mixture of primitive, degenerate and specialised features. Some of which are the following.

Primitive Features

Branchiostoma is regarded as a primitive chordate because it possesses several primitive chordate characters in an unmodified form. It also exhibits several non chordate features as relics from its ancestors. Some of the notable primitive features are persistent notochord, absence of a distinct and specialized head , absence of paired fins or limbs, enterocoelous