Phylum : Chordata
Sub Phylum : Cephalochordata
Type : Amphioxus
This group comprises an assemblage of small fish like marine organisms belonging to two genera, Branchiostoma and Asymmetron. Amphioxus, the lancelet (genus Branchiostoma) has been the object of much study because it illustrates the three fundamental chordate characters in simple form. Thus, it may be considered as a blue print of the phylum. Amphioxus has a long notochord which persists throughout life, a dorsal hollow nerve cord and many gill slits in the pharynx
Distribution :- Amphioxus is a marine organism having an almost world wide distribution. It is found on the sandy shores of tropical and warm temperate seas.
Habit and Habitat :- Amphioxus is usually found buried in the sand with the fore or anterior end projecting outside. In this portion, it draws small organisms into the mouth along with the current setup by the ciliary apparatus at the fore end of the body. Occasionally, it emerges out of the burrow and swims about by the sinuous movements of the body. As soon as it senses some disturbance it buries itself in sand again either by the head end or by the tail end.
External features :-
Amphioxus is a small translucent animal and 0.5 to 3 cm long. Thebody is long laterally compressed and pointed at both ends (hence called lancelet). It lacks a head. The body bears fins. The dorsal fin runs along the whole length of the mid dorsal line. It expands around the tail as caudal fin or tail fin and then continues ventrally as a ventral fin, extending for aboutone-third of the length of the body. The dorsal and ventral fins are supported by fin rays. The anterior two-thirds of the body is triangular in section. It is because the two lateral sides project down as flaps called metapleural folds. The posterior one-third is oval in out line. The metapleural folds are continuous anteriorly with the oral hood. Below the anterior end of the body, there is a median funnel shaped cavity called the vestibule or preoral chamber. The vestibule is surrounded by a membrane called oral hood. The oral hood is provided with 22 delicate, but stiff tentacle-like processes called oral cirri, guarding the opening. The mouth is situated at the bottom of the vestibule in the middle of a membrane called velum. The velum is fringed with 12 velar tentacles directed backwards into the pharynx. On the inner side of the oral hood, the epithelial lining is thrown into ciliated finger-shaped processes. As the movements of these processes cause a flow of water towards the mouth and give the appearance of wheel-like movement, the tract is called wheel organ. On the roof of the oral hood there is a ciliated pit called the Hatschek’s pit. The anus lies in front of the tail fin, being placed slightly towards the left side.
Atrium :- In Amphioxus, the gill slits instead of opening directly to the outside, open into a special cavity called the atrium. It is a space between the pharyngeal wall and the body wall. It is lined by ectoderm and is enclosed by the lateral extensions of the body wall. It surrounds the pharynx laterally and ventrally but not dorsally. The cavity encroaches into the bodycavity almost completely replacing the coelom. The atrium opens outside by a small aperture called the atriopore, lying in front of the anus. The atrium gives protection to the gills, preventing them from being choked up with sand.
Body wall :- The body wall is soft, thin and translucent. It is composed of outer epidermis and inner dermis. The epidermis consists of a single layer of columnar epidermal cells, bearing at places sensory hairs and the unicellular glands. The dermis consists of an outer layer of connective tissue (cutis) containing fibres and an inner thick layer of matrix (sub cutis) invaded by few fibres, blood channels and nerve endings. Beneath the dermis is the muscular layer arranged segmentally as muscle blocks, called the myotomes or myomeres. The number of myotomes may vary from 60 or more. The muscles are thickened dorsally than ventrally.
Coelom :- Coelom of amphioxus is distorted and restricted by the immense size of the atrium.
Skeleton :- The skeleton consists of a peculiar tissue called notochordal tissue and of gelatinous material. The chief supporting structure is the notochord. Other skeletal structures are connected with the oral hood, the fins and Pharynx.
Digestive system :– The mouth lies at the bottom of the vestibule. It leads into a large laterally compressed chamber, the Pharynx which occupies the most anterior part of the body cavity. The wall of the pharynx is perforated by numerous gill slits (as many as 180 or more) on each side. From the pharynx the narrow tubular midgut or intestine extends backwards to the anus. On the ventral side of the intestine is a large diverticulum, the liver diverticulum or hepatic caecum.
Running along the inner wall of the pharynx both on the dorsal and ventral middle lines are two grooves lined with cilia ; the dorsal groove is the hyperbranchial groove and ventral groove is the endo style. The dorsal groove leads into the intestine. The ventral groove is composed of four tracts of mucous glands separated by tracts of ciliated cells. The glands secrete a sticky mucous which entangle the food materials. Anteriorly the endo style and the hyperbranchial grooves are connected by two ciliated tracts, which encircle the pharynx just behind the mouth. These ciliated tracts are called peripharyngeal bands. The inner wall of the pharynx is lined with cilia.
Feeding mechanism :- Amphioxus feeds on minute organisms found suspended in the water. While feeding, a continuous current of water enters the mouth and expelled out through atriopore via pharynx and atrium. The water current is maintained by the actions of cilia in the wheel organ and the gill bars of the pharynx. The velar tentacles behind the mouth fold across and strain off the sand particles. Small organisms are brought into the endostyle and get entangled in the mucous. The cilia drive them along the gill bars and peripharyngeal bands into hyper branchial groove and thence to the intestine. Digestion takes place mainly in the midgut. Digestive enzymes are secreted in the midgut diverticulum and poured into the midgut. The digested food is also absorbed in the midgut.
Respiration :- Gaseous exchanges take place in the gill bars when the water passes through the gill slits. The oxygen diffuses into the blood flowing in the channels of gill bars and the carbon di oxide from the blood diffuses into the running water.
Blood vascular system :- There is no specialized heart. The blood is colourless. Respiratory pigments are also absent. There is a median contractile vessel (ventral aorta) in the ventral wall of the pharynx below the endostyle. It ends anteriorly by branching in the snout. The ventral aorta sends paired afferent branchial vessels into the primary gill bars. From the gill bars (Primary and secondary bars) the blood is collected by efferent-branchial vessels which open into the right and left lateral dorsal aortae one on each side of the pharynx above the hyperbranchial groove. The lateral dorsal aortae unite behind the pharynx forming a median dorsal aorta which runs behind and gives branches and breaks up into capillaries in the intestine.
The blood from the intestine and the posterior region of the body is collected by a median subintestinal vessel, which lies below the intestine. This vessel extends forward to break up into capillaries in the liver diverticulum forming hepatic portal system. From the liver the blood is collected by the hepatic vein which joins the ventral aorta. There are two cordinal veins, one on each side of the dorsal body wall, which collects blood from the muscles and body wall. These are connected with the ventral aorta by the cuvierian ducts.
Excretory system :– The excretory organs are the paired nephridia (about 90 pairs) of ectodermal origin. These are segmentally arranged on each side of the pharynx in its dorsolateral walls. Each nephridium is a closed, bent tube. Each tube has an anterior vertical and posterior horizontal limb. The vertical limb lies in the coelomic canal of the primary gill bar. It ends blindly. The horizontal limb projects into the dorsal coelom. It opens to the exterior through nephridiopore. Both limbs of the nephridia are provided with groups of flame cells or solenocytes. A nephridium bears about 500 solenocytes. Each solenocyte is a hollow cell with a flagellum hanging down from the wall. There are blood vessels passing to the nephridia from afferent branchial vessels. The nitrogenous waste matter is collected by the solenocytes from the blood as well as coelomic fluid and is passed on to the body of the nephridium by the flagella. From this it is discharged into the atrium. It finally goes through atriopore.
Nervous system :- The nervous system of amphioxus consists of a median, hollow nerve cord lying above the notochord and below the fin rays. It extends from the anterior to the posterior end of the body. It is contained in a canal called neural canal. A small longitudinal cleft, the dorsal fissure extends throughout the length of the nerve cord. The nerve cord is not differentiated into the brain and spinal cord.
Reproductive system :- In amphioxus the two sexes are separate, but there is no sexual dimorphism. There are about 26 pairs of gonads arranged segmentally. These are simple hollow sacs and are mesodermal in origin. There are no gonoducts. The gonads bulge conspicuously into the atrial cavity. When the germ cells ripe, the sacs burst and the genital elements are set free into the atrial cavity from where they pass outside by atriopore. Fertilization and development take place in sea water
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