EXTERNAL CHARACTERS OF AMPHIOXUS:
Amphioxus belongs to Cephalochordate. In 1778 Amphioxus was first discovered and described by Palias . He called it a Molluscan animal. In 1834, it was described as a chordate animal by Costa. He called it ‘Branchistoma’. In 1836, Yarrell gave th name Amphioxus.
Amphioxus lives in shallow marine waters and burrows in sand. It is commonly seen in the Indian ocean. Most of its body is burned. Only its anterior part is left outside. It is active during night It is 3.5 to 6cms, in length lts body is whitish in colour. The body is laterally compressed. It is pointed at both ends. This spindle shaped body is useful for burrowing and swimming. The anterior end of the animal will project anteriorly as snout. It is called rostrum. Below this o al hood is present. The body is divisible into trunk and posterior short tail. Head is absent. Mouth is a wide opening present at the anterior end of the oral hood. It bears 10 to 11 pairs of buccal cirri. They test the purity of water. The oral hood encloses a cup like vestibule.,
The first two thirds of the body is triangular in section. The last on third body is nearly oval in section
AMPHIOXUS- FINS :
On the dorsal side dorsal fin is ‘present. It is connected with a caudal fin. Caudal fin is connected with ventral fin which extends up to atriopore.
The dorsal and ventral fins are supported by small rectangular fin- ray boxes There is a single row of Fin ray box in the dorsal fin. Two rows are present in the ventral fin.
Ventral surface of the anterior two-thirds of the trunk is nearly flat and its lateral margins are produced into a pair of thin folds, the meta pleural folds or lateral fins.
AMPHIOXUS- BODY WALL:The body is covered by skin.It has two regions, a-Epidermis. b. Dermis.
a) Epidermis : The epidermis is very thin layer of the cells. it is single layered. The epidermal cells are columnar and present on a basement membrane. They are ciliated or non ciliated. it has an outer cuticle covering.
The cuticle is made of a substance resembling chitin. The epidermis contains mucous cells and sensory cells.
b) Dermis : It is composed of connective tissue. it shows an outer thin layer with fibres and an inner thick spongy layer with connective tissue, blood vessels and nerve fibres.
Myotomes : The myotomes are thick and are in the dorsal and dorso-lateral regions. They are arranged in a linear series. They ‘are V shaped blocks. There are about 60 pairs of myotomes. The myotomes of the left side alternate with those of the right side. Each myotome is covered by connective tissue, called myocomtnata.
All the body muscles of Amphioxus are striated. These myotomes are helpful in swimming.
AMPHIOXUS- OPENINGS ON THE BODY:
. There are 3 openings on the body of Amphioxus.
1 . Mouth, 2. Atriopore and 3. Anus.
1 . Mouth : The mouth is very wide and lies at the anterior end of the trunk.
2. Atriopore : The atriopore is a small aperture situated in front of the ventral fin.
3. Anus The anus lies at the base of caudal fin on the ventral side.