- Clear cytoplasm occupying the animal pole will mainly become epiderm& ectoderm.
- The granular yolky cytoplasm near the vegetal pole will become endoderm.
- The crescent shaped cytoplasm of the posterior side will become mesoderm.
- Opposite to this mesodermal crescent another crescent is present which becomes presumptive nervous system. The lower part will develop into notochord.
1. Head, 2. Middle piece and 3. Tail .
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:
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:
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.
All the body muscles of Amphioxus are striated. These myotomes are helpful in swimming.
- Mouth: The mouth is very wide and lies at the anterior end of the trunk.
- Atriopore: The atriopore is a small aperture situated in front of the ventral fin.
- Anus: The anus lies at the base of caudal fin on the ventral side.
- Pallas described this animal first. He named it as Umax lancealatus, He included this in Mollusca.
- In 1834, costa described its chordata features.
- In 1836, Yarrel named it as Amphioxus.
- Presence of dorsal tubural nerve cord.
- Presence of a long notochord from anterior end to posterior end on the dorsal side. Because, it extends to the cephalic region. Hence it is called Cphalochordata.
- Gill silts are present in the
- Presence of post anal tail.
- The presence of liver diverticulum.
- The development of hepatic portal system.
- Presence of Myotomes and which are useful for locomotion.
- The presence of dorsal, caudal and ventral fins.
- The excretory system contains protonephrldia.
In the chordate world, the presence of solenocytes is not reported. But, in Amphioxus solenocytes are associated with Nephridium.
- Absence of heart and kidney.
- Absence of paired limbs or paired fins.
- Absence of distinct head.
- Absence of distinct paired sense organs.
- Gonads are without gonoducts.
- Because of its ciliary mode of feeding, the pharynx is elaborated with many gill slits.
- Oral hood is well developed for ciliary mode of feeding.
- Because of its ciliary mode of feeding the atrium is very well developed.
- Presence of gill slits In pharynx.
- Presence of Endostyle in Pharynx.
- Presence of Ciliry mode of Feeding.
- Presence of atrium.
- Absence of test.
- Absence of distinct heart
- Presence of notoçhord and nervecord in the adults.
- Presence of Myotomes in the adult.
- Presence of paired nephridia like annelids.
- Presence of flame cells like Helminths.
- Presence of soft body and slug like appearance like molluscans.
- Body is fish-like and is useful for burrowing and swimming.
- It has a head.
- It shows a tail.
- Appendages are absent.
- Dorsal, caudal and ventral fins are present.
- Body-wall shows one- cell thick, non-ciliated epidermis, dermis, connective tissue, striated muscle and parietal peritoneum.
- It has no exoskeleton.
- Notochord extends from the anterior end to posterior end.
- Enterocoelic coelom is present. However, reduced in the pharyngeal region by atrium.
- Alimentary canal is long. It includes a large pharynx with many gill-slits ciliary mode of feeding is developed.
- Gillss will perform respiration.
- Circulatory system is closed.
- Heart and respiratory pigments are absent.
- Hepatic portal system is present.
- Excretory system shows paired protonephridia with solenocytes.
- Brain is not present
- Two pairs of cerebral and several pairs of spinal nerves are present.
- Sexes are separate. Gonads are metamerically arranged and with out gonoducts.
- Asexual reproduction will not take place.
- Fertilization is external.
- Cephalochordata Classification:
- The sub-phylum Cephalochordata includes a single class- Leptocardii, which has single family, Branchiostomidae.The family contains only two genera Branchiostoma and Asymmetron
- SUB-PHYLUM: UROCHORDATA
- SUB-PHYLUM: CEPHALOCHORDATA
- SUB-PHYLUM: VERTEBRATA