amphioxusamphioxus

Introduction

Organisms under Subphylum Cephalochordata:

• Has no distinct head

• But has a notochord that extends to the head part of the body.

Introduction:

Amphioxus:

- means “sharp at both ends”

- a living amphioxus is semitransparent but becomes opaque when immersed in preserving fluids.

Amphioxus and the Vertebrates

No cephalizationNo paired sense organs

Has notochord that remains throughout lifeHas no vertebral column

Notochord

Has pharyngeal slits but in large numbers

Dorsal, hollow nervous systemNo brain or cranium

Metameric muscles (myomeres) but extendes to the anterior tip of the head

Two-layered skin – epidermis and dermisBut outer layer is one-cell thick

Well-developed circulatory systemLack blood cells, muscular heart

Pulsating blood vessels

Is coelomateBut gretly restricted

External morphology of amphioxus

MYOMERES

•Lie immediately under the thin skin throughout the entire length of the body

•Provide locomotion

MYOSEPTA• a connective tissue partition to which longitudinal muscle bundles attach •It separates the myomeres

GONADS•Visible through the body wall and bulge into the water-filled atrium, into which sperm or eggs are shed

ATRIOPORE

•It is where the excess water , gametes and metabolic wastes exits

BUCCAL CIRRI

•Partially strain the water as it enters the vestibule and monitor it chemically

Dorsal and ventral fin rays: maintains balance during locomotion

EYESPOT OCELLI

WHEEL ORGAN

GILL BARS

GILL SLITS

GILL SLITS : where water goes through; where strings of mucus travelling across them trap tiny food particles

GILL BARS : supports between gill slits

OCELLI : light-sensitive organ; it assist in orienting the animal as it burrows in the sand

WHEEL ORGAN : retrieves some of the heavier food particle that miss the mouth, and it directs these through the mouth and into the pharynx along with the water stream

Internal Structure of Amphioxus

Parts and Function

Notochord:  protects and support the dorsal nerve cord; serves as the major skeletal support throughout life

Nerve cord: serves as central nervous system.

Pharynx: passage way for water, food and air.

Atrium: serves as a collecting chamber for respiratory water that has passed over the gills.

Intestine: major site for digestion of food.

Anus: excretion of waste

Anterior end

Vestibule: collecting chamber for sea water

Oral hood: serves as entrance and storage

Velum: works as valve and filter; surrounds the mouth

Velar tentacles: prevent undesirable objects from entering the digestive cavity

Amphioxus spend much of their time buried in gravel or mud on the ocean bottom

When feeding, they let the anterior part of the body project from the surface of the gravel so that they can filter food particles from water passing through their gill slits.

FEEDING:

Once the food is in the pharynx it is processed as follows.

Feeding:

Hypobranchial groove or endostyle

Hypobranchial groove or endostyle

Epibranchial groove

Epibranchial groove

They burrow into sand using rapid movements of the body

The continuity of the notochord to the very tip of the rostrum may be an adaptation for burrowing in sand.

The animals swim by contracting the muscle blocks, or myomeres, that run from end to end on each side of the body.

The blocks on each side are staggered, producing a side-to-side movement of the body when swimming.