Amphioxus :- V.L.S. Anterior Region
- The vertical longitudinal section shows buccal cirri, wheel organ, velum and some pharyngeal region.
- In a carmine stained section, prominent dorsal structures are fin rays, notochord and nerve cord.
- Dorsal fin is low, continuous and supported by fin rays.
- Nerve cord or spinal cord lies just above the notochord. It contains anterior and posterior pigmented spots and anteriorly swollen as
- Notochord lies just above the nerve cord forming axial skeletal rod. It extends antero-posteriorly.
- Anterior end projects as the rostrum. Important ventral structures are oral hood, vestibule, wheel organ, pharynx and atrium.
- Oral hood is clearly seen with oral cirri, which help during feeding by turning inwards to prevent sand particles from passing into buccal cavity. Oral hood guards the vestibule or buccal cavity. At the
hinder wall of vestibule lies a vertical partition called velum with velar tentacles.
- In front of velum is a peculiar wheel organ which helps in driving a current of water loaded with food particles into the mouth.
Identification: Since the above section has oral cirri and all above features, hence it is Amphioxus V.L.S. region.
Amphioxus :- T.S. Passing Through Oral Hood
- At the anterior end of Amphioxus is a mid-ventral opening encircled by frilled membrane, called oral hood. The T.S. Passing through the oral hood shows body wall, dorsal fin ray, nerve cord, notochord,
vestibule and oral hood, etc.
- Body wall is composed of epidermis, dermis or cutis and muscle layer.
- Epidermis is covered by a non-pigmented and iridescent cuticle. Unlike other chordates, the Amphioxus epidermis is very thin. Dermis is indistinct.
- Below epidermis and dermis is a thick longitudinal muscle layer. The cut segmental blocks or myotomes are very distinct, separated by myosepta. The muscle fibers in anterior half section are directed upwards while in posterior half, backwards. Below muscle layer is coelom.
- Dorsally below the epidermis is a dorsal fin ray.
- Dorsal tubulated glandular nerve cord having a central canal or neurocoel and below it notochord are clearly seen. The notochord is composed of chordal or fibrous sheath, which encloses vacuolated
notochordal cells filled with homogeneous liquid.
- Ventrally, section shows a large stomodaeum, oral hood and cut part of buccal cirri in a circular manner. Oral hood contains lymph spaces.
- Dorsal wall of buccal cavity has a sensory Hatscheck’s groove.
Identification: Since the section has buccal cirri and all above features, hence it is T.S. passing through oral hood of Amphioxus.
Amphioxus :- T.S. Passing Through Pharynx
- Pharynx is a large elongated, sac-like respiratory and digestive organ, extending from behind velum upto the intestine. T.S. passing through anterior pharynx shows body wall layers, dorsal rm ray, nerve cord, notochord, large cut pharynx with endostyle and metapleural folds.
- Body wall is composed of cuticle, epidermis, dermis and muscle layer.
- Cuticle and epidermis are thin-layered and indistinguishable. Below epidermis the dermis is also thin-layered.
- More than three-fourth of the section from dorsal side contains thick; cut, segmental muscle bundles or myotomes separated by transverse myosepta. The first three myotomes have side muscle fibres
whlle in posterior, half the muscle fibers are backwardly directed.
- Dorsally, just beneath epidermis, is the dorsal fin ray.
- Below dorsal fin ray is nerve cord and beneath nerve cord is notochord. Notochord is surrounded by notochordal sheath and filled with vacuolated notochordal cells.
- Ventral half of the section contains the large pharynx surrounded by atrial cavity and perforated by gill slits. It contains longitudinal rows of cilia in the form of an epipharyngeal groove mid-dorsally and an endostyle enclosing an endostylar canal, midventrally. The ciliated grooves direct food material towards oesophagus. The sides of the pharyngeal cavity contain several gill arches. Pharynx is adapted for ciliary feeding.
- Two metapleural folds with metapleural cavity are seen posteriorly.
- In some sections through pharynx, midgut diverticulum or liver is also seen. Other structures seen are dorsal aorta, coelomic spaces, gill arches, ventral vessel and transverse muscle fibers.
Identification : Since this section shows epipharyngeal groove, gill slits and all above features, hence it is T.S. Amphioxus through pharynx.
Amphioxus :- T.S. Passing Through Ovaries
- T.S. passing through above region shows body-wall layers, nerve cord, notochord, pharynx, midgut (liver) and ovaries.
- Body wall is composed of cuticle, epidermis, dermis and muscle layer. Cuticle, epidermis and dermis are very thin and indistinguishable. The musculature consisting of longitudinal fibers is well developed. First four segmental myotomes are thick and separated by myosepta. Last two myotomes are comparatively thinner. Muscle fibres in first three myotomes are directed sideways and upwards while muscle fibres in last three myotomes are backwardly directed.
- Dorsal tin ray is present just beneath the mid-dorsal epidermis.
- Below fin ray is nerve cord containing neurocoel. Notochord below nerve cord with chordal sheath enclosing vacuolated notochordal cells filled with homogeneous fluid.
- Ventral part of section contains two ovaries.
- Ovaries, enclosed in coelomic sac, contain several ova and are found from 25-51 segments.
- Pharynx, surrounded by atrial cavity, contains gill slits, epipharyngeal groove dorsally and endostyle ventrally. Two metapleural folds are seen ventro-Iaterally.
- In some sections midgut diverticulum or liver is also seen.
- Other structures seen are ventral vessel sub-endostyler coelomic caudal and coelomic space.
Identification : Since the section contains ova and all above characters, hence it is T.S. of female through ovaries of Amphioxus
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