Anatomy Lancelet

1 anatomy

1.1 nervous system
1.2 visual system

1.2.1 joseph cells , hesse organs
1.2.2 frontal eye


1.3 digestive system
1.4 other systems

anatomy

1. brain-like blister 2. notochord 3. dorsal nerve cell 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill s slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum

lancelets typically 5 centimetres (2.0 in) long, or 7 centimetres (2.8 in) @ longest. have translucent, fish-like body, without paired fins or other limbs. relatively poorly developed tail fin present, not swimmers. while possess cartilage-like material stiffening gill slits, mouth, , tail, have no true skeleton.

nervous system

in common vertebrates, lancelets have hollow nerve cord running along back, pharyngeal slits , tail runs past anus. vertebrates, muscles arranged in blocks called myomeres.

unlike vertebrates, dorsal nerve cord not protected bone simpler notochord made of cylinder of cells closely packed form toughened rod. lancelet notochord, unlike vertebrate spine, extends head. gives subphylum name (cephalo- meaning relating head ). nerve cord larger in head region in rest of body, lancelets not appear possess true brain. however, developmental gene expression , transmission electron microscopy indicate presence of diencephalic forebrain, possible midbrain, , hindbrain.

visual system

lancelets have 2 known kinds of light-sensing structures, joseph cells, , hesse organs, frontal eye , lamellar body. frontal eye thought light-sensitive, although has not been confirmed electrophysiological measurement. lamellar body, possible homologue of pineal body, once thought light-sensitive, although has been called question. of these organs , structures located in neural tube, frontal eye @ front, followed lamellar body, joseph cells, , hesse organs.

joseph cells , hesse organs

joseph cells bare photoreceptors surrounded band of microvilli. these cells bear opsin melanopsin. hesse organs (also known dorsal ocelli) consist of photoreceptor cell surrounded band of microvill , bearing melanopsin, half enveloped cup-shaped pigment cell. peak sensitivy of both cells ~470 nm (blue).

both joseph cells , hesse organs in neural tube, joseph cells forming dorsal column, hesse organs in ventral part along length of tube. joseph cells extend caudal end of anterior vesicle (or cerebral vesicle) boundary between myomeres 3 , 4, hesse organs begin , continue tail.

frontal eye

the frontal eye consists of pigment cup, group of putative photoreceptor cells (termed row 1), 3 rows of neurons (rows 2–4), , glial cells. frontal eye, expresses pax6 gene, has been proposed homolog of vertebrate paired eyes, pigment cup homolog of rpe (retinal pigment epithelium), putative photoreceptors homologs of vertebrate rods , cones, , row 2 neurons homologs of retinal ganglion cells.

the pigment cup oriented concave dorsally. cells contain pigment melanin.

the putative photoreceptor cells, row 1, arranged in 2 diagonal rows, 1 on either side of pigment cup, symmetrically positioned respect ventral midline. cells flask-shaped, long, slender ciliary processes (one cilium per cell). main bodies of cells lie outside of pigment cup, while cilia extend pigment cup before turning , exiting. cells bear opsin c-opsin 1, except few carry c-opsin 3.

the row 2 cells serotonergic neurons in direct contact row 1 cells. row 3 , 4 cells neurons. cells of 4 rows have axons project left , right ventrolateral nerves. row 2 neurons, axon projections have been traced tegmental neuropil. tegmental neuropil has been compared locomotor control regions of vertebrate hypothalamus, paracrine release modulates locomotor patterns such feeding , swimming.

digestive system

lancelets have oral cirri, thin tentacle-like strands hang in front of mouth , act sensory devices , filter water passing body. water passes mouth large pharynx, lined numerous gill-slits. ventral surface of pharynx contains groove, called endostyle, which, connected structure known hatschek s pit, produces film of mucus. ciliary action pushes mucus in film on surface of gill slits, trapping suspended food particles so. mucus collected in second, dorsal, groove, , passed rest of digestive tract. having passed through gill slits, water enters atrium surrounding pharynx, exits body via atriopore.

both adults , larvae exhibit cough reflex clear mouth or throat of debris or items large swallow. in larvae action mediated pharyngeal muscles while in adult animal accomplished atrial contraction.

the remainder of digestive system consists of simple tube running pharynx anus. hepatic caecum, single blind-ending caecum, branches off underside of gut, lining able phagocytize food particles, feature not found in vertebrates. although performs many functions of liver, not considered true liver homolog of vertebrate liver.

other systems

lancelets have no respiratory system, breathing solely through skin, consists of simple epithelium. despite name, little if respiration occurs in gill slits, solely devoted feeding. circulatory system resemble of primitive fish in general layout, simpler, , not include heart. there no blood cells, , no haemoglobin.

the excretory system consists of segmented kidneys containing protonephridia instead of nephrons, , quite unlike of vertebrates. unlike vertebrates, there numerous, segmented gonads.