The Oncology of Lancelet

Humans are of the phylum Chordata, and the majority of the animals in this phylum possess a flexible rod supporting their dorsal region, but in this post, I am not going to looking into Humans, possibly I will do that in another post, and I will be expecting to have you around. Today, I will be looking at lancelet / Amphioxus Branchiostoma lanceolatum.

Amphioxus is a marine organism that lives beneath the bottom of the body of water, buried in the shallow sand of the sea. Being easily mistaken for fishes in the river, Amphioxus are about 8cm long and are regarded as one of the origins of cordates.

Taxonomy of Amphioxus

There are about 30 species in this phylum, this subplylum gives an insight into the evolution of cordates/vertebrate sharing morphology, anatomy, and genomic characteristics with the last common ancestors of chordates.

Taxonomy Of Amphioxus
Kingdom	Animalia
Subkingdom	Bilateria
Infrakingdom	Deuterostomia
Phylum	Chordata
Subphylum	Cephalochordata
Order	Amphioxiformes
Family	Branchiostomatidae
Genus	Branchiostoma
Species	Branchiostoma lanceolatum

(itis.gov)

Other species includes; Branchiostoma africae, Branchiostoma belcheri, Branchiostoma californiense, Branchiostoma floridae, Branchiostoma indicum, Branchiostoma malayanum, to mention a few.

Habitat of Amphioxus

Lancelet inhibits the sublittoral sands of subtropical, and tropical estuaries, open coasts, coastal lagoons, and river deltas. While Lancelets are found in shallow sand in seas and water bodies. They burrow into the sand of in the sea with the rapid movement of their body. They burrow shallowly, into sea sands with the exceptions of the specie Asymmetron inferum which goes in depth up to 225 Meters.

Amphioxus Anatomy

Chordates, specifically Amphioxus are identifies by the presence of Notochord, Dorsal Nerve Cord, Pharyngeal Silts, and Post-anal tail. Lancelet are quite fishlike, with no bones or vertebral column. They also do not have a head structure, although, they possess a head, with no eyes, a mouth part, and a brainlike structure which is a swelling of the dorsal nerve cord.

Externally, Lancelet have a pointed posterior and anterior end, with a length within the range of 5cm to 8cm, and a flattened body from both side of view. The body can be differentiated into the head body and tail, with the head being snout in nature. Just right below the Snout is the oral hood formed by the dorsal and the lateral projection accompanied by about 20 oral cirri/tentacles which hides the mouth. On each Oral Cirrus is a stiff rod which is projected from inside the oral ring. The Anus is at the left ventral fin, not far from the tail of the Amphioxus.

Wikimedia common

The round Atriopore is located close to the anterior end of the ventral fin, not far from the posterior end of the Lancelet. The Atriopore allows the atrium to open to the exterior. Laterally, the body has numerous gill-slits, which are covered by the lateral fold of the body, partially making it virtually not visible from the outside. The body also posses a dorsal fin, which extends along the dorsal side of the body and connected to the caudal fin around the tail. On the opposite side is the ventral fin, which is extends from the caudal fin and the atriopore. Both fins (dorsal and ventral) are connected to a connective tissue called fin-ray boxes. Between the oral hood and the antiopore, is a part of the body that does not participate in swimming, called the Meta-Pleural fold. The Meta-pleural fold is used for burrowing the sand for the Lancelet.

Going into the internal anatomy of Amphioxus, I will be starting with the body. The body wall of Amphioxus is made up of a single cell layer of epidermis. The cells are ciliated in the young stage, but start to possess cuticles in the adult stage. The body is covered with sensory hairs which serves as receptor cells. In adult, the cutis beneath the epidermis are made up of fibres and the sub-cutis possesses a gelatinous matrix with fibres.

In the Lancelet, the notochord, situated above the alimentary canal, and on the mid-dorsal line, and extends from the tail to the rostrum. The esophagus is a short, narrow, ciliated tube, which is opened by the pharynx called the foregut.

Unlike vertebrates, Amphioxus does not possess an intestine, but it possesses a ciliated tube known as the gut. The gut devides the body into two regions, the Midgut, and the Hindgut. When it comes to feeding, the Lancelet filters water as they pass through the pharyngeal cavity. Once the food is filtered, the sensilliary papillae which i found in the buccal cirri and velar tentacles allows for tasting and sizing the food particle. Large food particles and other toxins are expelled.

Digestion occur both intracellularly and extracellularly, with intracellular digestion, the Hepatic diverticulum is responsible, while for extracellular digestion, the midgut is responsible. The Hepitic Diverticum which functions like the liver is referred to as the digestive gland in Branchiostoma. It is ciliated, and the Hepitic Diverticum is a large out-pouching found in between oesophagus and the midgut, lying into the pharynx right . The epitic Diverticum secretes an enzyme which contains Lipase and protease which serves as a digestive enzyme which are transported to the lumen of the midgut. In respiration as well, water enters into the pharyngeal cavity, thereby allowing for the extraction of oxygen dissolved in the water by the gill-bar. Since the gill-bars are made up of blood vessels which circulates close to the surface, allowing the absorbsion of oxygen and the expulsion of Carbon dioxide.

When it come to reproduction, the sexes in lencelets are different. The Gonad, situated at the ventro-lateral side of the pharyngeal region, is a pouch-like organ. The gamates develop from the gonad walls. Fertilization is done in the sea water, same as the development from egg. The gamates are discharged through the atriopore along ith water current.

Conclusion

Looking at Lancelets, it is true to say that they are vertebrates closest with an organized structure of anatomy, close to vertebrate ancestors and similar to the early stage of human fetus.

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https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-chordata

https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=685568#null

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https://www.biologydiscussion.com/animals-2/phylum-chordata/classification-of-subphylum-cephalochordata/40513

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2493399/

https://www.nature.com/articles/s41586-018-0734-6

https://www.biologydiscussion.com/animals-2/branchiostoma-distribution-structure-and-systematic-position/40549

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1458426/

https://earthlife.net/inverts/cephalochordata-or-lancets

https://www.frontiersin.org/articles/10.3389/fcell.2021.668025/full