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Lancelet

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Lancelet
Branchiostoma lanceolatum.png
Scientific Classification
  • Kingdom: Animalia
  • Phylum: Chordata
  • Subphylum: Cephalochordata
  • Class: Leptocardii
  • Order: Amphioxiformes
Families
Japanese Lancelet
Japanese Lancelet.jpg

Lancelets, or amphioxus', are little known creatures. They look like small eels and can easily be mistaken for them, but they are not vertebrates.[1] They are used as a food source in China, but evolution scientists use them as a basis for the evolution of vertebrates. There are about thirty different species of lancelets, but they all have similar characteristics.[2]

Anatomy

Illustration of a Lancelet's anatomy
‎ On the outside a lancelet appears to be a very simple creature. They have a thin fish-like body that can grow from five to seven inches long.[3] Their skin is translucent and is responsible for most of the creature's respiration.[4] A median fin runs along the dorsal side of the creature, and extends all the way back to the posterior end of the animal. There is also a ventral fin on the underside of the lancelet.[5]

The animal has several openings on its body. At the anterior end there is the mouth opening with several cirri that act as filters for food and as sensory input. The openings also include pharyngeal slits on both lateral sides of the animal that are used when the animal is feeding. The anus, which is used for the excretion of waste, is located on the ventral surface of the animal toward the posterior end.[6] Also located on the ventral side of the animal and a little more anterior than the anus, is the atriopore, which is used to release water from the amphioxus' body.[7]

The lancelet is a filter feeder and has several organs to aid in consumption and digestion. Several ridges on the inside of the mouth, also called the wheel organ, have small cilia that beat in order to suck water into the lancelet. While the water is being sucked in, oral cirri surrounding the mouth filter the water. The water is then carried down to the atrium: an organ made up of the gill slits and metapleural folds (folds created by the body). Mucus in the atrium traps food particles while the water exits out of the atriopore. The food is then moved to the iliocolonic ring, and the hepatic cecum secretes digestive enzymes to break down the food. After that paired nephridia distributes the nutrition throughout the body.[8]

The pharyngeal slits and the skin are responsible for the respiration of the lancelet. The slits running along the side of the creature have filaments that line the slits and carry blood vessels. The blood runs through these filaments, and it then exchanges gases with the enviroment.[9]

The lancelet's nerve system runs through a dorsal hollow nerve cord that starts at the head and ends at the base of the tail. This cord grows from a mid-dorsal ectoderm in the creature's body. [10] The lancelet's nervous inputs include an eyespot on the head, paired motor nerves on the dorsal and ventral side of the body, and a neurocel in the head that is sometimes referred to as the brain of the creature.[11]

Reproduction

Cleavage in Lancelet embryo (2 and 4 cell).

Lancelets have separate sexes with about the same number of males and females in a population.[12] The fertilization is external with females releasing eggs of about 0.00394 in diameter and males releasing simple spermatozoa. When the sperm and eggs meet a larva is produced.

The larva is an asymmetrical creature that has a large mouth on the left side of its body, and a single row of gill slits. Later a second row of gill slits forms, and after twelve to fifteen gill slits appear the young creature sinks to the bottom. The atrium then forms and more gill slits form on both sides. The larval stage can last from several weeks to several months, and it becomes fully mature after a year.[13]


Ecology

Lancelets are found in the coastal waters of temperate and tropical regions.[14] They can be found in China, Japan, Madagascar, Africa and several other locations around the world.[15] Lancelet populations can be found bunched extremely close together with over 9,000 creatures per square meter. They are not endangered in any form or way.[16]

Though lancelets are effective at swimming both backwards and forwards they spend most of their time with their body buried in the sand. Depending on the coarseness of the sand, lancelets will assume different feeding positions. They will have their body buried with their head sticking out in coarse sand, or they will lie on the bottom in smoother sand. With their head sticking out of the sand, a mucus net on their gill slits and their cilia are able to capture plankton floating by. [17] They prefer plankton with plant cells, specifically diatoms.[18]

Lancelets in Evolution

Many believe that the lancelet is the starting point for vertebrate evolution.[19] Lancelets share many characteristics with chordates: A notochord, dorsal nerve cord, branchial basket, and a post anal tail. Because of these characteristics scientists believe that vertebrates evolved from a chordate similar to the lancelet. [20]

Since Lancelets are soft bodied creatures it is difficult to find a fossilized lancelet. Though there have been fossils which scientists think are lancelets, but they can not know for sure.[21] Palaeontologist's theories are based on fossilized creatures, and even then fossils can be difficult to interpret. Scientists have many theories on how lancelets could have evolved into other creatures, but the fossil record shows no proof.[22]

The Biological Sciences Collegiate Division at the University of Chicago now has adopted the song It's a Long Way from Amphioxus as their theme song. The song was first performed by Sam Hingon: a folk musician and marine biologist. The song is sung to the tune It's a Long Way to Tippary ,but the lyrics are different.[23]

A fish-like thing appeared among the annelids one day.
It hadn't any parapods nor setae to display.
It hadn't any eyes nor jaws, nor ventral nervous cord,
But it had a lot of gill slits and it had a notochord.

It's a long way from Amphioxus. It's a long way to us.
It's a long way from Amphioxus to the meanest human cuss.
Well, it's goodbye to fins and gill slits, and it's welcome lungs and hair!
It's a long, long way from Amphioxus, but we all came from there.

It wasn't much to look at and it scarce knew how to swim,
And Nereis was very sure it hadn't come from him.
The mollusks wouldn't own it and the arthropods got sore,
So the poor thing had to burrow in the sand along the shore.

He burrowed in the sand before a crab could nip his tail,
And he said "Gill slits and myotomes are all to no avail.
I've grown some metapleural folds and sport an oral hood,
But all these fine new characters don't do me any good.

He sulked awhile down in the sand without a bit of pep,
Then he stiffened up his notochord and said, "I'll beat 'em yet!
Let 'em laugh and show their ignorance. I don't mind their jeers.
Just wait until they see me in a hundred million years.

My notochord shall turn into a chain of vertebrae
And as fins my metapleural folds will agitate the sea.
My tiny dorsal nervous cord will be a mighty brain
And the vertebrates shall dominate the animal domain.[24]

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