<< Chapter < Page Chapter >> Page >

The post-anal tail    is a posterior elongation of the body, extending beyond the anus. The tail contains skeletal elements and muscles, which provide a source of locomotion in aquatic species, such as fishes. In some terrestrial vertebrates, the tail also helps with balance, courting, and signaling when danger is near. In humans, the post-anal tail is vestigial, that is, reduced in size and nonfunctional.

Click for a video discussing the evolution of chordates and five characteristics that they share.

Chordates and the evolution of vertebrates

Chordata also contains two clades of invertebrates: Urochordata and Cephalochordata. Members of these groups also possess the four distinctive features of chordates at some point during their development.

Urochordata

Members of Urochordata    are also known as tunicates ( [link] ). The name tunicate derives from the cellulose-like carbohydrate material, called the tunic, which covers the outer body of tunicates. Although adult tunicates are classified as chordates, they do not have a notochord, a dorsal hollow nerve cord, or a post-anal tail, although they do have pharyngeal slits. The larval form, however, possesses all four structures. Most tunicates are hermaphrodites. Tunicate larvae hatch from eggs inside the adult tunicate’s body. After hatching, a tunicate larva swims for a few days until it finds a suitable surface on which it can attach, usually in a dark or shaded location. It then attaches via the head to the surface and undergoes metamorphosis into the adult form, at which point the notochord, nerve cord, and tail disappear.

Photo A shows tunicates, which are sponge-like in appearance and have holes along the surface. Illustration B shows the tunicate larval stage, which resembles a tadpole, with a post anal tail at the narrow end. A dorsal hollow nerve cord run along the upper back, and a notochord runs beneath the nerve cord. The digestive tract starts with a mouth at the front of the animal connected to a stomach. Above the stomach is the anus. The pharyngeal slits, which are located in between the stomach and mouth, are connected to an atrial opening at the top of the body. Illustration C shows an adult tunicate, which resembles a tree stump anchored to the bottom. Water enters through a mouth at the top of the body and passes through the pharyngeal slits, where it is filtered. Water then exits through another opening at the side of the body. A heart, stomach and gonad are tucked beneath the pharyngeal slit.
(a) This photograph shows a colony of the tunicate Botrylloides violaceus . (b) The larval stage of the tunicate possesses all of the features characteristic of chordates: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. (c) In the adult stage, the notochord, nerve cord, and tail disappear. (credit: modification of work by Dann Blackwood, USGS)

Most tunicates live a sessile existence on the ocean floor and are suspension feeders. The primary foods of tunicates are plankton and detritus. Seawater enters the tunicate’s body through its incurrent siphon. Suspended material is filtered out of this water by a mucous net (pharyngeal slits) and is passed into the intestine via the action of cilia. The anus empties into the excurrent siphon, which expels wastes and water. Tunicates are found in shallow ocean waters around the world.

Cephalochordata

Members of Cephalochordata    possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage ( [link] ). The notochord extends into the head, which gives the subphylum its name. Extinct members of this subphylum include Pikaia , which is the oldest known cephalochordate. Pikaia fossils were recovered from the Burgess shales of Canada and dated to the middle of the Cambrian age, making them more than 500 million years old.

Extant members of Cephalochordata are the lancelets , named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders.

Questions & Answers

what's marketing
Mirabel Reply
what are hormones
Mirabel
what is meant by th word photosynthesis
MARTHA Reply
it is the process where by chloropherious plant manufacture their food with the presence of sunlight , chlorophyll and water etc.
Kosoe
what is biology
stallon Reply
is the study living things
Zhayma
and non living things
Zhayma
what is water circle?
Faith Reply
please i asks whether this biology is for university
Sky Reply
for secondary
stallon
Copulation means coming together of male and female in the present of sexual Intercourse.
LEKAN Reply
what are the adaptive features of nervous system
Ridhwan
Please what is ovulation
Adusei Reply
What is cell division?
Adusei
What is copulation?
Adusei
bhaiya didi Gate mat
qaisar
bajali ji ka matlab Aur Payal Chaudhary ka matlab
qaisar
please tell me adaptive features of nervous system
Ridhwan
what is a metaborism
Beatrice Reply
this is a specialized part of the cell eg Nucleus
David Reply
what are adaptive features of nervous system
Ridhwan
what are the organelles?
Faith Reply
what is reproduction
mana Reply
reproduction is the process by which living organisms give rise to young ones of their own kind
Miriam
What is evolution
Wengelawit
the production of new forms of life over time as documented in the fossil record.
mana
hmm
Marvin
give two parasites where secondary host is water snail?
Kevin Reply
what is treats
Raih Reply
what are the organelles in cell that involves in protein sythenis
Rita Reply
what is a melanin?
Judith Reply
what is telophase
Elphas
melanin in that black color we posse in our skins
Marvin
it is a substance that gives the colour black to the hair
Kosoe
melanin is a dark brown or blacksubstance that is a natural part of people's skin, hair, and eyes
The

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask