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Introduction to the Cephalochordata
It's a long way from amphioxus. . .
The Cephalochordata are a small subphylum (about 28 species) of of small, 5 to 10 centimetres long, but
fascinating animals. visually they look like simplified fishes, and they are quite capable of swimming
short distances. Normally however they prefer to spend their time in the sand
of marine shores. All known species are marine. They occur all around the
world in both temperate and tropical waters. They have long fascinated
biologists because they exhibit all four basic characteristics of the phylum
Chordata (a dorsal nerve cord, a notochord, a post annul tail and pharyngeal gill
slits) in the adult stage of their lives. Among the 28 known species one occurs
off European shores (Branchiostoma lanceolatum) which was the original
lancelet, and four occur of the coasts of North America.
Known as lancelets or as amphioxus (from the Greek for "both [ends]
pointed," in reference to their shape), cephalochordates are small, eel-like,
unprepossessing animals that spend much of their time buried in sand.
However, because of their remarkable morphology, they have proved
crucial in understanding the morphology and evolution of chordates in
general -- including vertebrates.
The anatomy of a cephalochordate is diagrammed at left. Note that
cephalochordates have all the typical chordate features. The dorsal nerve
cord is supported by a muscularized rod, or notochord. The pharynx is
perforated by over 100 pharyngeal slits or "gill slits", which are used to
strain food particles out of the water. The musculature of the body is
divided up into V-shaped blocks, or myomeres, and there is a post-anal
tail. All of these features are shared with vertebrates. On the other hand,
cephalochordates lack features found in most or all true vertebrates: the
brain is very small and poorly developed, sense organs are also poorly
developed, and there are no true vertebrae.
Water is taken in through the mouth, drawn in by the beating of cilia
located on the wheel organ, a set of ridges lying inside the mouth. The
water is first filtered by the oral cirri, slender projections that surround
the opening of the mouth, clearly visible on the photograph at the top of
the page. It then passes through the gill slits. These gill slits are enclosed
by folds of the body wall, the metapleural folds, to form a body cavity
known as the atrium. Food particles in the water are trapped by mucus,
while water passes through the slits and out of the atrium through the
atriopore, located towards the posterior end. The rest of the digestive
system is fairly simple: a pouch or hepatic caecum secretes digestive
enzymes, and actual digestion takes place in a specialized part of the intestine known as the
iliocolonic ring. Cephalochordates also have a well-developed circulatory system and a simple
excretory system composed of paired nephridia. The sexes are separate, and both males and
females have multiple paired gonads. Eggs are fertilized externally, and develop into freeswimming, fishlike larvae.
Since cephalochordates have no hard parts, their fossil record is extremely sparse. However,
fossil cephalochordates have been found in very old rocks indeed, predating the origin of the
vertebrates. The famous Middle Cambrian Burgess Shale of British Columbia has yielded a few
fossils of Pikaia, which appears to be a cephalochordate (although the fossils are still being
restudied). More recently, Yunnanozoon, from the Early Cambrian of south China, was reported
to be a cephalochordate, the earliest known (Chen et al., 1995). These fossils show that the
chordate lineage appeared very early in the known history of the animal kingdom, and they
strengthen the case for an origin of true vertebrates from a cephalochordate-like ancestor.
Today, amphioxus may be extremely common in shallow sandy environments: at Discovery Bay,
Jamaica, up to five thousand individuals per square meter of sand have been reported. In some
parts of the world, amphioxus are eaten by humans or by domestic animals; they are important
food items in some parts of Asia, where they are commercially harvested.
The body is designed with three openings; a mouth, guarded by tentacles called buccal cirri and through
which water enters, an atriopore towards the posterior (tail) end of the animal through which water
exits and behind this an anus through which waste products exit. The Cephalochordata are filter feeders,
like the Urochordates, with most of the body being occupied by the large pharynx which exists within
the atrial cavity. Water, drawn in through the mouth by the action of numerous beating cilia, passes
through many small slits in the pharynx before exiting via the atriopore. The inside of the pharynx is
lined with a thin layer of mucus which is secreted by an organ called an endostyle found along the
ventral surface of the pharynx. This mucous is kept constantly moving up across the inner walls of the
pharynx by more beating cilia until it meets the dorsal groove of the pharynx. From here it is swept back
towards the animals stomach where, it, and the small particles of matter which have become trapped in
it are digested.
Cephalochordates have a closed blood system, with a main ventral and paired dorsal aorta connected by
an intermeshing series of smaller vessels. There is no central heart, but instead a series of brachial
hearts where each of the main branches of the system meets the ventral aorta. The blood of
Cephalochordates lacks hemoglobin and is colorless.
Cephalochordates have a simple dorsal nerve cord enclosed in a sheath of collagen fibers which runs the
length of the body below the notochord. From this arise a series of smaller nerves which serve the rest
of the body. There is no brain or cranium such as we will find in the Vertebrata.
Cephalochordates have a series of metameric (one after another and partially overlapping) blocks of
muscles arranged along the body. These blocks of muscles are shaped like chevrons and are called
myomeres. This is a structure we will come across again in the vertebrates, particularly the fish. It is the
rhythmic contracting of these blocks of muscle which allow the animal to flex its body in the sinusoidal
movement that allows it to swim like a fish. In the image below I have colored each myomere differently
to show how they overlap.
Single myomere
Longitudinal
Single myomere
Transverse
Several myomeres
Longitudinal
Cephalochordates are gonochoristic, meaning the sexes are separate. Both sperm and eggs are released
into the water and fertilization is therefore external. From these eggs hatch small ciliated larvae, these
soon change into what is called amphioxis larvae (it looks a bit more like the adult form), both of these
larval forms are pelagic, meaning the live in the water not on the sea floor. The amphioxis larvae feeds
on plankton until it is large enough to metamorphoses into an adult, whereupon it moves to the sea
floor to take up its fairly sedentary life style.
The Cephalochordata represent a very primitive example of the basic Chordate plan, and in some ways
resemble what scientists think the original ancestor to the vertebrates may have looked like. However in
their current form it is generally agreed that they represent a side branch of evolution rather than a
direct ancestor.