What are Cephalochordates?

Introduction:

The Cephalochordates (from Greek: "head" and, "chord") are the animals in the phylum Chordate subphylum, Cephalochordata. They are also called lancelets. Cephalochordates possess all the 5 derived characteristics of chordates, that all chordates possess at some point in their lives during their larval or adulthood stages and some retain them throughout their lives. These characteristics comprise a notochord, dorsal( hollow) nerve cord, pharyngeal slits, and a, muscular post-anal tail. Cephalochordates are expressed themselves in contemporary oceans by the Amphioxiformes and are normally found in hot and warm temperate and tropical seas throughout the world. Due to the existence of a notochord, the adult organism is capable to swim and abide the tides of harsh and coastal environments, but they are probably found within the residue of these communities and neighborhoods.

 The Cephalochordates are the segmented marine animals that have extended and elongated bodies having a notochord that expands throughout the length of the body from head to tail. They are very small, only a few centimeters in length, and due to their absence of a mineralized skeleton,  and due to this their presence in the fossil record is minimal. and that's why we don't know much about their history.

 But the Cephalochordates are thought to have separated from the other chordate subphylum about 700 to 650 million years ago. The basis of their phylogeny is on the combination of studies of extinct and existent species.

Scientific Classification:

Kingdom:        Animalia

Superphylum: Deuterostomia

Phylum:         Chordata

Subphylum:   Cephalochordata

Morphology:

The Cephalochordates is derived from the  Greek: “head” because the notochord prolongs from almost near the tip of the tail into the anterior of the body.  They are called a-craniates because they do not possess the braincase/cranium which is present in vertebrates. The pharynx is surrounded by the atrium ( a large coelom or cavity with one exit at the lower side of their bodies.),having many gill slits. The function of the atrium is to protect the gills. Tunicates also possess an atrium, but its development is likely independent from the atrium of the cephalochordates.

The bodies of cephalochordates, like fishes and other vertebrates, are primarily made up of repeating units or segments that comprise blocks of muscles called metameres. These segments also extend to the nerves that provide the myotomes and to some body cavities (coelom),  the excretory structures, and other parts of the body. These segments are thought to give more effective body coordination during movement and locomotion. These segmentations of both vertebrates and cephalochordates are so identical that they were almost definitely present in the joint ancestor of both groups. The tunicates and hemichordates have no clear signs of ever having segments. Segments are also present in other animals, including the annelids, worms, and arthropods, but the segments in these organisms have a different type of composition and most probably have a  separate evolutionary history. 

A particular“secondary” body cavity (coelom), is also present in cephalochordates and is well-developed which forms a channel of cavities and coeloms that retains the internal organs in vertebrates and many other animals. And it grows in the form of outpouchings in the embryo's gut.

Feeding and movements:

Cephalochordates can swim both forward and backward and motion and they can also move fastly through the gravel or stone in which they live. Their behavior is simple, largely being a case of locating a suitable habitat and escaping from predators. Larvae filter small organisms out of the water for their feed, at the time when they metamorphose into the adult, they also feed on coarser materials present on the bottom.

The adults' filter crustaceans and small organisms from the water by drawing water into the mouth. The tentacle-like cirri around their mouth, protect the mouth and form a grid that keeps out sand and other large particles.

Evolution:

The cephalochordates make convincing measures for the common ancestor of the chordates and vertebrates. They evidently show a collateral branch on the lineage of vertebrates. cephalochordates and vertebrates have many similar, unique features that suggest that they are sister groups and are more closely related to each other than other chordates.

These characteristics include the segmented musculature and its innervation, the pattern of circulation of fluids, and many biochemical features. The atrium in cephalochordates and tunicates are thought to have evolved independently because there is little proof that the two form a single evolutionary lineage.