Sub-Phylum Cephalochordata (Lancelets) Objectives

i. To understand the characteristics of Cephalochordate ii. To understand the function and physiology of Cephalochordate iii. Understand the differences and similarities between Urochordate and Cephalochordate

General features

The lancelets are also called cephalochordates because the notochord extends from near the tip of the tail well into the anterior of the body. Because they do not have the braincase, or cranium, of a vertebrate, lancelets are often called acraniates. The pharynx, with its many gill slits, is surrounded by the atrium, a large cavity with a single exit on the lower surface of the body. The atrium protects the gills. Tunicates also have an atrium, but its evolution is probably independent of that of the cephalochordrate atrium. Lancelets are members of a single genus, known by the names, Amphioxus (double pointed) and Branchiostoma (gill mouth).

The bodies of lancelets, like those of fishes and other vertebrates, are largely made up of serially repeated units (segments) that include blocks of muscles called metameres. This segmentation also extends to the nerves that supply the myotomes and to some body cavities, excretory structures, and other parts. Segmentation is thought to provide more effective body coordination during locomotion. The segments of vertebrates and cephalochordates are so similar that they were almost certainly present in the common ancestor of the two groups. Tunicates and hemichordates have no clear indications of ever having possessed segments. Annelids, arthropods and other animals have segments, but these segments have a different composition and probably a separate evolutionary origin.

A distinct “secondary” body cavity (coelom), like that which contains the internal organs in vertebrates and many other animals, is well developed and forms a system of cavities and spaces. Like the coelom of hemichordates, echinoderms, and a few other animals, it develops as outpouchings in the gut of the embryo.

Cephalochordates are the marine lancelets: slender, laterally compressed, translucent animals about 5 to 7 cm in length that inhabit the sandy bottoms of coastal waters around the world. Lancelets originally bore the generic name Amphioxus, later surrendered by priority to Branchiostoma. Amphioxus is still used.

Lancelets are distributed throughout the world along tropical and temperate coasts. They inhabit soft bottoms ranging from sand to coarse shelly sand or gravel in shallow coastal water. Lancelets lie buried beneath this substrate, often with their mouths protruding above the surface, allowing them to take in water laden with food. In China, lancelets are sometimes eaten and even support a small fishing industry.

They are small, fishlike marine invertebrates; they probably are the closest living relatives of the vertebrates. In most vertebrates, bony vertebrae or cartilaginous tissue eventually replaces the embryonic notochord; among cephalochordates, the notochord is retained into adulthood and is never replaced by vertebrae. There are about 20 species in two families, each with a single genus. Branchiostoma was formerly called Amphioxus, a name that is retained as an informal term. The other genus is Epigonichthys, also called Asymmetron. The genus Asymmetron is sometimes retained for some species. The cephalochordate fossil record extends back to about 525 million years ago during the early part of the Cambrian Period.

Four species of amphioxus are found in North American coastal waters. Amphioxus is especially interesting because it has the four distinctive characteristics of chordates in simple form.

GENERAL CHARACTERISTICS: 1. Body is fish-like and is useful for burrowing and swimming. 2. It has a head and it shows a tail. 3. Appendages are absent. 4. Dorsal, caudal and ventral fins are present. 5. Body wall shows one- cell thick, non-ciliated epidermis, dermis, connective tissue, striated muscle and parietal peritoneum. 6. It has no exoskeleton. 7. Notochord extends from the anterior end to posterior end. 8. Enterocoelic coelom is present. However, reduced in the pharyngeal region by atrium. 9. Alimentary canal is long. It includes a large pharynx with many gill-slits ciliary mode of feeding is developed. 10. Gills will perform respiration. 11. Circulatory system is closed. 12. Heart and respiratory pigments are absent. 13. Hepatic portal system is present. 14. Excretory system shows paired protonephridia with solenocytes. 15. Brain is not present 16. Two pairs of cerebral and several pairs of spinal nerves are present. 17. Sexes are separate. Gonads are metamerically arranged and without gonoducts. Fertilization is external. No asexual reproduction

Water enters the mouth, driven by cilia in the buccal cavity, then passes through numerous pharyngeal slits where food is trapped in mucus, which is then moved by cilia into the intestine. Here the smallest food particles are separated from the mucus and passed into the hepatic caecum (liver diverticulum) where they are phagocytized and digested intracellularly. As in tunicates, the filtered water passes first into an atrium, then leaves the body by an atriopore (equivalent to the excurrent siphon of tunicates).

The closed circulatory system is complex. The flow pattern is remarkably similar to that of primitive fishes, although there is no heart. Blood is pumped forward in the ventral aorta by peristaltic-like contractions of the vessel wall, then passes upward through branchial arteries (aortic arches) in the pharyngeal bars to paired dorsal aortas which join to become a single dorsal aorta. From here the blood is distributed to the body tissues by microcirculation and then is collected in veins, which return it to the ventral aorta. Lacking erythrocytes and haemoglobin, their blood is thought to transport nutrients but play little role in gas exchange.

The nervous system is centered around a hollow nerve cord lying above the notochord. Pairs of spinal nerve roots emerge at each trunk myomeric (muscle) segment. Sense organs are simple, unpaired bipolar receptors located in various parts of the body. The “brain” is a simple vesicle at the anterior end of the nerve cord.

No other chordate shows the basic diagnostic chordate characteristics as clearly as amphioxus. In addition to the four hallmarks of chordate features, amphioxus possesses several structural features that suggest the vertebrate plan. Among these are; hepatic caecum, a diverticulum that resembles the vertebrate pancreas in secreting digestive enzymes, segmented trunk musculature, and the basic circulatory plan of more advanced chordates.

Lancelet sexes are separate, and asexual reproduction does not occur. Eggs and sperm are shed directly into the water, where fertilization occurs. The early stages of development strikingly resemble those of both tunicates and vertebrates. A larva is produced that is similar in structure to the adult but is peculiarly asymmetrical. The larvae spend much of their time feeding in the open water but can be found on the bottom. After growing and developing, they metamorphose into the adult form and complete their life history in the substrate.

Lancelets can swim both forward and backward and can move rapidly through the gravel in which they live. Their behaviour is simple, largely being a matter of locating the proper habitat and escaping from predators. Larvae filter small organisms out of the water; at the time when they metamorphose into the adult, they also feed upon coarser materials deposited on the bottom. The adults filter small organisms from the overlying water by drawing a current into the mouth. The tentacle-like cirri around the mouth form a grid that keeps out sand and other large particles.

Figure: External morphology of Branchiostoma