Invertebrate chordates

The phylum Chordata is chiefly of interest because it includes the human taxonomic group, the vertebrates. Classic classifications of Chordata also include several lesser-known groups termed the lower chordates. Chordates as a whole are characterized by the presence of a longitudinal cartilaginous stiffening rod (the notochord), a single tubular dorsal nerve cord, and perforations in the pharynx comparable to gill slits. The subphylum that includes humans, the Vertebrata, is further characterized by the presence of a bony skeleton that forms an internal support and protects vital structures such as the spine and brain. In land-dwelling vertebrates, gill slits are present only in the embryonic stages. The other two subphyla within the Chordata and making up the lower chordates are the Tunicata (formerly Urochordata), or tunicates, and the Cephalochordata, or Lancelets, all of which are small marine organisms. In the past, an additional group, the Hemichordata (acorn worms and pterobranchs), were also considered chordates. They possess a dorsal nerve cord and gill slits but do not have a notochord. They were reclassified to a separate phylum, although they are considered close relatives of chordates. Additionally, a group of fossil organisms with echinoderm affinities have been interpreted as chordates and classified in the subphylum Calcichordata (synonymous with stylophorans). Most specialists in this group (generally known as cornutes and mitrates) consider them to be echinoderms, however, as their chordate affinities are based entirely on soft-part reconstructions.

Sea Squirts

The subphylum of lower chordates Tunicata (formerly Urochordata) contains about 3,000 species in four classes. These organisms, commonly called tunicates, little resemble chordates in their adult form—only the tadpolelike larval stage possesses distinct chordate characters. In the class Ascidiacea, there are several groups of filter-feeding invertebrates called sea squirts or ascidians. Adult sea squirts are primarily sessile (attached to the ocean bottom) marine organisms found in coastal salt waters worldwide. They are sack-shaped, range from a few millimeters to a few centimeters in length, and have two siphons, inhalant and exhalant, extending from the upper surface. The outer part of the body is called the tunic. This is a protective structure made of proteins and polysaccharides, which is often quite thick and may vary from a soft, delicate consistency to one that is tough and similar to cartilage. Within the tunic, much of the space is taken up by the pharynx (an expanded part of the digestive tube), which has small perforations in its wall, creating a net. Small, hairlike structures called cilia create currents that pull water in through the inhalant siphon. The water then passes through the openings in the wall of the pharynx into the atrium (the surrounding cavity), and from there, it passes out through the exhalant siphon. Food particles are trapped on the pharynx wall by a mucus sheet, which moves constantly to the midline of the pharynx and then posteriorly to the gut, where digestion takes place. This is a very effective system, and it can filter out particles only one to two micrometers in diameter.

Sea squirt larvae, unlike the adults, are tadpole-shaped and mobile. They are generally very small and are mobile for only a few hours before settling and becoming sessile adults. The tail of the larva has a notochord, which acts in much the same way as the vertebral column of vertebrates. As it is flexible but of a fixed length, it will not shorten when muscles on either side contract; it, thus, makes them antagonistic, bending the tail from side to side. Above the notochord is a dorsal nerve cord that swells anteriorly into a light detector and an organ sensitive to tilting. The larvae tend to swim down and away from light—behavior that takes them to sites such as overhanging rock faces that are suitable adult habitats.

Thaliacea, Larvacea, and Cephalochordata

The Thaliacea and Larvacea are both planktonic (that is, they float near the surface of the ocean). The Thaliacea class consists of salps, pyrosomes and doliolids, are colonial forms that may reach two meters in length. Their inhalant and exhalant siphons are at opposite ends of the body, and water is pumped through by rhythmic muscular contractions. This class has no tadpolelike larvae, but instead develops directly into the adult form. The Larvacea do develop from larvae, but they retain the tail as a permanent organ. Water is filtered through a mucus sheet in the pharynx as in other tunicates, but in this group, the entire animal is surrounded by a delicate gelatinous “house” that is probably homologous to the sea squirt’s tunic. The house has mesh at the inhalant and exhalant openings that help to concentrate food particles; this concentrate is then passed through the mucus sheet, and food particles are trapped. The house is continually shed and replaced, probably to counteract clogging of the filters, and may not last more than a few hours.

The subphylum Cephalochordata includes about thirty species of organisms, sometimes called amphioxi, in the class Leptocardii, often referred to as amphioxus. These are small, fusiform, and rather fishlike organisms up to seven centimeters long that live in sandy and shelly bottoms in shallow coastal waters. They burrow head down in coarse sediments and can filter-feed even when buried by filtering the water that penetrates between grains.

In cephalochordates, the notochord extends almost to the snout. This is further forward than in fishes, and it may aid in burrowing by stiffening the snout. The swimming muscles are arranged in myomeres (muscle segments) down the body and are similar to those found in fish, though simpler in shape. A hollow dorsal nerve cord runs above the notochord and is enclosed in a tube of collagen fibers that enclose the cord in a way similar to the vertebrae in fish. There is no anterior swelling of the nerve cord that might be comparable to the brain in vertebrates. The nerve cord, however, sends a ventral motor nerve and a dorsal sensory nerve to each myomere, an arrangement identical to that found in vertebrates. As it is in the urochordates, the pharynx is pierced by numerous slits, and food particles are trapped by a mucus sheet that moves across them and back to the gut. The blood system is more complex, however, and is similar in general arrangement to that of fish; although there is no heart, blood is propelled by pulsations of some of the vessels.

Hints About Vertebrate Origins

The cephalochordates and urochordates are particularly interesting for what they suggest about vertebrate origins. Although it has been suggested that vertebrates evolved from various invertebrate groups, such as annelids (segmented worms) or cephalopods (squid and octopuses), it is now recognized that fundamental patterns of development distinguish chordates and echinoderms from mollusks and advanced segmented invertebrates. These differences involve the way in which cells divide and the relative potential of the cells.

In mollusks and annelids, spiral cleavage results in cells that are nested between one another in successive rows. They are also determinate—that is, the fate of each cell is predetermined, so that the removal of one results in the developmental failure of part of the organism. In chordates and echinoderms, however, the cells are directly above one another in layers (radial cleavage), and if a cell is removed, adjacent cells will compensate for its loss (indeterminate cleavage). In both groups, cells initially form a ball, termed the blastula, and cells at one end grow to form a second layer of tissue, the endoderm, which forms the lining of the gut. The external layer, or ectoderm, forms the outer surface of the body. In the mollusk-annelid group, the opening in the blastula (the blastopore) is retained as the mouth, while in the echinoderms and chordates it becomes the anus.

Though this information points to a close relationship between echinoderms and chordates, there are too many differences for them to make convincing vertebrate ancestors. The larvae are similar to those of lower chordates, but the adults are quite different. The same can be said for hemichordates, although they are probably more closely related to chordates than echinoderms are. Adult urochordates are also too specialized to be suitable candidates for the vertebrate ancestor, and the same can be said of cephalochordates, although amphioxus does show many vertebrate-like features. Some scientists assert that the echinoderms, hemichordates, and lower chordates must have diverged from vertebrate ancestors no later than the lower Cambrian period (about 550 million years ago). This is substantiated by the presence of a fossil cephalochordate in the Burgess Shale, which is Middle Cambrian in age.

It has been suggested that the lower chordates show how the vertebrate ancestor may have arisen by a process of neoteny or pedogenesis, in which larval characteristics may be retained in the adult. Sexual development is accelerated, and the development of other organ systems is arrested, so that the nonreproductive larvae of the ancestor become the reproductive adults of the descendant. In this case, a notochord and tail muscles are found in sea squirt larvae but not in the adults; however, they are retained in the Larvacea. Further retention of larval features could have given rise to cephalochordates and, by a further step, to vertebrates.

Paleontologist Richard Jefferies proposed the Calcichordate Hypothesis in the 1960s based on his study of fossil records. The theory suggests that vertebrates, tunicates, and lancelets evolved from carpoids, and chordates were closely related to echinoderms. However, many scientists disagreed with his assessment, and by the late 2010s, the theory was largely disregarded.

Studying Chordates

Lower chordates, small marine organisms, are often difficult to study because of their size and delicacy. The gelatinous covering of the Larvacea, for example, is so easily damaged as to be almost impossible to observe. Many modern techniques, however, have been developed to aid in the study of organisms such as these. Cinematography is used in studies of movement, and high-speed photography is particularly useful; the film can then be shown at a much slower speed to enable detailed analyses of complex movements. Electromyography can also be used to trace muscular action, as it follows the electrical changes that take place in muscles when they are active. The lower chordates are all filter feeders, and it is possible to carry out experiments that show how effective they are at removing small particles from the water. Sea squirts can be placed in a dish containing a suspension of colloidal graphite, and their filtering ability can be seen as a function of the rate at which the water clears. In addition, the size of particles can be varied to show how efficient the filtering apparatus is; this has shown that sea squirts can remove particles as small as one to two micrometers in diameter, although the diatoms on which they normally feed are closer to two hundred micrometers in diameter.

Fossil lower chordates are extremely rare; however, a probable cephalochordate, Pikaia, is known from the Burgess Shale of British Columbia, which is dated as Middle Cambrian (530 million years ago). The Burgess Shale contains a variety of soft-bodied organisms preserved as films of carbon on the bedding surfaces. Study is difficult because the material is compressed and because it is the same color (black) as the rock that contains it. Specimens can be prepared by picking rock away with needles. Details can then be studied by observing the specimens under low-angle light, which picks out differences in reflectivity of the rock surfaces and carbon films, or by immersing specimens in water or alcohol, which also enhances differences between the fossil and the surrounding rock.

These techniques make it possible to determine which characters are important in determining relationships within the lower chordates. Studies on relationships rely heavily on a methodology called phylogenetic systematics, or cladistics. In this taxonomic method (taxonomy is the study of interrelationships), only advanced characters shared between species (termed synapomorphies) are used to develop a picture of relationship. These relationships are expressed as branching diagrams termed cladograms (klados is Greek for “branch”), hence the name cladistics. Studies using this technique have advanced understanding of the relationship between lower chordates and the vertebrates. The picture is by no means clear, however, and much still waits to be done.

Understanding the Route of Evolution

The lower chordates are of particular interest for the light that they shed on the way in which the group to which humans belong, the vertebrates, may have developed and when this may have occurred. Vertebrates differ from lower chordates in possessing both an internal skeleton and a brain; however, there are limited fossil records of the earliest members of the group. The earliest known vertebrates are fish that are found in rocks of the Ordovician period (450 million years ago) in Australia, South America, and North America. These animals had an external bony armor, but knowledge of both their external appearance and their internal anatomy is restricted by poor fossil preservation. It is clear, however, that these animals were already relatively advanced and, hence, that a fairly long period of vertebrate development is not represented in the fossil record. As the lower chordates are the nearest relatives of the vertebrates, they can provide some information on how this development may have taken place.

It has been suggested that the process of neoteny, or pedogenesis, might explain the development of vertebrates. In this process, development of adult characters is retarded, and the organism reaches sexual maturity while still in the larval stage. This process may already have operated in the lower chordates, as the urochordates show chordate characters in the tadpolelike larval stage only, whereas the more advanced cephalochordates retain the chordate characters in the fishlike adult stage. It is easy to see, therefore, how a continuation of this process could lead, by small morphological changes, to organisms similar to the larvae of modern lampreys, jawless fish that represent the most primitive modern vertebrates.

It is clear, however, that vertebrates did not evolve directly from cephalochordate ancestors. Although the modern cephalochordate amphioxus shows many features that one would expect to find in a vertebrate ancestor, it also has a number of basic differences that are inconsistent with a position on the direct evolutionary lineage of the vertebrates. In particular, the presence of a notochord extending to the anterior end of the rostrum and the lack of a clearly differentiated head make cephalochordates unlikely ancestors of organisms with large brains. It seems more likely, therefore, that both vertebrates and cephalochordates represent divergent lineages from a common ancestor, probably of urochordate type. The presence of the fossil cephalochordate Pikaia in rocks of Middle Cambrian age (530 million years ago) indicates that the division had already taken place then and that the first vertebrates must have been present sometime in the early Cambrian period.

Principal Terms

Chordata: A phylum of organisms characterized by the presence of a notochord, a dorsal nerve cord, and gill slits

Deuterostomes: Echinoderms, hemichordates, chordates, and the extinct Vetulicolia, a group linked by features of cell development including retention of the blastopore as anus

Lower Chordates: A group within the Chordata that shows chordate characteristics in the larvae but is separated from vertebrates by the lack of a skeleton

Neoteny: A process by which larval features are retained into the reproductive adult stage

Notochord: A flexible stiffening rod found in primitive chordates

Protostomes: Annelids, mollusks, flatworms, and arthropods, a group linked by features of cell development, including retention of the blastopore as the mouth

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