There are certain animals which have difficulties in qualifying in all particulars as vertebrates but are, nevertheless, classified as chordates. These interesting connecting links between vertebrates and invertebrates may be called “borderline chordates,” or Protochordates. They comprise three subphyla, namely: hemichordata, urochordata, and cephalochordata.
Dolichoglossus kowalevskyi, found on the Atlantic coast (Fig. 8), may be taken as a representative of the Hemichordata, of which there are only a few genera. This small, fragile, wormlike animal has no common name, because it is not commonly known, an additional reason for resorting to the use of a scientific name. Its body is divided into three general regions: (1) a proboscis used to push aside the sand as the animal works its way along; (2) a short collar; and (3) the trunk, making up by far the greater part of the body.
It lives buried in mud at various localities along the Atlantic seashore where at low tide its burrows may be identified by peculiar coiled piles of castings similar to those deposited by earthworms (Fig. 9).
The hemichord is of special interest to the biologist since, unlike any true worm, it has the three major chordate characteristics. Numerous, U-shaped, pharyngeal gill slits are present in the anterior trunk region. There is a dorsal nerve cord which usually possesses merely a few isolated small cavities in the collar region only. A ventral nerve strand, more or less comparable to that of invertebrates, is connected with the dorsal cord by a nerve ring in the collar region. Finally, an outgrowth from the dorsal side of the digestive tract of the collar region, extending forward into the proboscis, may represent the notochord.
Although hemichordates are wormlike in many structural details, the few chordate features just mentioned suffice to indicate their problematical position between invertebrates and vertebrates.
The Tornaria larvae of hemichords resemble corresponding stages of echinoderms so much that the Tornaria was for a long time thought to belong to some member of that phylum. The similarities of these two larval types have served as a basis for considering the echinoderms as the invertebrate group most closely related to the chordates.
Different species are found widespread in similar habitats the world over. For example, in 1896 during the Harriman Expedition, Dr. W. E. Ritter, an authority on this group of primitive animals, discovered among other kinds along the shore of Alaska a new genus of hemichords, probably the most primitive of them all, which he christened Harrimania in honor of the expedition.
The Urochordata are tunicates or ascidians, so called not only from a peculiar baglike outer lifeless envelope, or “tunic,” with two openings but also because their general appearance suggests an “askidion,” the Greek name for a primitive wine sac made of goatskin.
These degenerate representatives of the chordates are all marine organisms, living for the most part a sedentary life. Molgula manhattensis, DeKay, a common “sea squirt” found along the Atlantic shore attached to piles and other objects, may be taken as. a typical species.
The only structures conspicuous externally are two projections each of which bears a terminal opening (Fig. 10). Water is continuously taken into the body through one of these openings (the mouth) and discharged through the other (the atriopore). Internally, the mouth leads into a pharynx, an enlargement which occupies a considerable portion of the body cavity of the animal. The pharynx is permeated by numerous gill slits which open into the atrial cavity, a space almost completely surrounding the pharynx and opening to the outside through the atriopore. Beyond the pharynx the digestive tract continues for a short distance as a small tube, which usually makes one loop and then opens into the atrial cavity. Located on the dorsal side of the pharynx, between mouth and atriopore regions, is a solid mass of nerve tissue, the neural ganglion. There is no notochord. Thus only one of the three main chordate characteristics is present in the adult.
It is the early life of the tunicate, however, that gives the real clue to its unmistakable relationship to true chordates, which it so little resembles when superficially examined. The egg develops into a free-swimming tadpole-like larva (Fig. 11), showing in its locomotor tail - an unmistakable notochord and a single tubular dorsal nerve cord. This is the reason for the name of the group, “Urochordata” (uro, tail; chorda, notochord).
After the larva swims about for a time and increases in size, it settles down on a suitable support and enters upon its lifetime of stationary existence. The tail, no longer needed for locomotion, is absorbed into other tissues of the body. The central nervous system, reduced to a simple dorsal ganglion, permits only an over-all contraction of the entire body in response to any stimulation. The pharynx, on the other hand, enlarges greatly and becomes the only one of the three chief chordate characteristics to remain well developed in the adult. The tunicate thus sacrifices most of its birthright of chordate characteristics, that is, notochord and tubular nerve cord.
Some tunicates as the result of budding are colonial in habit, living connected together in more or less dependence upon each other, a state of affairs not uncommon among invertebrates but which does not occur elsewhere among chordates. The compound or colonial manner of life is shown, for example, by Botryllus (Fig. 12), a small tunicate that grows in starlike slippery patches over the surface of seaweeds floating in the shallow waters of the seashore. The incurrent openings of the several individuals in the colony are separate and arranged in a circle around a common excurrent opening.
Other colonial tunicates, such as the beautiful transparent “chain salpas,” are pelagic, or free-swimming in habit, forming elongated rafts of barrel-shaped glassy-clear individuals which float near the surface of the ocean, usually many miles off shore.
Most primitive of all these humble cousins of the vertebrates are the microscopic Appendicularia, tiny ghostlike creatures of the vast ocean waters, that live a life of complete freedom and do not relapse, like other tunicates, into unambitious sedentary degeneration, but remain larval “tadpoles” throughout life. Animals such as these, which never “grow up” but become sexually mature while in the larval stage, are said to be paedogenic.
The Cephalochordata, so named because they have a notochord extending into the head region, include only two genera, Branchiostoma and Asymmetron. This subphylum is also called Acraniota because of the absence of a cranium, or brain case. If these animals had a cranium they would have nothing to put in it, for they are brainless little creatures whose nerve cord fails to enlarge at its anterior end into anything like a brain.
Amphioxus (Branchiostoma), the most widely known member of the group, has had enough written about it to fill more than one ponderous tome. For a century or more it was looked upon as a most primitive chordate, a “fish in the making,” illustrating the beginnings of many great things. More recent work has cast considerable doubt upon this point of view, especially when some of the highly specialized features of amphioxus are taken into consideration. Yet this animal has apparently retained a great many relatively simple features, despite the fact that it has probably evolved a long way from the actual ancestors of the chordates. If we direct our attention away from the few specialized structures of amphioxus and toward its many important basic features, as we must always do in any type study, we find that this animal shows us the chordate plan reduced to almost its lowest terms.
Amphioxus dwells in the shallow waters of tropical and semi-tropical seas in locations as far apart as the Bay of Naples, the coast of Peru, Japan, the Indian Ocean, California, the Philippines, West Indies, Australia, North Carolina, Hawaii, Maldive Islands, and China. This wide distribution suggests the great antiquity of the type which, in spite of its restricted means of locomotion, has had time to spread to the uttermost tropical comers of the earth.
Off the coast of southern China, north of the Island of Amoy, amphioxus is so abundant that it forms an important food fishery which has been worked by the Chinese for centuries. Professor S. F. Light writes in Science for July 27, 1923: “Here on this little strip of coast about 400 fishermen, using 200 small boats, are engaged for from two to four hours on the ebb tide of every calm day during the nine months from August to April of each year in dredging for amphioxus for the market. The catch averages about 2600 pounds, well over a ton for each calm day during the nine months of the fishing season and a total of hundreds of tons of amphioxus are taken during the year!”
It is not as a source of Chinese food, however, that the chief interest in amphioxus lies, but because in its development and structure this little animal points the way to the rise of the complicated conditions found in higher vertebrates. It will be necessary later on, when tracing the origin of various vertebrate organs, to go back repeatedly to the stage presented by amphioxus.
Amphioxus, or the “lancelet” as it is commonly called, is an elongated, semi-transparent, fishlike animal, two or three inches in length when fully grown, and somewhat pointed at either end, as its Greek name (am phi, both; oxus, sharp) indicates. In habit it is largely sedentary, lying buried in the sandy bottoms with its anterior end projecting. It is a poor swimmer, coming to rest by lying on its side when not burrowing into the sand.
At the anterior end, guarding the mouth, is a circle of bristle-like cirri attached to the edge of the buccal funnel within which is a whirlpool of cilia, the “wheel organ ” which helps to direct the microscopic food into the mouth (Fig. 13).
Running along the entire dorsal side of the body, then around the tail end and forward on the ventral side is a continuous median fin. This fold of the skin, as it passes around the tail end, expands to form a conspicuous caudal fin. As the fold reaches the region of the atrial pore, anterior to the anal opening, it divides like a letter Y, and extends forward in ventro-lateral folds on either side of the body. It is out of persisting parts of similar folds, which are laid down temporarily in the embryos of fishes, that fins are formed (Fig. 14).
Of all known lower chordates, amphioxus shows the best development of diagnostic chordate features. Extending the entire length of the body just dorsal to the digestive tract is a notochord (Fig. 15). Above the latter is the dorsal, hollow central nervous system which, instead of enlarging anteriorly into a brain, actually is more slender near its anterior end than throughout most of its length. Not far from the mouth, the digestive tract becomes greatly enlarged into a pharynx, the wall of which is penetrated by 26 pairs of long, narrow gill slits. Beyond the pharynx an uncoiled intestine of small diameter leads to an anus located on the left side of the mid-ventral region a short distance from the posterior end of the body. Thus, a post-anal tail is present.
Cilia of the wheel organ and pharyngeal wall ordinarily maintain a continuous flow of water through the pharynx and gill slits. This water not only acts as a respiratory current but also brings in numerous small organisms which serve as food.
Extending the entire length of the pharynx are two grooves, a ventral endostyle and a dorsal epipharyngeal groove. Both are heavily ciliated. In the endostyle numerous glandular cells secrete mucus which is carried anteriorly by ciliary action. Other cilia transport small streams of the mucus dorsally along the inner surfaces of the gill-bars into the epipharyngeal groove. In the latter the cilia carry the mucus posteriorly into the intestine. Microscopic organisms brought into the body by the so-called respiratory current are trapped by the mucus and eventually carried into the intestine. The lining of the latter is also ciliated (an invertebrate characteristic).
As in tunicates an atrial cavity, a specialized feature probably associated with the animal’s particular habits of life, surrounds the pharynx. Water, passing from the gill slits into this atrial cavity, is discharged to the outside through a relatively small atriopore located about two-thirds of the way back on the ventral side of the body. Thus the delicate respiratory membranes are removed from the surface of the body where they would be seriously damaged, if not destroyed, as the animal pushes its way through the sand.
Growing out ventrally from the intestine is a blind sac, the liver diverticulum, lined with glandular epithelium and supplied with a network of blood vessels that represents the beginnings of a vertebrate hepatic portal system, since the blood from the food tube has to pass through this capillary strainer in the liver diverticulum before going forward to the gills and thence over the body.
There is no heart present, but a pulsating ventral blood vessel, lying below the pharynx, is larger than the other blood vessels and is prophetic of the future ventral heart of vertebrates.
The gonads are segmental and can be seen from the outside through the transparent body wall. The germ cells are discharged directly into the atrial cavity from which they are carried to the outside by the respiratory current. The sexes are separate.
Unlike urochordates and hemichordates, cephalochordates probably represent simple advancing forms, and not ones whose simplicity is the result of degradation.