Earlier in this chapter we made reference to these near-vertebrates. Unless you are a frequenter of certain rocky seashores or have fished for the lancelets in China, you have probably never seen a protochordate. In fact, few people even know of their existence. Zoologists, on the other hand, are well aware of them, but in many ways the protochordates are mystery animals to scientists as well as laymen.
The protochordates are not fishes, although they live in water. Everyone has heard about “missing links” and how they provide key information about the course that evolution has taken. By rights, protochordates should be missing links. They have, at some time during their lives, a rod-shaped structure (notochord), the forerunner of the backbone. Thus they stand between the animals without backbones (the jellyfish, shellfish, starfish, insects, worms and all the rest) and those with them (the fishes, amphibians, reptiles, birds and mammals). But instead of solving the problem of how some back-boneless creatures through eons acquired backbones, the protochordates have simply created new problems of their own. Instead of being a key, they turned out to be a headache. In other words, careful study of these animals has failed to reveal the course of evolution.
But the exact location of the protochordates in the scale of life is mainly the concern of scientific specialists. These animals are also interesting in their own right because of the most peculiar lives they lead. They may be separated into three principal groups: the sea-squirts, the acorn worms, and the lancelets.
Sea-squirts are aptly named, for if you touch one it is likely to send forth a couple of small jets of water. Most likely you would mistake the creature for a bit of marine plant life or at best some sort of sponge. It is a small, sac-like object fastened at one end to a rock and bearing two small holes at the other. The body is encased in a jacket or tunic which gives it the appearance of a small bag and from which it gets the name of tunicate.
Sea-squirts or tunicates live only in the sea, many of them on rocks that may be exposed at low tide, others in deeper water, and still others in the water itself, floating free. One of the fascinating things about the fixed sea-squirts is that they start out life as tiny, free-swimming, tadpolelike creatures which soon settle down, lose the tail and gradually assume the typical adult tunicate shape – or lack of it if you will. They spend the rest of their lives in one spot, more like plants than animals. Like plants, some tunicates can reproduce themselves by budding. Whole colonies arise from the buds of a single individual. Tunicates feed on microscopic organisms which they filter out of the water.
Acorn worms live in the mud and are seldom seen. Like the sea-squirts, they are strictly marine. They may be only two inches in length or as long as two feet, but they are rarely any bigger around than an ordinary pencil. At the front end, they have a rounded proboscis which fits into a collar, and it is from this structure that they get their popular name. In the manner of earthworms, although they are not worms at all, these animals swallow quantities of earth or mud from which they digest the organic matter, and, like earthworms, they leave their castings on the surface near the entrance to their burrow.
The lancelets are transparent, fishlike animals, two inches or less in length, which inhabit sandy beaches in many parts of the world. They spend most of their time hidden in the sand, only the snout projecting above the surface. Occasionally they dart about, but they soon return, usually burrowing tail first. Although they look a good deal like a small fish, lancelets do not have eyes or ears, nor do they have a brain, heart or skeleton. They feed on microscopic organisms which they strain out from a current of water flowing through the mouth and out past the gills.
Despite their small size, lancelets are so abundant at one spot near Amoy in southern China that they are fished for food. It has been estimated that about 35 tons are taken during a single season. This is equivalent to more than a billion individuals.