All ribbon worms are highly muscular but lack the co-ordination of nerve and muscle that higher worms have. There is a brain consisting of simple nerve ganglia. Some have primitive hearing organs, and the characteristic slits along the sides of the head (suggestive of a mouth) seem to contain important organs of sensation. Although there are a few hermaphroditic species, in most ribbon worms the sexes are separate. There is, however, a strong tendency toward asexual reproduction, and associated with this is a habit of breaking up into many pieces when handled. The fragments then regenerate complete worms. Professor Wesley Coe of Yale University found that a certain species of ribbon worm could be cut repeatedly until eventually miniature worms less than one one-hundred thousandth the volume of the original were obtained. An adult can live a year without food, according to Professor Coe, compensating for lack of nourishment by diminishing in size.
The ribbon worms are unique in the possession of an extensible weapon called a proboscis, enclosed in a sheath and capable of being suddenly everted, hurled out, and coiled around the prey, which is then drawn back toward the mouth. In many species the proboscis is armed with a sharp lance, or stylet, which if lost is quickly replaced by another held in reserve. All ribbon worms are carnivorous, and many prey on the bristle worms.
THE ANNELID (ringed, or segmented) worms include several classes, one of which, the Polychaeta (many bristles) includes most marine annelids. Many of the polychaetes, or bristle worms, are active swimmers that make their living as predators; others are more or less sedentary, building tubes of various sorts in which they live, either feeding on detritus in sand or mud or on plankton which they strain from the water. Some of these worms are among the most beautiful creatures of the sea, their bodies shining with iridescent splendor, or adorned with feathery crowns of tentacles in soft and beautiful colors.
In their structure they represent a great advance over lower forms. Most of them possess a circulatory system (although the blood worm, Glycera, much used as bait, has no blood vessels but a blood-filled cavity between the skin and the alimentary canal) and so are able to dispense with the thinness of body of the flatworms, for the blood flowing through vessels transports food and oxygen to all parts of the body. The blood is red in some, green in others. The body consists of a series of segments, several of the anterior ones being fused to form the head. Each segment bears a pair of unbranched, unsegmented paddle-like appendages for crawling or swimming.
Bristle worms include many diverse forms. The familiar nereids, or clam worms, often used for bait, spend most of their lives in crude burrows among stones on the sea bottom but emerge to hunt or, in swarms, to spawn. The sluggish scale worms live under rocks, in muddy burrows, or among the holdfasts of seaweeds. The serpulid worms build variously shaped limy tubes from which only their heads emerge; other worms, like the beautifully plumed Amphitrite, form mucous tubes under rocks or crusts of coralline algae or on muddy bottoms, and a worm of colonial habit, Sabellaria, uses coarse sand grains to build elaborate structures that may be several feet across. Though honeycombed with the burrows of the worms, these massive dwelling places are strong enough to bear the weight of a man.
THE ARTHROPOD (jointed foot) phylum is an enormous group, comprising five times as many species as are included in all the rest of the animal phyla combined. The arthropods include the crustacea (e.g., crabs, shrimps, lobsters), the insects, the myriopods (centipedes and millipedes), the arachnids (spiders, mites, and king crabs) and the tropical, wormlike Ony-chophora. All marine arthropods belong to the class Copepod Crustacea except for a scant handful of insects, a few mites and sea spiders, and the king crabs.
Whereas the paired appendages of the annelids are simple flaps, those of the arthropods possess multiple joints and are specialized to perform such varied functions as swimming, walking, handling food, and gaining sensory impressions of the environment. Whereas the annelids interpose only a simple cuticle between their internal organs and the environment, the arthropods protect themselves by a rigid skeleton of chitin impregnated with lime salts. This, in addition to being protective, has the advantage of giving a firm support for the insertion of muscles. On the other hand there is the disadvantage that, as the animal grows, the rigid outer covering must be shed from time to time.
The crustaceans include such familiar animals as crabs, lobsters, shrimps, and barnacles, as well as less-known creatures like the ostracods, isopods, amphipods, and copepods, all of which are important or interesting for one reason or another.
The ostracods are unusual arthropods in that they are not segmented but are enclosed in a two-part carapace, or shell, flattened from side to side, and opened and closed by muscles like a mollusk’s shell. The antennae act as oars and are extended through the opened carapace to row the little animal through the water. Ostracods often live in seaweeds or in sand on the ocean floor, usually being quiet by day and coming out to feed at night. Many marine ostracods are luminous and as they swim about emit little puffs of bluish light. They are one of the chief sources of phosphorescence at sea. Even when dead and dried they retain the phosphorescent quality to an astonishing degree. Professor E. Newton Harvey of Princeton University says in his authoritative volume Bioluminescence that during the Second World War Japanese army officers used dried ostracod powder in advanced positions where use of flashlights was prohibited—by adding a few drops of water to a little powder in the palm of the hand, they could obtain enough light to read dispatches.
Copepods (oar-footed) are very small crustaceans with rounded bodies, jointed tails, and oarlike legs with which to propel themselves jerkily. In spite of their minute size (from microscopic to half-inch) the copepods form one of the basic populations of the sea, and are food for an immense variety of other animals. They are an indispensable link in the food chain by which the nutrient salts of the sea are eventually made available (via plant plankton, animal plankton, carnivores) to larger animals such as fishes and whales. Copepods of the genus Calanus, known as “red feed,” redden large areas of ocean surface and are eaten in prodigious numbers by herring and mackerel and also by certain whales. Birds of the open sea such as petrels and albatrosses are plankton feeders and sometimes subsist largely on copepods. In their turn, the copepods graze on diatoms, eating sometimes as much as their own weight in a day.
Amphipods are small crustaceans that are flattened from side to side, while isopods are flattened from upper to lower surface. The names are a scientific reference to the kinds of appendages possessed by these small creatures. The amphipods have feet that can be used both for swimming and walking or crawling. The isopods, or “equal-footed” animals, have appendages that show little difference in size and shape from one end of the body to the other.
On the shore the amphipods include the beach hoppers, or sand fleas, that rise in clouds (leaping, not flying) from masses of seaweed when they are disturbed, and others that live offshore in seaweed and under rocks. They eat fragments and bits of organic debris and are themselves eaten in great number by fish, birds, and other larger creatures. Many amphipods wriggle along on their sides when out of water. Sand hoppers use their tails and posterior legs as a spring and progress by leaps; other species swim.