The internal skeleton of Sea cucumbers (Holothuroidea) has been diminished to the point that it consists only of minute bits of crystalline matter in the skin. Most species lie on their sides on the ocean bottom, thus developing a definite upper and lower surface of the body. Some that have tentacles can burrow as well as travel by contracting their muscles. Species with tube feet can walk slowly by means of the tube feet’s tiny suckers.
The tentacles are modified tube feet that surround the animal’s mouth. They are used to trap small particles of food and thrust them into the mouth. Certain species eject a white fluid that rapidly congeals into a sticky mass and serves to entangle a would-be predator. Others can cast away their internal organs, perhaps to temporarily satisfy a predator while the sea cucumber makes its escape. The organs are regrown later.
Although lacking arms, Sea urchins (Echinoidea) have five double rows of tube feet and long spines that move on ball-and-socket joints. Their bodies can be spherical, heart-shaped or flattened, with a crystalline, chalky skeleton of flattened plates fitted together like a mosaic and lying just under the skin. The tube feet and spines allow the urchin to creep over rocky surfaces or through the sand. Spines of some species contain poison glands.
The needle-like and brittle spines are a hazard for swimmers and skin divers. They pierce the skin and break off, discharging a mild venom that can be painful. Sea urchins may be plant-or meat-eaters, but will scavenge for almost anything where food is hard to find. In some, the mouth, which is on the underside of the body, has teeth that project to scrape algae and other food off rocks and even dig hiding places in rock or coral. In some countries parts of the sea urchin are considered a real delicacy.
Comb jellies (Ctenophora) are important elements of plankton. They resemble delicate, transparent jellyfish, and most of them drift freely in the water. The name comb jelly comes from eight rows of tiny, comblike structures on their bodies that are actually rows of cilia matted together. These combs beat in the water to propel the animals forward. Some species have two long tentacles that can be withdrawn into the body. Comb jellies feed on other plankton that they catch with special sticky cells, either in their tentacles or on the body surface itself in species without tentacles. Many comb jellies are luminescent, and some emit brilliant flashes of color at night.
Swimming with undulating movements, the graceful comb jelly Venus’s girdle (Cestum veneris) looks like a transparent belt.The comb jelly (Eurhamphaea vexilligera) displays the eight rows of combs that give its name.
Seemingly floating in space, a comb jelly moves through the water and displays its luminescence.
Moss animals (Bryozoa) are small, colonial bottom dwellers which fix themselves to a variety of surfaces in the water: other animals, or rocks, seaweeds, twigs or pilings. Some are microscopic, but some others grow to be three feet (0.9m) long. Some colonies look like flat sheets of moss; others grow into shapes like fans or twigs. Each individual in a colony has a tough, horny or chalky outer covering and a mouth surrounded by tentacles. Cilia on the tentacles make water currents to direct bits of food to the mouth. When upset the animal retracts its tentacles.
A fleshy stalk protrudes from the beak and anchors the animal. The stalks of a few lamp shells can move and help the animals burrow in mud or sand. Some lamp shells have limy shells with long stalks; others have horny shells with fleshy, footlike projections. Lamp shells feed by opening their shells and drawing water in by beating their cilia, which are attached to a body part that filters food from the water.
Sea squirts (Tunicata) attach themselves to the sea bottom or rocks, seashells, the backs of certain crabs, pilings and ships’ hulls. They are found in shallows as well as in fairly deep water. Sea squirts are shaped like hollow jars and get their name from the fact that they contract and squirt water through their siphons when touched. Though sea squirts resemble sponges, their appearance is deceptive, for they are related to, and in fact may be distant ancestors of , the higher animals with backbones, including humas. In the larval stage the sea squirt swims freely and looks like a tadpole. Along the underside of the larva’s nerve cord is a notochord, a gelatinous rod that is a kind of primitive backbone. This rod is also found in fish, animals that do have backbones.
In the transition from larva to adult, the notochord disappears, being absorbed into the body after the larva has attached itself to something of a sucker on the front of its head. The sea squirt’s body is attached to the sea bottom or some other object by a stalk. At the other end are two siphons: one developed from the tadpole larva’s mouth to draw water into the body, the other to expel it. As water passes through its mouth the sea squirt removes the food and oxygen from it before it is passed out of its body. The inlet and outlet siphons of the sea squirts (Clavellina picta) in this tiny colony show up quite plainly at the top.
The lancelet (Branchiostoma) as an eel-like animal that stands between two dominant groups that makes up the animal kingdom: vertebrates and invertebrates. It has no distinct head, but it has the beginnings of eyes, a mouth, a fin extending along most of its body and a notochord that is well developed and is widely regarded as the evolutionary beginning of the backbones, which is found in the vertebrates.
The lancelet (Branchiostoma) as an eel-like animal that stands between two dominant groups that makes up the animal kingdom: vertebrates and invertebrates. It has no distinct head, but it has the beginnings of eyes, a mouth, a fin extending along most of its body and a notochord that is well developed and is widely regarded as the evolutionary beginning of the backbones, which is found in the vertebrates. It is believed that the lancelet is related to ancestors of the vertebrates and bridges the gap between the sea squirts and the most primitive fishlike vertebrate. The lancelet usually lies buried in the sand of coastal waters by day with only its mouth exposed. Food particles are filtered from the water by tentacles that cover the mouth. At night the animal emerges to swim with fishlike movements of its long body.