Animals with no backbone are called invertebrates. They are by far the largest group of animals, making up most of the life on Earth. Instead of a bony skeleton, their bodies use other substances for support or protection, such as fluid or shell.
Arachnids
The class Arachnida includes a diverse group of arthropods: spiders, scorpions, ticks, mites, harvestmen, and their cousins. Scientists describe over 100,000 species of arachnids. In North America alone, there are about 8,000 arachnid species. The name Arachnida derives from the Greek aráchn?, which means spider. The vast majority of arachnids are spiders.
Most arachnids are carnivorous, typically preying on insects, and terrestrial, living on land. Their mouthparts often have narrow openings, which restricts them to eating liquefied prey. Arachnids provide an important service, keeping insect populations under control.
Although technically the word arachnophobia refers to a fear of arachnids, this term is widely used to describe a fear of spiders.
Molluscs
Molluscs have a wide range of body types, but they all have a protective shell. Most molluscs are water-loving creatures, but quite a few are land-based, such as some slugs and snails.
Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills’ cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.
Insects
These small creatures have three pairs of legs and a body divided into three parts. They use two feelers on their head to touch, smell and taste. Many insects also have wings. Adult insects typically move about by walking, flying, or sometimes swimming. As it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles, composed of the front & rear on one side with the middle on the other side. Insects are the only invertebrates to have evolved flight, and all flying insects derive from one common ancestor. Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming. Some species, such as water striders, are capable of walking on the surface of water. Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large, well-organized colonies. Some insects, such as earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate, or rub their wings together, to attract a mate and repel other males. Lampyrid beetles communicate with light.
Jellyfish
Jellyfish have drifted along on ocean currents for millions of years, even before dinosaurs lived on the Earth. The jellylike creatures pulse along on ocean currents and are abundant in cold and warm ocean water, in deep water, and along coastlines. But despite their name, jellyfish aren’t actually fish—they’re invertebrates, or animals with no backbones.
Jellyfish have tiny stinging cells in their tentacles to stun or paralyze their prey before they eat them. Inside their bell-shaped body is an opening that is its mouth. They eat and discard waste from this opening.
As jellyfish squirt water from their mouths they are propelled forward. Tentacles hang down from the smooth baglike body and sting their prey.
Jellyfish stings can be painful to humans and sometimes very dangerous. But jellyfish don’t purposely attack humans. Most stings occur when people accidentally touch a jellyfish, but if the sting is from a dangerous species, it can be deadly. Jellyfish digest their food very quickly. They wouldn’t be able to float if they had to carry a large, undigested meal around.
Crustaceans
Crustaceans are a very diverse group of invertebrate animals which includes active animals such as the crabs, lobsters, shrimp, krill, copepods, amphipods and more sessile creatures like barnacles.
Crustaceans are some of the most important marine life to humans – crabs, lobsters and shrimp are widely fished and consumed around the world. They may also be used in other ways – crustaceans like land hermit crabs may also be used as pets, and marine crustaceans may be used in aquariums.
In addition, crustaceans are very important to other marine life, with krill, shrimp, crabs and other crustaceans serving as prey for marine animals such as whales, pinnipeds, and fish.
Starfish and sea urchins
Starfish are among the principal predators of the intertidal zone. Although several species are found locally, almost all that are seen in the Bartlett Cove area are the five armed mottled starfish, Evasterias. Starfish are echinoderms, related to sea urchins and sea cucumbers. When exposed at low tide a starfish will be inactive, but if it is picked up and turned over, it is frequently possible to see what it has been eating. A starfish feeds on such animals as mussels and snails by everting its stomach or turning it inside out, through its mouth and into the shell of the prey. Digestion takes place outside the starfish’s body.
Sea urchins are sometimes exposed at very low tides in Bartlett Cove. During such tides large numbers of urchins can be seen in the lagoon channel just below the low tide line where they dominate large areas. Sea urchins, relatives of the starfish, are plant eaters and account for the absence of algae from the channel or adjacent areas. Sea urchins play a very important role in the marine environment by determining the amount and type of plants that are present in many areas. Some larger algae can be seen in the quiet waters of the lagoon near the channel. These algae probably survive because sea urchins don’t like the still muddy water outside of the fast-flowing channel.
Sponges
Sponges have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes. Sponges were first to branch off the evolutionary tree from the common ancestor of all animals, making them the sister group of all other animals.
Sponges are worldwide in their distribution, living in a wide range of ocean habitats, from the Polar Regions to the tropics. Most live in quiet, clear waters, because sediment stirred up by waves or currents would block their pores, making it difficult for them to feed and breathe. The greatest numbers of sponges are usually found on firm surfaces such as rocks, but some sponges can attach themselves to soft sediment by means of a root-like base.
Worms
Worms have long, soft, segmented bodies. They breathe through their skin, so they have to keep it damp if they live on land. Bristles on their body help them to move around.
Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda(tapeworms) which reside in the intestines of their host. When an animal or human is said to “have worms”, it means that it is infested with parasitic worms, typically roundworms or tapeworms. Lungworm is also a common parasitic worm found in various animal species such as fish and cats.
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