Sea anemone

Sea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. As cnidarians, sea anemones are closely related to corals, jellyfish, tube-dwelling anemones and Hydra.

A sea anemone is a small sac, attached to the bottom by an adhesive foot, called a basal disc, with a column shaped body ending in an oral disc. The mouth is in the middle of the oral disc, surrounded by tentacles armed with many cnidocytes, which are cells that function as a defense and as a means to capture prey. Cnidocytes contain cnidae, capsule-like organelles capable of everting, giving phylum Cnidaria its name . The cnidae that sting are called nematocysts. Each nematocyst contains a small vesicle filled with toxins—actinoporins—an inner filament and an external sensory hair. When the hair is touched, it mechanically triggers the cell explosion, a harpoon-like structure which attaches to organisms that trigger it, and injects a dose of poison in the flesh of the aggressor or prey. This gives the anemone its characteristic sticky feeling.

The poison is a mix of toxins, including neurotoxins, which paralyze the prey, which is then moved by the tentacles to the mouth/anus for digestion inside the gastrovascular cavity. Actinoporins have been reported as highly toxic to fish and crustaceans, which may be the natural prey of sea anemones. In addition to their role in predation, it has been suggested that actinoporins could act, when released in water, as repellents against potential predators. Certain clownfish are not affected by their host anemone's sting.

The internal anatomy of anemones is simple. There is a gastrovascular cavity (which functions as a stomach) with a single opening to the outside which functions as both a mouth and an anus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut. A primitive nervous system, without centralization, coordinates the processes involved in maintaining homeostasis as well as biochemical and physical responses to various stimuli. Anemones range in size from less than 1¼ cm (½ in) to nearly 2 m (6 ft) in diameter.They can have a range of ten tentacles to hundreds.

The muscles and nerves in anemones are much simpler than those of other animals. Cells in the outer layer (epidermis) and the inner layer (gastrodermis) have microfilaments grouped together into contractile fibers. These are not true muscles because they are not freely suspended in the body cavity as they are in more developed animals. Since the anemone lacks a skeleton, the contractile cells pull against the gastrovascular cavity, which acts as a hydrostatic skeleton. The stability for this hydrostatic skeleton is caused by the anemone shutting its mouth, which keeps the gastrovascular cavity at a constant volume, making it more rigid.

Mollusca

The molluscs (British spelling) or mollusks (American spelling) are members of the very large and diverse phylum of invertebrate animals known as Mollusca. There are some 112,000 species within this phylum. The scientific study of molluscs is known as malacology.

The word mollusc comes from the French mollusque, which originated from the Latin molluscus, meaning thin-shelled, from mollis, soft.

Molluscs range from minute snails and clams (micromollusks) to large organisms such as squid, cuttlefish and octopus, which are among the most neurologically-advanced invertebrates.

There are a wide variety of molluscs which are valued by humans as seafood or for their decorative shells. The edible species include many kinds of clams, snails, squid and octopuses.

The vast majority of molluscs live in marine environments, and many of them are found intertidally, in the shallow subtidal and on the continental shelf. Species of octopus and squid live throughout the ocean depths and some species of clam and limpet live in the abyssal depths of the oceans around hot vents.

Not all molluscs are marine: two taxonomic groups or classes, the bivalves and the gastropods, also contain freshwater species. Only the gastropods have representatives that live on land: the land snails and slugs.

Sponge

The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera,porifera translates to "Pore-bearer". They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. Sponges represent the simplest of animals. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. However, recent genomic studies suggest they are not the most ancient lineage of animals, but may instead by secondarily simplified.

There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered.

Echinoderm

Echinoderms (Phylum Echinodermata, from the Greek for spiny skin) are a phylum of marine animals found at all ocean depths. Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period, and contains about 7,000 living species, making it the second largest grouping of deuterostomes, after the chordates; they are the largest phylum without freshwater or terrestrial representatives.

The Echinoderms are important both biologically and geologically: biologically because few other groupings are so abundant in the biotic desert of the deep sea, as well as the shallower oceans, and geologically as their ossified skeletons are major contributors to many limestone formations, and can provide valuable clues as to the geological environment. Further, it is held by some that the radiation of echinoderms was responsible for the Mesozoic revolution of marine life.

Two main subdivisions of Echinoderms are traditionally recognised: the more familiar, motile Eleutherozoa, which encompasses the Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchin and sand dollar) and Holothuroidea (sea cucumbers); and the sessile Pelmatazoa, which consist of the crinoids. Some crinoids, the feather stars, have secondarily re-evolved a free-living lifestyle.

A fifth class of Eleutherozoa consisting of just two species, the Concentricycloidea (sea daisies), were recently merged into the Asteroidea. The fossil record contains a host of other classes which do not appear to fall into any extant crown group.