Mollusca

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Mollusk is a common name for members of a phylum of soft-bodied animals (Latin mollus, “soft”), usually with a hard external shell. The mollusks represent a diverse group of marine, freshwater, and terrestrial invertebrates, including such varied forms as snails, chitons, limpets, clams, mussels, oysters, octopuses, squid, cuttlefish, tusk shells, slugs, nudibranchs, and several highly modified deep-sea forms. They all have one anatomical feature in common, the presence of a shell at some stage in the life cycle. Although most mollusks have a shell as adults, the octopus, squid, and deep-sea forms do not. They do however have a small, shell-like structure, called a shell gland, present for a short time during embryonic development. The mollusk phylum is the second largest in the animal kingdom, after the arthropods.

Scientific classification: Mollusks make up the phylum Mollusca. In the class Aplacophora, the body is wormlike. No shell exists, only a tough mantle, and the foot has virtually been lost. The three orders of the class Polyplacophora (chitons) have a series of eight shell plates (valves) in a row and are well adapted to clinging on rocks. The mainly fossil Monoplacophora is now known to have one living genus, Neopilina, discovered in deep water in 1952. The animal has a single flat shell and multiple gills. The class Bivalvia have a shell divided into two valves, and they feed with their gills. As a consequence the head is poorly developed. Members of the class Scaphopoda (tusk shells) have a long, tapered, slightly curved shell and live on sandy bottoms. Members of the class Gastropoda (snails and slugs) are asymmetrical and have only one shell or, as in slugs, are shell-less. The three subclasses of the Gastropoda are the Prosobranchia (mostly marine snails, with three orders), Opisthobranchia (sea slugs and their allies, with eight orders), and Pulmonata (lunged mollusks, largely freshwater and terrestrial, with two orders). The class Cephalopoda are modified by reduction of the foot and shell and the development of arms around the mouth. The two subclasses are Nautiloidea ( Nautilus, with four gills and other archaic traits such as an external shell) and Coleoidea (octopuses, squid, and cuttlefish, with two gills and other advanced traits).

Mesozoa

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Mesozoa (Greek mesos, “middle”; zōion, “animal”), phylum or superphylum of life forms, sometimes thought to be transitional from unicellular to multicellular organisms. The body consists of a layer of outer cells surrounding internal reproductive cells; it contains no real organs. Except at the time of dispersal, mesozoans live as internal parasites of marine invertebrates. Some authorities consider them degenerate flatworms; others deny that they are animals. The group contains about 50 species placed in two classes or orders.

Loricifera

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Loriciferan, name of a group of tiny marine animals, first identified in 1974, and established as a new phylum in 1983 by Danish zoologist Reinhardt Kristensen. Loriciferans are unusual, nearly microscopic creatures 0.25 to 0.33 mm (0.01 to 0.013 in) long. They are generally oval in shape. The head ends in a beaklike conical mouth that can be retracted into the body. The mouth is surrounded by nine rings of bristly scales, the first set of which point forward, the rest backward. The body is encased in several hard plates that resemble a corset, from which the group gets the name Loricifera, meaning “corset-bearer.” At the hind end of the body is an anus. Much of the body cavity of adults is filled with either developing eggs or testes. There is a specialized immature form known as a Higgins larva. It has a pair of oarlike scales, called toes, at its hind end which are used for locomotion. Little is known about the reproduction and life history of loriciferans.

Scientific classification: Loriciferans make up the phylum Loricifera. The first discovered loriciferan is classified as Pliciloricus enigmatus.

Cycliophora

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Symbion, a tiny marine organism so different from any other that it has given rise to a new phylum (see Classification). Symbion (meaning “living with”) is a commensal, a harmless companion that lives on the mouth hairs of the Norwegian lobster and feeds on the scraps the lobster leaves behind after its messy meals. Symbion's distribution is unknown but may coincide with that of its lobster host.

Symbion takes on different forms throughout its strange life cycle. The most common form is the feeding stage, which is about 0.1 mm (0.004 in) long. The body is urn shaped and attached to the lobster by a short stalk and an adhesive disk. On the other end is a funnel-like mouth topped with a ring of microscopic hairs, or cilia. The mouth leads to an S-shaped esophagus, or throat, and a U-shaped digestive system. The first part of the U forms a stomach lined with ciliated and secretory cells; farther along it becomes an intestine, also lined with cilia. Because of the U shape of the digestive system, the anus, or excretory opening, is near the mouth. The two-lobed brain is situated between the funnel base and the anus. The outer surface of the animal is layered and sculptured with pentagonal shapes. Symbion replaces its mouthparts and nervous system several times during its life by forming an internal bud consisting of a new digestive tract and brain. As this bud matures, it eventually replaces the old structures.

The reproductive cycle is unusual and complicated and is an important reason for putting Symbion in its own phylum. Symbion reproduces in two different phases: asexual budding and sexual fertilization. In the asexual phase, the feeding stage forms an internal bud containing an embryo that develops without fertilization into a Pandora larva, a type of free-swimming larva. The Pandora larva escapes and settles on the same lobster host, developing into another feeding stage.

Symbion's discovery was announced by the Danish scientists Peter Funch and Reinhardt Mobjerg Kristensen in 1995. They documented its curious life cycle and provided evidence to distinguish it from other species to which it may be related, such as rotifers, moss animals, and entoprocts, small marine and freshwater animals that Symbion resembles.

Scientific classification:Symbion pandora is the sole member of the phylum Cycliophora, or “wheelmouth” animals. It is also the only member of the family Symbiidae, order Symbiida, and class Eucycliophora.

Hemichordata

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Acorn Worm, common name for simple, wormlike marine animals in the hemichordate phylum. They are of special interest because of their close relationship to chordates. This connection is evident in the adult anatomy. Some representative acorn worms have gill slits, traces of a supporting structure resembling a notochord, and a tubular nerve cord, which are features characteristic of vertebrates. The larval stages of acorn worms, however, are very much like those of echinoderms such as starfish, indicating a remote common ancestry of echinoderms and vertebrates.

The hemichordates are divided into two classes comprising about 50 species. The first class, the acorn worms, consists of animals that average 10 cm (4 in) in length, although some species may be up to 1.5 m (up to 5 ft) long. They construct burrows, commonly U-shaped, in sand of shallow seafloors using an extendable, muscular proboscis attached to a thick collar that resembles an acorn—hence the name. They secrete a slime that collects food particles on the proboscis and collar, but some species filter sediments and sand through a complicated pharynx with many gill slits. The second class consists of small, usually colonial animals of the deep sea. They are not worm-shaped but stout, and they usually construct tubes. Food is captured by tentacles that project from the tube. The body is much simplified, and gill slits are reduced to one pair or none.

See also Balanoglossus.

Scientific classification: Acorn worms make up the class Enteropneusta in the phylum Hemichordata. The other class of hemichordates is Pterobranchia.