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.

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