Phylum Phoronida

Phylum Phoronida
Phylum Phoronida (fo-ron´i-da) (L. Phoronis, in mythology, surname of Io, who was turned into a white heifer) contains approximately 10 species of small, wormlike animals that live on the bottom of shallow coastal waters, especially in temperate seas. They range from a few millimeters to 30 cm in length. Each worm secretes a leathery or chitinous tube in which it lies free, but which it never leaves. The tubes may be anchored singly or in a tangled mass on rocks, shells, or pilings or buried in sand. They thrust out the tentacles on the lophophore for feeding, but if the animal is disturbed, it can withdraw completely into its tube.

The lophophore consists of two parallel ridges curved in a horseshoe shape, the bend located ventrally and the mouth lying between the two ridges (Figure 22-1). Horns of the ridges often coil into twin spirals. Each ridge carries hollow ciliated tentacles, which, like the ridges themselves, are extensions of the body wall.

Cilia on the tentacles direct a water current toward a groove between the two ridges, which leads toward the mouth. Plankton and detritus caught in this current are carried by cilia to the mouth. The anus lies dorsal to the mouth, outside the lophophore, flanked on each side by a nephridiopore (Figure 22-1). Water leaving the lophophore passes over the anus and nephridiopores, carrying away wastes. Cilia in the stomach area of the U-shaped gut aid in food movement.

The body wall consists of cuticle, epidermis, and both longitudinal and circular muscles. The protocoel is present as a small cavity in the epistome; it connects to the mesocoel along the lateral aspects of the epistome. A septum separates the metacoel from the mesocoel. Phoronids have a closed system of contractile blood vessels but no heart; the red blood contains hemoglobin within nucleated cells. There is a pair of metanephridia. A nerve ring sends nerves to the tentacles and body wall; a single giant motor fiber lies in the epidermis; and an epidermal nerve plexus supplies the body wall and epidermis.

There are both monoecious (the majority) and dioecious species of Phoronida, and at least one species reproduces asexually. The cleavage pattern is radial, and development is regulative. Coelom formation is by a highly modified enterocoelous route, but the blastopore becomes the mouth. A free-swimming, ciliated larva, called an actinotroch, metamorphoses into an adult, which sinks to the bottom, secretes a tube, and becomes sessile.

Phoronopsis californica is a large, orange form about 30 cm long found along the west coast of the United States. Phoronis architecta is a smaller (approximately 12 cm long) Atlantic coast species that has a very wide distribution.