MARINE WORM DIVERSITY:
Free-living polychaetes, such as Nereis, actively seek out their food and shelter. Other types of polychaetes remain stationary, living in tubes, burrows, or crevices and obtain food by extending their tentacles out into the water. Two ways of attaining food with tentacles are exemplified by the fan worm Sabella, and the burrowing polychaete Amphitrite.
Color the fan worm in the upper left. You may wish to use a bright color for the mass of radioles, as they are very colorful reds and oranges in life. Next, color the enlargement of the tube and radioles. The tube is opaque, but for purposes of illustration it is shown as transparent so you can color the tan body segments of the worm as it resides in its tube.
Fan worms are conspicuous members of wharf piling and coral reef communities. Their beautiful fans, which resemble a flower, are actually a group of tentaclelike projections called radioles, extending from the prostomium (Plate 27). The radioles have lateral branches, or pinnules, covered with fine cilia. When the fan worm emerges from its parchmentlike tube, the radioles spread in a funnel-shaped crown around the mouth. The microscopic cilia beat in unison and create a current, bringing water up through the crown of radioles and out the top. Food particles carried in the current are trapped by cilia and moved down the pinnule to a food groove that runs the length of the radiole.
Now color the diagrammatic enlargement of the radiole and pinnules.
The ciliated food groove carries food toward the mouth; at the base of the radiole, the various food particles are sorted by size. Large particles are rejected; special ciliary tracks on the palps carry these back into the excurrent stream flowing up and out the center of the crown of radioles. The smallest particles are carried to the mouth and consumed. The medium particles are carried to a special ventral sac, where they are stored for use in tube building.
At the base of the radiole crown is a fleshy fold of tissue called the collar, which holds the fan worm securely at the top of its tube. Mucus, secreted from both the collar glands and the glandular ventral plate, is mixed with the medium-sized particles stored in the ventral sac to create a thin thread of tube material. The fan worm rotates slowly, and as it lays down this material along the edge of its papery but flexible tube, gradually repairs and lengthens it.
Color the off-white tentacles of Amphitrite, as well as its red gills and all of the tan body segments. The drawing at the lower right illustrates the three ways a tentacle can transport a food particle to the region of the mouth, where the particle is sorted and either eaten or rejected.
The polychaete Amphitrite lives in semi-permanent burrows on mud flats or in rock crevices in the intertidal zone; it feeds on organic material deposited on the surface of the substratum. Amphitrite possesses a multitude of hollow, distensible, ciliated, prostomial tentacles that reach out over the substratum until a food particle is found. Tentacles move food particles to the mouth one of three ways, depending on the particle size. If the particle is very small, the tentacle forms a shallow ciliated food groove that carries the food to the mouth. For somewhat larger particles, the action of the ciliated food groove is augmented by peristaltic contraction along the length of the tentacle. Very large particles are wrapped securely by the tentacle which is then retracted, carrying the food to the mouth. Each tentacle is pulled separately through the folded upper lip, which sorts by size and rejects the particle or pushes it into the mouth.
Bright red gills filled with blood are located near the head region of the worm. Both the gills and the tentacles may be injured or removed by predators, but the worm is capable of regenerating these structures. The elastic tentacles of Amphitrite can reach out over a wide area: a worm 4 cm (1.5 in) long can cover a diameter of 20 cm (8 in) with a radiating maze of tentacles.