MARINE WORM DIVERSITY:
The marine worms include several groups of animals that occur in a great variety of sizes, shapes, and colors.
Color each worm as it is discussed in the text. Begin with the flatworm and notice that the pharynx, located on the under or ventral side, is visible from the top through the thin body.
The free-living flatworms range in size from almost microscopic to 60 cm (24 in) in length. The great majority of the 3000 known species are marine, and most of these are bottom dwellers, living in sand or mud under rocks and algae. The marine flatworm illustrated here depicts a generalized form (such as Notoplana), and is commonly found worldwide in the rocky intertidal zone. It lives on the underside of large boulders, and glides along the rock by means of the cilia on its ventral surface. Notoplana has two dark eyespots at the anterior of its brownish-gray body. The dark area located on the midline of the body is the only opening of the digestive tract. What appears to be ruffled curtain is the retracted pharynx, which can be extruded for food gathering (shown in the illustration). The flatworm shown here is a nocturnal predator, feeding on small molluscs, crustaceans, and other invertebrates.
The ribbon worms (over 600 species) are generally thought to be related to flatworms. They are capable of tremendous lengthwise extension; a worm that measures 20 cm (8 in) when contracted can stretch to over a meter (3 ft)! The ribbon worm shown here lives in a parchment-like tube among the algae, mussels, and other organisms on low intertidal and subtidal rocks and pilings along the west coast of North America.
Unlike the flatworms, ribbon worms possess a complete digestive tract (both mouth and anus). Ribbon worms have a food gathering device called an eversible proboscis, which, unlike the eversible pharynx of polychaete worms (Plate 27), is not an extension of the digestive system. When not in use, the proboscis is retracted inside a cavity above the digestive tract. The proboscis can be
everted (shot out) anteriorly to coil around the worm’s prey. A very sticky mucus is secreted from the proboscis to aid in the capture of the prey. In some ribbon worm species, the proboscis may be longer than the rest of the worm, and may be equipped with piercing barb, or stylet, and poison glands. Ribbon worms are generally nocturnal carnivores, feeding on other worms, molluscs, crustaceans, and small fish.
The peanut worms, or sipunculids, are a group of about 300 species. They live burrowed in mud or sand flats, in muddy crevices between rocks, in coral crevices, in abandoned shells of gastropods, or in the tubes of polychaete worms. Peanut worms range in size from 0.3 to 72 cm (0.08 – 28 in); the average length is about 10 cm (4 in). Their bodies consist of two basic sections: the rounded, bulbous trunk and the narrower introvert. The introvert is the anterior portion of the worm’s body, and can be retracted into the trunk. The mouth is at the tip of the introvert and is surrounded by ciliated tentacles that are used in filter or deposit feeding. A peanut worm with a 5-cm (2 in) trunk can extend its introvert out 15 cm (6 in) in search of food, while it stays safely in a crevice.
The nematodes or round worms are the most common of the worm groups in both the terrestrial and marine realms. However, they are frequently overlooked because most are a few millimeters or less in length. Almost all nematodes look very similar. The worm is encased in a translucent, elastic cuticle which gives it a smooth, unornamented appearance. The mouth, surrounded by three blunt lips, opens at the anterior end of the worm. The posterior end tapers to a point. Nematodes are extremely successful parasites. This cuticle is impervious to noxious chemicals such as stomach enzymes, and many nematodes are intestinal parasites of vertebrates. They also parasitize plants, feeding on the sap and cell fluids. Many species are free-living, occurring by the millions in terrestrial soils as well as in marine muds and sands where they ingest the organic material contained in the substrate.