Marine Invertebrate: Ribbon Worms

 

    Also called Nemertean worms, ribbon worms can reach great lengths of at least 160 ft (50 m), although many are small and inconspicuous.  While they are commonly slightly flattened, the longest are cylindrical and are often called bootlace worms.  The majority of ribbon worms live in the sea under the rocks, among undergrown or in sediment, and some are parasitic.  A few species live inside the shells of mollusks and crabs.

    Nemertean worms have long, unsegmented bodies with strong muscles in the body wall that can shorten the worm to a fraction of its full length.  Unlike flatworms, ribbon worms have blood vessels and a complete gut with mouth and anus.  It is often difficult to distinguish between the front and rear end of the worm, but most species have many simple eyes at the front.  The most characteristic feature of these worms is a strong, tubular structure called a proboscis that lies in a sheath above the gut.  It can be thrust out by hydrostatic pressure, either through the mouth or a separate opening, and is used to capture prey.  In some species, the probiotic is armed with a sharp styler.

    Most marine ribbon worms have separate sexes and their numerous, sample gonads produce either eggs or sperm.  These are usually shed into the sea through pores along the sides of the body.  Some species cocoon themselves together in a mucous net where the eggs are duly fertilized.   In some types of ribbon worms, the eggs develop directly into juvenile worms, while others initially hatch into juvenile worms, while others initially hatch into various types of larvae.  The long, fragile bodies of ribbon worms tend to break easily but they have the useful ability to regenerate any lost parts.  Some species even use regeneration as a method of asexual reproduction, where the body breaks up into several pieces and each piece develops a new head and tail.

From Ocean: American Museum of Natural History