You may think that if you have seen one worm, you have seen them all. In fact there are many different sorts of worms, some so completely unlike the others that they are placed in their own major groups. The invertebrate phylum Sipuncula is one such group. These tiny (1-6 cm) marine worms known as peanut worms are so different from other worms that their origins are mysterious and their relationships with other worms still undiscovered.
They get their common name from their peanut-like appearance. Their most notable characteristic is a long extendible proboscis called an introvert because it can be drawn into the body of the worm when it is disturbed. The body of the worm then takes on a peanut shape. When the introvert is extended, the worm uses tiny tentacles at its end to gather food particles from the water. The tentacles are powered by a hydraulic system and pick up sand, mud, diatoms, protozoa and detritus, which are passed down the mouth to a simple one-way gut consisting of a pharynx, stomach, short intestine and rectum.
There are about three hundred species of peanut-worms found worldwide in the oceans. They are mostly sediment dwellers but a few have found refuge in the shells of molluscs. The majority are found in shallow temperate waters but some have made it to the abyssal depths or survive in icy polar waters.
Peanut worms are unsegmented but they have a body cavity known as a coelom. This was an important evolutionary step because it freed the gut from the movements of muscles. All higher animals have a coelom. Peanut worms scarcely qualify as 'higher animals' but they do share this characteristic with us. The coelom in these animals is used to mature the gametes, which are produced in the coelomic lining. Sexes are separate in the peanut worms and fertilisation is external. Gametes are released into the water where they are fertilized and then develop into free-living 'trochophore' larvae before maturing and settling into the adult bottom-dwelling form.
Because they are soft-bodied, peanut worms are not common in the fossil record but a few are known from the Cambrian Period when many other worm groups also appear including the annelids, pogonophora, echiura and priapulids. Sipunculids appear to be an offshoot of the molluscs and because they share a coelom with the priapulids are often grouped with them as 'minor coelomate protostomes'. Genetic analysis may eventually clear up their origins and relationships.
Although not common, asexual reproduction by fission is also known in the peanut worms. They can divide themselves in two by transverse fission and then the two halves generate new body parts.
Other characteristics of these worms include a simple brain and a ventral nerve chord, like other protostomes (where the blastopore in the blastula develops into a mouth). There is no circulatory or respiratory system, with oxygen simply passing by diffusion from water to tissues. There are however simple nephridia for removing waste particles from the body, which are excreted as ammonia. The body wall consists of an outer cuticle, epidermis, both circular and longitudinal muscles and a peritoneum.
For burrowing animals, sipunculids have a lot of sense organs including nurosensory cells and sensory buds plus ciliated chemoreceptors on the tentacles. They even have simple eyes. The nervous system is similar to annelids,
Sipunculids are of little importance to humans, although it has been reported that some Chinese make a jelly from these worms that is considered a delicacy. For the most part though, peanut worms have been quietly living their lives out for hundreds of millions of years, relatively unchanged but managing to compete against higher and more developed organisms for all that time.
References: Meglitsch, P. Invertebrate Zoology