Marine worms: a heterogeneous group of mats!

What we have collocated here with the name of marine worms, is in reality a various group belonging to different taxonomic phyla. In my description I will only focus on phyla that are visible during snorkelling or diving. I will not describe at all microscopic or parasite taxa or that ones which are living only inside the interstitial spaces of marine sediments.

First of all, in comparison to the previous animals belonging to the coelenterate (cnidarians and ctenophores) presenting radial symmetry and 2 layers of tissue, we have to say that all marine worms present a bilateral symmetry and a body structure based on 3 layers of tissue. These acquisitions represent another important step in evolution, driving to more complex and evolutes forms.


The most primitive phyla of marine worms are grouped under the name Acoelomata because they don’t present an inner body cavity called coeloma, which will be the characteristic of following groups. Acoelomata have some primitive organs even if the digestive system is incomplete (without anus). They have a head and they move in a forward direction.

In this first group we recognize some important phyla. Let’s see them one by one!

Phylum PLATYHELMINTHES (flat worms)

Platyhelminthes are bilateral animals with head and tail. Of the 13000 existing species, only about 3000 species are free living (not parasites) and here is where we put our focus.

Free living flatworms belong to the Class Turbellaria. Most of the Turbellaria flat worms are marine and benthic animals. They live both on soft or hard substrates and they are usually predators or scavengers of animal tissue, especially bryozoans or tunicates. Few of them are herbivores, feeding on diatoms.

Turbellaria present the typical flatworm anatomy with 3 body layers and the space of the body filled with a soft tissue of cells called parenchyma. The bilateral structure permits cephalisation and the presence of a primitive ganglion of nerves and some primitive sensitive “organs” for light and chemical detection.

They can move thanks to synchronous beating of cilia or to muscular contractions in the most evolutes.

Most of them have beautiful and charming colours; with these they advise their potential predators that they are poisonous. In nature they can be confused with the Nudibranchia (that we will meet later) but, differently from them, flat worms are no more than 1 mm thick!

(Unfortunately we don’t have any Platyhelminthes photo at the moment. We will scrape up as soon as possible).

Phylum NEMERTEA (ribbon worms)

Nemerteans, like flatworms, have no body cavity and the space of body space is filled by parenchyma. Unlike flatworms, ribbon worms have a digestive tract with two openings, which is more efficient.

Locomotion uses the same strategy of flat worms, even if ribbon worms are normally much longer that the first ones and they can also change length. They present a well developed proboscis that can be rapidly extended by muscular contraction for prey capture. They are active predators and their favourite preys are small crustaceans, molluscs and annelids.

They are abundant in temperate water like in Mediterranean and are common in shallow waters, where they burrow in mud or sand, or live among algae or under rocks and shells. They are quite long, between 20 and 100 cm, even if it is not so easy to see them. (In fact also in this case we don’t have a photo! Give us some time!)

The next group of marine worms is represented by the Pseudocoelomates. It counts 9 phyla of aquatic animals which are characterized by the presence of a pseudocoeloma, a sort of cavity full of liquid in which organs are suspended. In this group the animals are microscopic and they live especially in the sediment or they parasite other organisms. In the Pseudocoelomates we can find Nematoda or round worms. There is anyway no reason to describe them because it is very difficult to see them during diving.

Another step forward is enterprise by the Coelomata, a taxa presenting a proper body cavity (coeloma) where the inner organs are suspended. This group is very successful and it presents the most evolutes marine organisms.


Phylum ANNELIDA (ring worms)

The most important phylum of marine worms is represented by the Annelida. With the Mollusc that we will describe later, Annelida are originated by protostomes (group of phyla where, during development from zygote, the mouth (stoma) is originated first (proto) than the anus).

Characteristic of this phylum is the body which is divided in a series of segments called metameres. Each segment presents a similar musculature, internal organs and body appendages. Annelida presents about 8700 species of terrestrial, marine and fresh water animals.

Only the class of the Polychaeta presents a significant number of marine species (about 5000 species). Polychaeta are relatively common in the intertidal zone and in the shallow water.


The segments, except the head and the tail ends, are poorly specialized and essentially identical among them. They contain similar elements of the organ system. Besides, each segment presents lateral structures for locomotion, called parapodia, doted with setae. Movement is due by the contraction of the hydrostatic skeleton and the parapodia.

In base to their way of life, Polychaetes are divided in two non taxonomic groups: the errant (surface crawlers) and the sedentary (burrowers and tube dwelling).

The first ones present very developed parapodia which they use to walk around on rocks, algae or corrals. They are active and voracious predators and they present jaws and sensory structures extremely developed.

The sedentary polychaetes are living both in the sediments or in a self produced protein or calcareous tube. The parapodia of the metameres are reduced and even assent. They present at the top of the head a sometime spectacular cluster of tentacles used for filter feeding.


Phylum ECHIURA (sausage worms)

Another group of protostomes marine worms is represented by the sausage worms. This is a small group of about 100 species of animals which live inside the sediment or in the rock scissors. Most of them live at big depth.

They have a long, not retractable proboscis that stretches out for a high length on the substrate. Echiura are detritus feeders: the detritus sticks on the proboscis mucus or it is filtered from the water by the proboscis itself.

A typical representative of Echiura in our waters is the original Bonellia viridis. This specie presents a big female, hidden among the rocks, which extends a long green proboscis, visible on the substrate. The male is microscopic, few mm of length, and it use to live as parasite on the big female body (bad male habit!!). Its only aim is to breed with the female (.... No comment!!)


.... it continues!....

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