Marine Worms
Group 2 - Marine Worms (Flatworms & Segmented Worms): In this section we discus Flatworms that make up the phylum Platyhelminthes. Most flatworms are internal parasites.
The phylum of Annelida are a more complex segmented worms, both terrestrial and marine. Most marine annelids are in the class Polychaeta, which include approximately 10,000-20,000 annelid species.
Choose an Invertebrate Category
5 Fun Facts - Flatworms
Flatworms have the simplest body structure of all worms. They are bilaterally symmetrical, meaning both halves of the organism are exactly the same. They only have on body opening, used to both take in food and expel waste. Free-living marine flatworms are brghtly colored and they can often be seen on coral reefs and on the ocean floor.
Characteristics: Flatworms do not have a respiratory system or circulatory system, but they do have a basic central nervous system and a brain.
Hermaphroditic: Flatworms are hermaphroditic and most can reproduce both asexually and sexually.
Asexual Reproduction: Many flatworms can clone themselves through fission. One halve of their body attaches to a hard surface while the other half pulls away, splitting the body into two or more pieces. The pieces then regenerate, forming two complete organisms.
Sexual Reproduction: Sexual reproduction most often occurs through cross-fertilization, where both individuals fertilize each other. Fertilized eggs are then laid as egg masses or developed inside the body, depending on the species.
Penis fencing: Some larger free-living flatworms engage in an activity known as penis fencing. When this occurs, the two flatworms fight to impregnate each other. The winner inseminates the loser, who then has the task of egg development.
5 Fun Facts - Segmented Worms
Most marine annelids (segmented worms) are in the class Polychaeta, and can be found throughout the oceans, at all depths and temperatures. While most species are typically less than ten centimeters in length, the largest of the polychaetes, the sand striker or bobbit worm (Eunice aphroditois), can grow up to three meters in length.
Characteristics: Annelids have segmented bodies and these sections are often covered in bristles called setae.
Movement: Annelids mov using longitudinal muscles that lengthen and shorten their segments and circular muscles that contract and expand their diameter. This combination makes them good at crawling or burrowing.
Asexual Reproduction: Most annelid species can reproduce asexually, either through budding or fission. The method of sexual reproduction depends on the individual species. Some polychaetes breed and reproduce only once during their lives. Others reproduce throughout their lives or through several breeding cycles.
Defense: Most annelids defend themselves by hiding. Polychaetes that live in tubes, such as Christmas tree worms (Spirobranchus giganteus) and feather-duster worms (family Sabellidae), will retract into their tubes at the first sign of danger.
Important role to play: Much like some flatworms, marine annelids play an important role in nutrient cycling by feeding on the bodies of dead animals. Theiur small boedies allows them to access all parts of a carcass, even inside its bones.