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Eurythoe laevisetis


Catherine Russo (2014)


Fact Sheet



Physical Description


Light Preference (Experiment)

Life History & Behaviour


Reproduction: Sexual

Reproduction: Asexual (Experiment)



Anatomy & Physiology

External Anatomy

Internal Anatomy

Evolution & Systematics

Biogeographic Distribution

Conservation & Threats

References & Links

Few studies have been done on the internal anatomy of Eurythoe laevisetis so most of the information presented in these sections have been based off textbook polychaete anatomy.

Figure 15: General internal anatomy of a polychaete worm (Hickman et al. 2012).

Circulatory System:

Most polychaetes have a closed circulatory system that is comprised of medial, dorsal and ventral longitudinal vessels that are connected to smaller vessels, capillary beds and gut lacunae. Blood flows anteriorly in the dorsal vessel towards the pygidium in the ventral vessel. The ventral vessel is located underneath the gut and has branches that supply blood to the branchiae, the epidermis and the body wall. From these areas blood flows back to the dorsal vessel via lateral vessels that are located in each segment (Edmonds et al. 2000).   


Most polychaetes possess two fluid systems - a coelom and a circulatory system. Both systems are vital for respiration and the excretion of waste products. The coelom is connected to the exterior of the body by nephridia and gonoducts that are usually arranged segmentally. The inner end of the nephridium opens up into the coelom located in the segment in front of it. Podocytes (used for filtration) are located between the coelom and circulatory system. When the pressure in the blood vessels is increased filtered vascular fluid containing waste products is pushed into the coelom. Waste is then excreted via the nephridium (Edmonds et al. 2000).

Nervous System:

In most polychaetes the cerebral ganglion is located in the prostomium. There are varying levels of complexity of the brain however it is assumed that Eurythoe laevisetis has a more complex brain because it is quite an active species. The cerebral ganglion is connected to the nerve cord via two cirum-oesophageal connectives. The brain is divided into three sections – the forebrain, the midbrain and the hindbrain. The forebrain contains palpal and buccal centres, the midbrain contains antennal and optical centres and the hindbrain contains the nuchal organ centres. The ventral nerve cord runs the entire length of the body and branches off into a ganglion in each segment. From the ganglion segmental nerves connect to the gut, muscles and body wall (Edmonds et al. 2000).

Sensory Organs:

Eurythoe laevisetis has a wide range of sensory organs. These include palps, antennae, eyes, statocysts and nuchal organs.
Palps and antennae are both found on the head of the worm with the palps being innervated from the forebrain and the antennae from the midbrain. Both play tactile roles in helping the fireworm determine its surroundings and find food (Edmonds et al. 2000).
Eurythoe laevisetis has four eyes that are arranged in a square shape. Each eye consists of two confluent depressions, a deep ventral cup and a shallow dorsal cup. The lenses have a large surface area and curve from the lateral to the dorsal surface of the prostomium. All of the eyes are heavily pigmented. Eyes allow the fireworm to detect light and movement and are useful for detecting predators, hunting for prey and finding suitable hiding places (Marsden & Galloway 1967).   
Statocysts are always found on the anterior end of the body and are dorsally situated. They are innervated from the cirum-oesophageal connectives. Statocysts act as gravity receptors and allow the fireworm to orientate itself. 
Nuchal organs are paired and ciliated and are innervated from the posterior section of the brain. They are situated on the caruncle of Eurythoe laevisetis and are used in chemosensory reception. They allow the fireworm to detect odors and are used when the fireworm is looking for food (Edmonds et al. 2000).