External Anatomy
Polychaetes are a diverse and abundant group of marine segmented worms
(Pamungkas, 2020). They range from less than 1 mm to more than 1 meter in length
(Hutchings et. al, 2014). Their body is divided into three main regions—the head or the
pre-segmental region, segmented trunk, and the post-segmental pygidium. The head
consists of the prostomium, peristomium, and pharynx. On the most anterior part of the
body is the prostomium. This can be retractile and usually bears the eyes, tentacles,
antennae, and palps. Depending on the species, some consists of one or two pairs of eyes.
Antennae and palps are used as sensory organs. Moreover, palps can also be used as
feeding appendages. On the posterolateral margin of the prostomium is a pair of
chemosensory structures called the nuchal organs. These organs are the only
synapomorphy of the Polychaeta that makes them distinguishable from other members of
Annelida (Verdonschot, 2015). Right next to the prostomium is the segment around the
mouth or the peristomium. This consists of a tentacular cirrus, present mostly in ciliary
feeders, that bears a crown of tentacles that may be opened like a fan and the proboscis.
Located on the anterior part of the digestive tract for feeding and sometimes used for
burrowing is the pharynx which is usually eversible. The head is followed by the trunk
that is composed of repeated body segments. These segments are composed of lateral flat
like projections called the parapodia on both of their sides. Furthermore, parapodia bear
chaetae. (Pamungkas, 2020). This bristle-like chaetae can be simple, compound, limbate,
capillary, bifurcate, trifurcate, pinnate, harpoon, pectinate, or spatulate. In addition, these
facilitate the locomotion, feeding, and tube-building of the polychaetes (Verdonschot,
2015). On the posterior end of the polychaetes is the pygidium or the tail. Unlike the
trunk, the pygidium is not segmented and can either be conical or broadly rounded. In
addition, this is the part that contains the dorsal or terminal anus (Pamungkas, 2020).
Moreover, cirri may also be present for some species (Verdonschot, 2015).
However, due to the abundance of polychaetes, many species vary in morphology
(Saeedi et. al, 2022). A few do not possess the parapodia that most of the polychaetes
have. Moreover, most polychaetes have gills like the marine genus Amphitrite. They have
three pairs of branched gills and tentacles that are long and extensible. Also, a marine
genus Arenicola has paired gill on certain segments of their trunk (Verdonschot, 2015).
Meanwhile, to distinguish the polychaetes from other annelids, Glassby and Timm (2008)
stated that it should have a head with sensory appendages, segmental parapodia with
chaetae, ciliated pits or patches (nuchal organs) on the back of the head. But the presence
of parapodia may be an exemption because Verdonschot (2015) stated that there are some
polychaetes that do not possess this.
Internal Anatomy
Aside from being segmented, the body wall of the polychaetes is made up of
circular and longitudinal muscle fibers. The moist and cellular cuticle that comes from
the epidermal epithelium surrounds these muscle fibers. In addition, different organ
systems are also present in polychaetes. Like all annelids, polychaetes have a brain or a
cerebral ganglion in their head. Their brain structure varies depending on the lifestyle
they have. The brain is then connected to the ventral nerve cord that runs the length of the
body of the polychaetes. Moreover, the digestive system of the polychaetes is made up of
the foregut, midgut, and the hindgut. The midgut is connected to the exterior part by the
short hindgut through the anus that is in the pygidium. Two fluid systems are also present
in polychaetes: coelom and circulatory system. These are involved in excreting waste
products. However, a circulatory system is not present in a lot of small polychaetes.
Furthermore, different kinds of sensory structures are found in the polychaetes. Palps and
antennae that are in the head are considered as sensory structures, but palps are also used
as feeding appendages for others. Satocysts are balanced sensory receptors. Nuchal
organs, on the other hand, are chemosensory structures that are found in nearly all
polychaetes. Also, polychaetes have a variety of epidermal sensory cells that are sensitive
to light and touch—such as lateral organs (Verdonschot, 2015).
Physiology
Most polychaetes' guts have been morphologically and physiologically modified
to get the maximum amount of organic matter from the mineral substance that is their
food. According to Verdonschot (2015), a study conducted by Kermace (1995) on
Arenicola marina showed this process of food intake in polychaetes. Meanwhile, gas
exchange happens by the diffusion of gas through the body wall.
The blood–vascular system of most polychaetes is well developed with blood
circulating in the anterior direction through the dorsal vessel. Part of the blood is
transported toward the ventral circulation via lateral vessels in most segments. Blood is
carried posteriorly by the ventral vessel. The parapodia, nephridia, body wall, and
stomach all have smaller vessels that transport blood. Hemoglobin, chlorocruorin, and
hemerythrin are respiratory pigments found in polychaetes’ blood. Furthermore,
polychaetes—particularly the Nereididae—have a strong osmoregulatory ability. The
blood and tissue fluids of these tolerant organisms are isotonic with the surrounding
environment (Verdonschot, 2015).
Feeding Behavior
Polychaetes have several feeding modes depending on their behavior and
lifestyle. There are surface and subsurface deposit feeders, filter feeders, carnivores,
omnivores, or herbivores (Checon et. al, 2017). Deposit feeders are those that consume
organic materials from their surroundings while filter feeders take their food from the
particles in the water column (Pamungkas, 2020). Most of the errant polychaetes are
predators and scavengers while those that are sedentary are mostly filter feeders and
some are deposit feeders that consume sediment particles. Other species—usually the
tube-living polychaetes—would use their tentacles to dip into the substrate and collect
some muck and would draw it into their mouths to digest the edible particles. On the
other hand, the predator type of polychaetes would swim through the water in search for
small worms or algae. Most of these have an extendible jaw-like structure to help in
catching prey. Moreover, they possess a muscular, eversible pharynx containing a pair of
toothed, opposing jaws. These jaws allow the prey to be captured or the algae to be teared
into pieces (Verdonschot, 2015). Eunicidae, one of the families of polychaeta, is an
example of species that was reported to include predation, scavenging, and herbivorous
behavior in their feeding. In addition, with the use of their intricate and incredibly hard
jaw mechanism, eunicids are known to drill the calcareous skeleton of hard corals; when
they are abundant, they serve as ecologically significant coral block destroyers and play a
significant role in bio erosion processes. (Diaz & Lopez, 2020).
Locomotion
Depending on the species, polychaetes vary in their locomotion traits. The
extension or retraction of the parapodia and the chaetae facilitates their movement. Their
movements include burrowing, slow creeping, fast crawling, and swimming
(Verdonschot, 2020).
Díaz, A., & López, E. (2020). Four New Records of Eunicidae (Annelida: Polychaeta) from the
Western Philippines Islands. Thalassas, 36(2), 321–332. https://doi.org/10.1007/s41208020-00229-5