p1: The aim of this chapter is to introduce you to the annelid Platynereis dumerilii
p1: more specifically its morphology and its body plan, its life cycle and
p1: its embryonic development. At the end of this chapter you will be able to recognise
p1: the main structure of the animal as well as its developmental stages.
p1: For many decades, Platynereis has been used as a wonderful model system
p1: to address various questions relative to diverse topics, such as chronobiology and
p1: ecology, but also developmental biology in an evolutionary context and regeneration.
p1: In addition to its important phylogenetic position within bilaterians and metazoans
p1: as we have seen in chapter one, its current success as a powerful lab model is notably
p1: due to the fact that we can complete its full life cycle in the lab quite easily
p1: as you will see in Chapter 3. The availability of many technical tools
p1: to study this species is also very advantageous, as you will see in Chapter 4.
p1: Platynereis also harbours a complex body plan with many tissues, structures, and
p1: organs, and is thus very useful to address their evolution, notably from a
p1: developmental biology perspective. I will thus present Platynereis from
p1: a morphological point of view. First of all, we'll have a look at its
p1: external morphology: its body plan.
p1: Platynereis' body plan is somewhat class- ical within the annelids and is composed of
p1: three main parts: the head, with a substantial cephalization;
p1: the metamerized trunk, composed of many iden tical units called segments with appendages;
p1: and the terminal part, the tail bearing the anus.
p1: Platynereis' worm size can be up to 90mm for around one hundred segments.
p1: Their colour is highly variable, from yellowish, reddish, and greenish and
p1: many more combinations.
p1: Let's first focus on Platynereis' head: you can observe here in these two
p1: pictures that have been taken respectively
p1: under a dissecting scope and under a scanning electron microscope.
p1: You can easily see that this important part of the animal contains many different
p1: structures that are sensory structures. They play crucial functions for the worms'
p1: life, and we will look at them in detail. First of all, Platynereis posess two pairs
p1: of brown eyes in a trapezoid arrangement only visible in the dorsal part of the worm.
p1: Those four eyes are localised on a specific structure of the head named the prostomium,
p1: which literally means "at the back of the mouth".
p1: Still on the prostomium are attached a pair of sensory antennae at the front of the head.
p1: Close to the antennae also at the front of the head, there is a pair of palps that are
p1: sensory too. The head is also composed of four pairs of long, sensory, tentacular cirri.
p1: Finally, all those structures are supported by a specific segment with no appendages,
p1: the peristomium, which literally means "surrounding the mouth."
p1: If we now have a look at the ventral face of Platynereis,
p1: you can easily recognise the already introduced structures: the antennae,
p1: the palps, the cirri, and below all of them a large structure: the mouth, or stomium.
p1: Inside the pharynx in the extension of the mouth, there is a pair of chitinous and
p1: denticulated jaws invaginated in the first segment of the worms. The reversible jaws
p1: will be evaginated to catch a prey.
p1: Let's now have a look at Platynereis' trunk.
p1: As said before, the trunk is composed of identical units or segments bearing
p1: appendages named parapods. On the dorsal image, you can see the feces of the worm
p1: inside the somewhat transparent body, as well as pigmented cells or chromatophores,
p1: which are shining at the surface of the epidermis. On the ventral picture, you can
p1: visualise the blood in red within each segment, and one of the main elements
p1: of the nervous system in annelids, the ventral nerve chord.
p1: Each segment of the trunk possesses a pair of appendages or parapods.
p1: Parapods are sensory and respiratory struc tures, also very important for locomotion.
p1: They notably allow the worm to crawl and swim, but also to secrete a cocoon where it
p1: spends most of its time. Parapods are biramous and thus composed of two parts:
p1: the notopodium in the dorsal side and the neuropodium in the ventral side.
p1: Each of them is composed of cirrus, a lobe, and a beam of extracellular chitinous
p1: structures named chaetae or bristles. Platynereis' chaetae are compound
p1: with various articles at the tip, and in annelid systematics, the shape of the
p1: parapods and the type of chaetae are informative characteristics
p1: to determine a species.
p1: The terminal part of the worm or tip of the tail is named the pygidium.
p1: The pygidium bears the anus and presents two anal or pygidial cirri in its ventral part.
p1: Let's now have a look at the internal structure of the worm.
p1: Basically Platynereis' body plan is a tube within a tube. I mean here that the
p1: segmented external morphology that we have seen before is also present internally.
p1: Each internal segment of the body is partially separated from the next one
p1: by an incomplete, inter-segmental piece of tissue that is called septa.
p1: Septa partially divide the body cavity, or coelom. Thus, within each segment,
p1: you will have a piece of nervous system, of musculature, of excretory system,
p1: of circulatory system, and of digestive system. We will now see some of them in detail.
p1: Concerning blood circulation, Platynereis has a closed circulatory system with
p1: two vessels, one dorsal and one ventral, connected together by a capillary network
p1: of ring shapes around the intestine. The dorsal vessel is the main pump
p1: and pumps the blood anteriorly, while the ventral vessel pumps the blood posteriorly.
p1: We can see in these videos the blood inside the dorsal vessel going from
p1: the tail to the head.
p1: In the ventral part, the blood flux is going to the left, towards the tail.
p1: Major vessels also irrigate the parapods thanks to a capillary network.
p1: Platynereis' nervous system's main element is a chord in the ventral position
p1: but not only. Platynereis indeed has a highly developed nervous system
p1: and a significant cephalization, with cerebral ganglia within the brain
p1: as visible in blue and green in the picture. As already mentioned, the trunk presents
p1: a large ventral nerve chord, with extensions into the segments and
p1: appendages corresponding to the peripheral nervous system which make it ladder-shaped.
p1: The tail is also highly innervated, with the ventral nerve chord that has extension
p1: into the two lobes of the parapods, and into the anal cirri and the pygidium.
p1: Let's now have a look at the musculature. Platynereis has two main types of muscles:
p1: longitudinal muscles and circular muscles.
p1: You can clearly see the circular muscles of the anterior part of Platynereis in the
p1: first picture. Note also the substantial musculature at the base of the tentacles
p1: that are highly mobile structures. In the second picture of a small worm,
p1: two large longitudinal muscles are visible. Appendages have also a very complex
p1: musculature, allowing the parapods to be very mobile.
p1: As you can see in this video, Platynereis can be very dynamic.
p1: Interestingly, Platynereis worms' body plan changes overtime as they age.
p1: Indeed, the morphological description we have seen so far corresponds to a long
p1: period of their life when the worms are juvenile or atoke. But at some point during
p1: their life cycle they will undergo sexual metamorphosis that implies drastic
p1: morphological changes. As you can see in this picture, sexual or epitoke worms are
p1: quite different from the juveniles.
p1: The main and obvious difference is their colour: while juveniles are mainly brownish,
p1: females are bright yellow as they are full of oocytes. The males are white
p1: in their anterior part, full of sperm, and red in the posterior part due to
p1: a lot of accessory blood capillaries.
p1: During sexual maturation, animals do not feed anymore and consequently, there are
p1: no longer feces visible.
p1: The whole intestine regresses and the trunk of the animal
p1: is progressively modified to become a bag full of gametes.
p1: But the morphological differences are not limited to the colour and the absence of gut.
p1: Indeed, Platynereis' eyes are also very different. They are much bigger and darker
p1: in the mature worms.
p1: Differences are also visible along the trunk of the animals that appears divided into
p1: two regions at around 25 segments from the head. The cirri of the parapods of the
p1: anterior part are modified, and more importantly, the posterior parapods
p1: are highly metamorphosed.
p1: This is notable in males as they develop a significant blood network
p1: and present accessory flaps.
p1: In both sexes the ordinary chaetae are replaced by paddle chaetae
p1: in the modified parapodia.
p1: Finally, while the terminal part of the female is not modified, the male pygidium
p1: presents extra papillae, allowing the sperm to be released in many directions.
p1: Later, when the mature male and female will meet each other, they will release gametes,
p1: allowing an external fertilization as you will see in Chapter 3.
p1: If you want more information about the embryonic development of Platynereis,
p1: I encourage you to consult the diagram showing its full life cycle, and for more
p1: detailed information you can also read the following important paper.