Trochophore Larva – Features, Structure, Metamorphosis, Evolutionary significance

What Is Trochophore Larva?

• A trochophore larva is a distinct developmental stage observed in many marine invertebrates, including annelids (such as worms) and mollusks (such as snails, clams, and octopuses). This planktonic organism is free-swimming and plays a critical role in the early life cycle of these animals. Characterized by its small size and translucent body, the trochophore larva is easily identified by the presence of a ring of cilia encircling its middle, which aids in locomotion.
• The most prominent feature of the trochophore larva is its cilia, which are hair-like structures that enable it to move through the water. These cilia not only facilitate swimming but also help in feeding by creating water currents that bring food particles towards the larva’s mouth. The movement of these cilia is essential for the larva’s survival and its interaction with the surrounding environment.
• In addition to cilia, trochophore larvae possess specialized sensory organs that play a key role in environmental perception. For instance, the apical tuft of cilia located at the top of the larva functions as a sensory organ, helping the larva detect changes in water currents. This allows the larva to navigate its aquatic environment more effectively. Trochophores also have an ocellus, a simple light-sensitive organ that provides rudimentary vision, helping the larva orient itself in response to light.
• The digestive system of the trochophore is relatively simple, yet highly functional for its size and lifestyle. It includes a mouth, stomach, and anus, allowing the larva to process food and eliminate waste. The simplicity of this system is appropriate for its role as a planktonic organism, where feeding and digestion occur quickly to sustain the larva’s high-energy lifestyle in the water column.
• Bilateral symmetry is another defining feature of the trochophore larva. This means that the organism can be divided into two identical halves along its length. Bilateral symmetry is a key characteristic in the evolution of many invertebrates, allowing for more efficient movement and coordination as the organism develops.
• From an evolutionary perspective, the trochophore larva holds significant importance. It is believed to represent an ancestral form common to several invertebrate groups. Its presence in the life cycle of both annelids and mollusks suggests that the trochophore may have evolved from a common ancestor, linking diverse marine invertebrate species. This evolutionary connection makes the trochophore an important model for studying the development and diversification of marine organisms.

Historical Retrospect of Trochophore Larva

The trochophore larva, a critical developmental stage in marine invertebrates, has a rich history of scientific discovery and nomenclature. Its identification and study have undergone several phases, with various researchers contributing to the understanding of this organism’s significance and evolutionary relationships. Below is a chronological overview of the key milestones in the history of the trochophore larva:

• Loven (1840): The Swedish naturalist, Loven, was the first to discover the trochophore larva. His initial observations identified this larva as a distinct form, and it was commonly referred to as “Loven’s larva” in honor of his work. This marked the beginning of the scientific study of the trochophore and its place in the life cycle of marine invertebrates.
• Semper (1859): Following Loven’s discovery, Semper introduced the term “Trochosphaera” to describe the organism. He likened it to a rotifer, drawing parallels between the trochophore and this group of microscopic animals. Semper’s work furthered the recognition of the trochophore as a significant larval form, though his naming was not widely adopted.
• Ray Lankester (1877): The British zoologist Ray Lankester played a pivotal role in formalizing the identity of the trochophore larva by giving it the name “Trochophore.” This term, which highlights the larva’s distinctive ring of cilia, gained widespread acceptance in the scientific community and became the standard nomenclature used in subsequent studies.
• Hatschek (1879): Shortly after Lankester’s work, Hatschek, another prominent biologist, supported the name “Trochophore,” solidifying its use in scientific literature. Hatschek’s contributions helped cement the trochophore’s status as a well-recognized and important larval form in the animal kingdom, particularly in relation to marine invertebrates.
• Hyman (1957) and Barnes (1980): In the mid-20th century, researchers like Hyman and Barnes sought to explore the evolutionary relationships of the trochophore larva with other groups of animals. Their efforts focused on understanding how this larval stage might serve as a link between diverse invertebrate lineages. Although their theories varied, both contributed to the broader understanding of the trochophore’s evolutionary significance.

Salient Features of Trochophore Larva

The trochophore larva is a small, bilaterally symmetrical, ovoid-shaped larval organism commonly found in the pelagic zone, although it can also be found in the epipelagic zone. This larval form is characterized by its simple structure and distinct morphological features, which are vital in identifying it across various invertebrate groups. Below are the key structural features of the trochophore larva:

• Body Structure:
  • The body of the trochophore is unsegmented and is distinctly divided into three main regions:
    1. Petrochal Region: This includes the apical plate, prototroch, and the area around the mouth.
    2. Pygidium: Located at the posterior, this region includes the telotroch and anal area.
    3. Growth Zone: This is the portion of the larva between the mouth and telotroch, which encompasses the main body of the larva.
• Apical Plate:
  • The apical plate is a distinctive feature of the fully grown trochophore larva, found at the apical portion of the body. It bears a tuft of cilia, referred to as the apical tuft of cilia, which plays a crucial role in locomotion.
• Ciliated Bands:
  • The presence of several encircled ciliated bands is one of the most important identifying features of the trochophore larva. These bands are essential for both locomotion and feeding:
    1. Prototroch: A preoral ciliated band located anterior to the mouth, above the equatorial region. This band is primarily responsible for locomotion.
    2. Post-Oral Ciliated Band (Metatroch): This band is located posterior to the mouth and functions in movement and food capture.
    3. Telotroch: This ciliated band is located in front of the anus and the pygidium.
    4. Neurotroch: A longitudinal band of cilia that traverses the body, contributing to the larva’s movement.
• Gut Structure:
  • The trochophore larva has a complete digestive system, consisting of four distinct regions:
    1. A mid-ventral mouth.
    2. A sac-like stomach.
    3. A long intestine.
    4. An anus at the posterior end of the body.
• Ganglion:
  • A single ganglion is located at the apical end of the body, considered rudimentary to the brain or cerebral ganglia. Additionally, a ventral nerve cord runs along the length of the body, providing nervous system connectivity.
• Mesoderm:
  • The mesoderm in the trochophore larva appears as an undifferentiated mass, located at the lower pole of the body. It is present in pairs and plays a role in the development of other tissues.
• Ectoderm:
  • The ectoderm contains ectodermal derivatives and scattered ectodermal elements that contribute to the larva’s outer structure.
• Absence of Coelom:
  • At the early larval stages, the trochophore does not have a fully developed coelom, although a prominent blastocoel is present between the ectoderm and endoderm. This gelatinous matrix layer plays a role in the larva’s structure and flexibility.
• Ocelli:
  • Some groups of trochophore larvae possess a pair of ocelli, which are small, light-sensitive structures located at the apical end. These structures are important for detecting light and aiding in orientation.
• Protonephridia:
  • The presence of protonephridia in the blastocoel, located at each side of the alimentary canal or gut, is another defining feature. These primitive excretory organs help in osmoregulation and waste removal.

Structures of the Trochophore Larva (Loven’s Larva of Polygordius)

The trochophore larva, the larval stage observed in archiannelids such as Polygordius, exhibits various distinct structures that allow it to swim, feed, and begin its journey toward adulthood. The following descriptions highlight these key features of the trochophore, based on the morphology of the organism.

• Marine Planktonic Nature: The trochophore larva is planktonic and typically lecithotrophic, meaning it derives nutrition from the yolk stored within the egg, which sustains it during early development.
• Body Structure: The body of the trochophore is bilaterally symmetrical, with the anterior (front) end being broader than the posterior (rear) end. This symmetry is a critical feature that contributes to its movement and organization.
• Digestive System:
  • Mouth: Located near the mid-ventral line of the body, the mouth is positioned to receive food particles from the surrounding water.
  • Alimentary Canal: The mouth leads to a sac-like stomach, which then extends into a narrow alimentary canal.
  • Anus: The alimentary canal opens to the exterior through an anal aperture located at the posterior tip of the body.
  • The walls of both the stomach and alimentary canal are lined with cilia, which assist in moving food particles and aiding in digestion.
• Ciliary Bands:
  • The trochophore has two prominent bands of cilia encircling its body. These ciliary bands are crucial for locomotion and feeding. In some species, a third band may be present.
  • Prototroch: The pre-oral circlet of cilia, or prototroch, is a strong, specialized band that encircles the body above the mouth. It is the primary locomotor structure.
  • Metatroch: Behind the mouth lies a second transverse ring of cilia known as the metatroch, which aids in further locomotion.
• Additional Ciliary Features:
  • Telotroch: Often, a second circlet of cilia forms around the pygidium, or anal region, called the telotroch. This band plays a role in movement and orientation.
  • Neurotroch: A longitudinal band of cilia may run along the mid-ventral part of the body in certain cases, referred to as the neurotroch.
• Lack of Metamerism: The trochophore exhibits no metamerism (segmentation), a feature that will later develop in the adult form. At this larval stage, the trunk is undifferentiated, though the rudiment of the future adult trunk appears as a small region at the posterior pole.
• Blastocoel: The body cavity at this stage is not a coelom but a spacious blastocoel that encloses the gut. The blastocoel serves as a simple body cavity that supports the internal structures of the larva.
• Internal Structures:
  • Protonephridia: The trochophore has a pair of protonephridia, simple excretory structures, located on each side of the gut. Each protonephridium consists of a hollow cell containing a flame of cilia, which assist in the excretion of waste products.
  • Mesenchyme and Muscles: Some mesenchyme (a type of connective tissue) and larval muscles are present within the blastocoel, allowing for basic movement and structure.
• Sensory Features:
  • Apical Sensory Plate: At the apical (top) pole of the trochophore, there is a thickened ectodermal area known as the apical sensory plate. This plate houses cells that are the precursors to the cerebral ganglia, important components of the nervous system.
  • Apical Tuft of Cilia: Emanating from the apical sensory plate is a tuft of long cilia known as the apical tuft. This structure helps in sensory functions and movement.
  • Ocelli: Many trochophores possess sense organs, such as ocelli, or simple eye spots, located beneath the apical plate. These structures are light-sensitive and assist in orientation and navigation.
• Body Regions:
  • Pretrochal Region: This region includes the area above the prototroch, which is part of the anterior region of the larva.
  • Growth Zone: The middle region between the mouth and the telotroch is referred to as the growth zone. This is the area where most of the development and elongation occur as the larva matures.
  • Pygidium: The posterior end of the larva, called the pygidium, includes the telotroch and anal area, contributing to locomotion and the elimination of waste.

Biology and Metamorphosis of Trochophore Larva

The trochophore larva represents an important early stage in the life cycle of various marine invertebrates, such as polychaetes, echiurans, and sipunculans. These larvae vary in their nutritional sources and their duration of free-swimming life. The biology of the trochophore larva, as well as the process of metamorphosis, provides insight into the transformation from a planktonic form to a more complex adult organism.

• Nutritional Strategies and Larval Types:
  • Planktotrophic Larvae: Certain trochophore larvae, including those of echiurans, polychaetes (such as Polygordius), phyllodocids, and serpulid fan worms, are planktotrophic. These larvae feed on plankton and other microscopic marine organisms in the water. The prolonged, free-swimming life of these larvae enables them to move through the marine environment and gather the necessary nutrients for their development.
  • Lecithotrophic Larvae: In contrast, some groups, such as sipunculans and certain polychaetes (like nereids and eunicids), produce lecithotrophic larvae. These larvae do not feed on external sources of nutrition but rely solely on the yolk stored in the egg. As a result, lecithotrophic larvae have a much shorter planktonic life compared to their planktotrophic counterparts.
• Metamorphosis in Trochophore Larva:
  • Initial Signs of Metamorphosis: The metamorphosis of the trochophore larva is best observed in species like Polygordius. The first indication of metamorphosis is the segmentation of mesodermal bands within the larva. This process marks the transition from a simple, undifferentiated larval form into a more structured organism.
  • Elongation of the Posterior Region: As metamorphosis progresses, the posterior end of the larva elongates rapidly. This elongation is externally marked by segmentation, which is a precursor to the formation of the adult structure.
  • Formation of the Prostomium and Peristomium: The region above the prototrochal ring undergoes further differentiation and becomes the prostomium. The area surrounding the prototroch, which is responsible for movement, then differentiates into the peristomium, a structure that will be important in the adult form.
  • Development of the Cerebral Ganglion and Nerve Cord: The apical sensory region, which contains the apical sense organ, evolves into the cerebral ganglion. This ganglion becomes integrated with the developing ventral nerve cord, an important component of the nervous system that will coordinate movement and other bodily functions in the adult worm.
• Internal and Structural Changes:
  • Mesodermal Splitting and Coelom Formation: Internally, the mesodermal bands split to form coelomic sacs. The coelom is a fluid-filled body cavity that will house various internal organs and contribute to the overall body structure of the adult worm.
  • Shifting of the Mouth and Anal Organization: As the larva metamorphoses, the mouth shifts forward, and the anal region undergoes gradual changes. These structural adjustments are necessary for the transformation into the adult form.
• Ciliary Band Disappearance and Growth:
  • During metamorphosis, the ciliary bands that were originally crucial for locomotion disappear. The larva no longer relies on these cilia for movement and starts to grow in size and length. This growth occurs as new segments are added, further differentiating the larva into a more complex organism.
• Post-Metamorphosis Development:
  • After completing metamorphosis, the young worm sinks to the ocean floor and adopts a burrowing lifestyle, a critical characteristic of its adult form. The transition from the free-swimming larva to the adult worm represents a significant developmental change, as the organism now begins to function in its final ecological niche.

Structures of the Trochophore Larva in Different Classes

The trochophore larva, while a common feature in many marine invertebrates, exhibits variation in structure depending on the taxonomic class. These variations reflect adaptations to the specific needs of each group. Below is a detailed description of the structures and features of trochophore larvae across different classes, including Polychaeta, Oligochaeta, and Hirudinea.

• Class Polychaeta:
  • The trochophore larva in the class Polychaeta presents various structural modifications, often distinguished by the presence of ciliary rings and sensory structures.
    • Neanthes (Nereis): The larva of Neanthes resembles the typical trochophore but includes a pair of eye spots, which serve sensory functions.
    • Psygmobranchus: This larva does not exhibit a blastocoel, and the ectoderm and endoderm are in direct contact except where separated by the larval mesoderm.
    • Lumbriconereis: The cilia in this larva are either spread evenly over the entire surface of the body or absent from specialized circlets. Such larvae are referred to as atrochal.
    • Nephthys: The larva of Nephthys shows two distinct circlets of cilia, one at the anterior (pre-oral) end and another at the posterior (peri-anal) end. This type of larva is termed a telotroch larva.
    • Amphitrochal Larvae: In some polychaetes, ciliary rings may be present on both the dorsal and ventral surfaces, referred to as amphitrochal larvae.
    • Chaetopterus: The larva of Chaetopterus displays rows of cilia that encircle the middle of the body. This arrangement is known as a mesotrochal larva, where pre-oral and peri-anal rings are absent.
    • Ophryotrocha: In the larva of Ophryotrocha, numerous ciliary circlets are found, each developing on a true mesodermal segment, leading to its classification as a polytrochal larva.
    • Mitraria: The larva of Mitraria shows provisional setae, which will eventually be replaced by permanent structures. Similarly, in the older larva of Nereis, lateral parapodial-like structures with setae begin to form.
• Class Oligochaeta:
  • In contrast to the diverse larval forms seen in Polychaeta, the class Oligochaeta does not exhibit a free-swimming larval stage. The development in this class proceeds directly, meaning that the organism does not undergo a distinct trochophore larval form. Instead, the young worm gradually develops into the adult without a free-swimming intermediate stage.
• Class Hirudinea:
  • Like the Oligochaeta, the Hirudinea class also lacks a free-swimming larval stage. The development is direct, meaning that there is no trochophore larva or other intermediary larval form. The organism undergoes a continuous developmental process from egg to adult without distinct larval transformations.

Affinities of the Trochophore Larva

The trochophore larva, a common developmental stage in several groups of invertebrates, exhibits striking similarities with the larvae of other taxa. This has led to various hypotheses regarding its phylogenetic significance and potential evolutionary relationships. However, despite these similarities, the unique characteristics of the trochophore larva, in combination with distinct differences, must be considered in order to assess its affinities with other larval forms.

• Affinities with Ctenophora:
  • Similarities:
    • Both the trochophore and ctenophore larvae have a pear-shaped body.
    • The ctenophore’s aboral sense organ, called a statocyst, is compared to the apical sensory plate of the trochophore larva.
    • Both forms exhibit sub-ectodermal radiating nerves and a prototroch derived from the fourth group of ciliated cells.
  • Discrepancies:
    • Despite the apparent similarities, the fundamental organization of these two groups is quite different.
    • The cleavage pattern in both larval forms is dissimilar.
    • The anus is absent in ctenophores, unlike in the trochophore larva.
    • Given these divergences, the trochophore larva cannot be regarded as directly related to ctenophores.
• Affinities with Muller’s Larva (Turbellarians):
  • Similarities:
    • Both the trochophore and Muller’s larva (especially that of Planocera) share similarities in developmental stage, the disposition of ciliated bands, and the presence of eye spots at the aboral end.
  • Discrepancies:
    • The absence of an anus in Muller’s larva is a key difference.
    • In Muller’s larva, the enteron opens into a single aperture, while in trochophores, the alimentary canal ends with an anus.
    • There are differences in the embryonic differentiation of the mesoderm.
    • Muller’s larva has a tuft of cilia at its caudal end, which is absent in the trochophore larva.
    • Due to these differences, the parallelism between the two groups cannot be fully justified.
• Affinities with Pilidium (Nemertini) Larva:
  • Similarities:
    • Both the trochophore and pilidium larva have a helmet-shaped body.
    • The ciliated ring between the oral and aboral ends in the pilidium larva represents the prototroch of the trochophore.
    • Similarities in the distribution and arrangement of the nerve ring and stomodaeum are also observed.
    • Both exhibit a schizocoelic mode of coelom formation.
  • Discrepancies:
    • The pilidium larva lacks an anus, which contrasts with the structure of the trochophore larva.
    • There are notable differences in the formation of the mesoderm, preventing a clear evolutionary link between these two forms.
• Affinities with Rotifera:
  • Trochosphaera, a larval form of the rotifer group, shows superficial resemblances to the trochophore larva.
    • Both share ciliated girdles, a similar nervous system (sometimes referred to as a ‘brain’), and comparable sense organs.
    • The anus and nephridia in both forms are similarly placed, and the curvature of the intestine also resembles that of the trochophore.
  • However, these resemblances are largely superficial and do not provide a solid basis for a phylogenetic relationship. Critical examination is necessary to determine if any deeper evolutionary connection exists.
• Affinities with Veliger Larva (Mollusca):
  • Similarities:
    • The pre-oral ciliated ring, ciliated tuft of flagella, and apical plate found in the veliger larva of mollusks resemble features of the trochophore larva.
  • Differences:
    • The similarities between the trochophore and veliger larvae are likely due to remote phylogenetic convergence, rather than direct evolutionary affinity.

Phylogenetic Significance of the Trochophore Larva

The trochophore larva holds a pivotal position in the study of invertebrate evolution. Its occurrence in several groups of invertebrates provides critical insight into the transition from animals with radial symmetry to those with bilateral symmetry. As such, the trochophore is viewed as a transitional form, potentially linking ancient radial-symmetry organisms with the more complex bilateral forms. Understanding these connections is essential in mapping the evolutionary history of various invertebrate groups.

• Evolutionary Role:
  • The trochophore larva is considered an important transitional stage in the evolutionary progression from radial symmetry, characteristic of groups like the ctenophores, to bilateral symmetry seen in more advanced invertebrate groups (e.g., rotifers).
  • Its structural similarities with larvae of different groups, such as echinoderms and hemichordates, further supports the idea that trochophore larva could represent a significant evolutionary link in the development of bilateral symmetry.
• Affinities with Other Groups:
  • The trochophore larva shares certain features with the larvae of echinoderms (specifically, the bipinnaria and pluteus larvae) and the tornaria larva of Balanoglossus, reinforcing the hypothesis that the trochophore may have played a role in the emergence of bilateral symmetry from radial symmetry.
  • These similarities include shared ciliated bands and developmental patterns, which strengthen the idea of a common evolutionary ancestry among these groups.
• Theories Regarding the Phylogenetic Relationship: Several theories have been proposed to explain the phylogenetic significance of the trochophore larva:
  1. Ctenophore-Polyclad Theory (Lang, 1881):
     • This theory suggests that the trochophore larva is related to the ctenophores, a group with radial symmetry. The idea is that a common ancestor may have given rise to both ctenophores and the trochophore larva, and through evolutionary changes, bilateral symmetry developed.
  2. Ctenophore-Trochophore Theory (Hatschek, 1878):
     • This theory, later modified by Hatschek, posits that the trochophore larva may have evolved directly from ctenophores, suggesting that the transition from radial to bilateral symmetry occurred via this intermediate larval form. This is a widely accepted theory among researchers.
  3. Planuloid-Coeloid Theory (L. Vongraff, 1882):
     • According to this theory, the trochophore larva arose from a hypothetical organism called “Trochozoon,” which served as a link between the early radial-symmetry organisms and those that developed bilateral symmetry.
• Modern Views:
  • The Ctenophore-Trochophore Theory is the most widely accepted explanation among modern workers. It suggests that the trochophore larva evolved from a common ancestor that exhibited radial symmetry, and this form ultimately gave rise to more complex bilateral organisms.
  • According to Salvini-Plawen (1973), annelids and echiurans are closely related due to their shared trochophore larval stage. In contrast, groups like flatworms, nemerteans, and entoproct larvae show no such larval similarities and are thus considered unrelated to the trochophore group.

Evolutionary significance of trochophore larva

The trochophore larva is a key morphological stage in the development of several invertebrate groups, especially within the phyla Annelida and Mollusca. Its evolutionary significance has been widely debated, as it appears to represent an important transitional form in the history of animal evolution, shedding light on the origins of bilateral symmetry from radial symmetry. The trochophore larva’s structure and similarities with other larval forms indicate its role in understanding the evolutionary connections among diverse invertebrate groups. Below are the key aspects of its evolutionary significance:

• Transitional Form Between Radial and Bilateral Symmetry:
  • One of the most crucial aspects of the trochophore larva is its position as a transitional form in the evolutionary lineage of animals. It is thought to have evolved from animals exhibiting radial symmetry, such as ctenophores, into animals with bilateral symmetry, which is seen in modern annelids, mollusks, and other related groups.
  • This notion supports the idea that bilateral symmetry might have evolved from the radial symmetry that characterizes more primitive organisms. The trochophore’s symmetry and structure suggest it is a link in the evolutionary history between these two major body plans.
• Theories Supporting Evolutionary Relationships:
  • Various theories have been proposed regarding the evolutionary origin and significance of the trochophore larva:
    1. Ctenophore-Polyclad Theory (Lang, 1881): This theory suggests that the trochophore might have evolved from ctenophores, a group of animals with radial symmetry. The theory implies that the trochophore is an intermediate form between ctenophores and other bilateral organisms.
    2. Ctenophore-Trochophore Theory (Hatschek, 1878): This revised version of the first theory posits a closer evolutionary link between ctenophores and trochophore larvae, proposing that the trochophore might have directly evolved from ctenophores.
    3. Planuloid-Coeloid Theory (Vongraff, 1882): This theory considers the planuloid stage (a hypothetical, ancestral form) as the precursor to the trochophore larva, which later gave rise to coelomates and more complex bilateral animals.
  • These theories highlight the potential role of the trochophore as a transitional stage in the emergence of bilateral symmetry. Though not universally accepted, the second theory—Ctenophore-Trochophore theory—has gained widespread recognition.
• Links with Other Larval Forms:
  • The trochophore larva exhibits similarities with several other larval forms, supporting the idea of shared evolutionary ancestry:
    1. Echinoderm Larvae (Bipinnaria and Pluteus): These larvae share morphological features with the trochophore, further bolstering the hypothesis that the trochophore is connected to the emergence of bilateral symmetry.
    2. Tornaria Larva of Balanoglossus: This larva, seen in hemichordates, also exhibits similarities with the trochophore, suggesting a possible common ancestor for these groups.
    3. Nematode Larvae: Certain nematode larvae exhibit features similar to the trochophore, underscoring the evolutionary continuity of this larval form across different phyla.
• Evolutionary Developmental Significance (Evo-Devo):
  • The study of trochophore larvae provides valuable insight into the field of evolutionary developmental biology, often referred to as “evo-devo.” By examining how the trochophore develops and comparing it across species, scientists can better understand how changes in developmental processes (such as symmetry) may have driven evolutionary transitions between body plans.
  • The presence of features like the prototroch (a ciliated band used for movement), and the well-defined mouth, stomach, and anus highlight the trochophore’s role in the development of basic body structures in bilaterally symmetrical organisms.
• Annelid and Mollusk Evolution:
  • In the context of the phyla Annelida and Mollusca, the trochophore larva is seen as a crucial stage in their development. The larva likely represents an ancestral form from which these groups diversified.
  • In particular, the trochophore is thought to be an intermediate form in the evolutionary lineage of annelids and mollusks. Its basic body plan, including the bilaterally symmetrical structure and the development of a gut, is seen as foundational to these groups’ evolutionary success.