Chapter 33: Protostome Animals
oMolecular phylogenies support the hypothesis that protostomes are a monophyletic group divided into two major
subgroups: the Lophotrochozoa and the Ecdysozoa.
oAlthough the members of many protostome phyla have limbless, wormlike bodies and live in marine sediments, the
most diverse and species-rich lineages—Mollusca and Arthropoda—have body plans with a series of distinctive,
complex features. Molluscs and arthropods inhabit a wide range of environments.
oKey events triggered the diversification of protostomes, including several lineages making the water-to-land transition,
a diversification in appendages and mouthparts, and the evolution of metamorphosis in both marine and terrestrial
An Overview of Protostome Evolution
Protostomes are bilaterally symmetric, triploblastic, coelomate animals.
Protostomes are a monophyletic group comprising two major lineages: Lophotrochozoa and Ecdysozoa (Figure 33.3).
What Is a Lophotrochozoan?
The 14 phyla of lophotrochozoans include molluscs, anelids, and flatworms.
The name lophotrochozoan was inspired by the presence of a feeding structure called a lophophore and a type of larva
called a trochophore. Only some phyla have these morphological traits.
A lophophore is a specialized structure that rings the mouth and functions in suspension feeding (Figure 33.4a).
Trocophores are a type of larvae common to several phyla of lophotrochozoa (Figure 33.4b).
What Is an Ecdysozoan?
The primary contrast between lophotrochozoans and ecdysozoans involves the methods of growth used by organisms
in each group.
Ecdysozoans grow by molting – shedding of the hard exoskeleton or soft cuticle (Figure 33.5).
web animation: Protostome Diversity
Themes in the Diversification of Protostomes
Protostomes have diverged into 22 different phyla that are recognized by distinctive body plans.
How Do Body Plans Vary among Phyla?
All protostomes are triploblastic and bilaterally symmetric, and all protostomes undergo embryonic development in a
The nature of the coelom varies among protostomes though (Figure 33.6).
Platyhelminthes are acoelomate, lacking a body cavity.
The pseudocoelom, which forms from an opening that originates between the ectoderm and mesoderm layers, arose
independently in rotifers and ecdysozoans.
Most protostome phyla have wormlike bodies with a basic tube–within–a–tube design.
Some phyla of protostomes had a drastic reduction of the coelom.
A fully functioning coelom has two functions: – providing space for fluids to circulate among organs
– providing a hydrostatic skeleton for movement
The Arthropod Body Plan
Arthropods have segmented bodies that are organized into prominent tagma (Figure 33.7a).
Arthropod locomotion is based on muscles that apply force against the exoskeleton to move legs or wings.
Arthropods have a spacious body cavity called the hemocoel that provides space for internal organs and circulation of
The Molluscan Body Plan
The mollusc body plan (Figure 33.7b) is based on three major components:
– the foot, a large muscle located at the base of the animal and usually used in movement
– the visceral mass, the region containing most of the internal organs and external gill
– the mantle, a tissue layer that covers the visceral mass and that secretes a shell in some species
In molluscs, the coelom's functions are replaced by the visceral mass and the muscular foot.
Variation Among Body Plans of the Wormlike Phyla
Echiurans have a proboscis, an extended structure which forms a gutter leading to the mouth (Figure 33.8a).
Priapulids have a toothed throat that can be everted to grab prey and then retracted to take in the food (Figure 33.8b).
Nemerteans have a barb–tipped proboscis that everts and spears or entangles the prey and retracts toward the mouth
The Water–to–Land Transition
The ability to live in terrestrial environments evolved independently in arthropods, molluscs, roundworms, and annelids.
The protostome groups that made the water–to–land transition already had hydrostatic skeletons, exoskeletons,
appendages, or other adaptations for support and locomotion that happened to work on land as well as water,
facilitating the transition.
To make the transition to land, new adaptations allowed protostomes to exchange gases and avoid drying out.
How Do Protostomes Feed, Move, and Reproduce?
After an array of body plans had evolved in animal phyla, subsequent diversification was largely driven by adaptations
that allowed animals to feed, move, or reproduce in novel ways.
Adaptations for Feeding
Protostomes include suspension, deposit, liquid, and food–mass feeders.
Arthropod mouth parts are very diverse, ranging from tubes to pincers that allow the various species to pierce, suck,
grind, bite, mop, chew, engulf, cut, or mash (Figure 33.9).
Adaptations for Moving In protostomes, variation in movement depends on variation in the presence or absence of limbs and the type of
Protostome movement includes walking, running, jumping, flying, gliding, crawling, and jet propulsion (Figure 33.10).
Adaptations in Reproduction
Protostomes can reproduce sexually or asexually, although sexual reproduction is the predominant mode in most
Many crustacean and insect species reproduce asexually by parthen