Chapter 15 – Polyzoa and Kryptrochozoa
CHAPTER 15
POLYZOA AND KRYPTROCHOZOA
CHAPTER OUTLINE
15.1.
15.2.
Some Evolutionary Experiments
A. During the Cambrian explosion, all major phyla and probably all smaller phyla developed.
B. Many phyla disappeared during mass extinctions.
C. Some phyla disappeared as failed evolutionary models, while others survived and thrived, or persisted
with only a small number of species.
Overview
A. This chapter describes six lophotrochozoan phyla (Figure 15.1).
B. Lophotrochozoans typically have a lophophore and/or a trochopore.
1. A lophophore is a crown of ciliated tentacles used as a feeding device. And used for respiratory
2. The thin walls of the lophophore also act as a respiratory surface.
C. Three phyla possess a lophophore: Ectoprocta, Brachiopoda, and Phoronida.
1. Data indicate the lophophore evolved twice, once in Ectoprocta and once in the common ancestor
of the other two phyla.
2. Consequently, Ectoprocta are in clade Polyzoa with two other taxa that all share ciliated tentacles:
Cycliophora and Entoprocta.
3. Brachiopod and Phoronida are in clade Brachiozoa.
D. Phylum Nemertea is usually depicted as the sister taxon to Brachiozoa, and together those three phyla
comprise clade Trochozoa.
1. Clade Trochozoa also contains the molluscs and annelids.
E. ‘Troch’ is named for the trochophore larvae. Have larvae stage
1. The larvae have a ring of large ciliated cells in front of the mouth, the prototroch.
2. The trochophore is likely homologous but the developmental patterns, molecular characteristics,
and morphologies of the animals covered here are all being investigated, and evolutionary
relationships may change.
Clade Polyzoa
A. The clade Polyzoa unites cycliophorans with entoprocts and ectoprocts (bryozoans) (Figure 15.1).
B. The close relationship between entoprocts and ectoprocts was originally proposed based on
morphological characteristics, but phylogenetic studies support the association.
C. Cycliophorans, which were discovered later, exhibit similarities with entoprocts.
Phylum Cycliophora
A. Cycliophorans live exclusively on the mouthparts of marine decapod crustaceans in the northern
hemisphere (Figure 15.2).
1. They attach to bristles with an adhesive disc on the end of an acellular stalk.
2. They feed by collecting bacteria or bits of food dropped from their lobster host on a ring of
compound cilia that surrounds the mouth.
B. Simple body plan: mouth leads to U-shaped gut ending with an anus that opens outside the ciliated
ring.
1. Body is acoelomate.
2. Epidermis is cellular and surrounded by a cuticle.
C. Life cycle has both sexual and asexual phases.
1. Feeding animals make internal buds called Pandora larvae, which become new feeding individuals
upon release; clone members occupy vacant areas on the lobster mouthparts.
2. In sexual reproduction a male larvae is released and settles atop another animal housing a female
larvae.
3. A male larvae produces secondary males with reproductive organs; internal fertilization occurs as
a secondary male mates with a female larva leaving the body of a feeding animal.
4. Once fertilization occurs, a chordoid larva develops inside the body of its mother, consuming it.
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Chapter 15 – Polyzoa and Kryptrochozoa
15.3.
15.4.
Phylum Entoprocta
A. Diversity
1. About 150 species of Entoprocts occur worldwide, usually in marine environments.
2. They are less than 5 mm long and mostly microscopic, resembling hydroid cnidarians (Figure
15.3).
3. All are stalked and sessile and are ciliary feeders.
4. Except for Urnatella, which are common freshwater species, all are marine.
B. Form and Function
1. The body or calyx is cup shaped and bears a circular crown of ciliated tentacles.
2. It attaches by a stalk with adhesive glands; tentacles and stalk are continuations of the body wall.
3. The body wall has a cuticle, cellular epidermis and longitudinal muscles
4. The 8–30 tentacles are ciliate on lateral and inner surfaces.
5. Tentacles can roll inward (cover & protect mouth and anus) but cannot be retracted into the calyx.
6. The gut is U-shaped with both mouth and anus opening within the circle of tentacles, hence its
phylum name.
7. Long cilia on the sides keep a current bringing in food and food particles; short cilia on the inner
surfaces capture food and direct it to the mouth.
8. A pair of protonephridia for excretion in the gelatinous parenchyma that fills the pseudocoel.
9. A well-developed nerve ganglion is on the ventral side of the stomach.
10. There are no circulatory or respiratory organs.
11. Some are dioecious, many are monoecious (often protandrous hermaphrodites).
12. Fertilized eggs develop in a brood pouch between gonopore and anus.
13. Modified spiral cleavage leads to a have trochophore-like larva.
Phylum Ectoprocta (Bryozoa) (Figures 15.4 − 15.6)
A. General Characteristics
1. Contains aquatic animals that often encrust hard surfaces.
2. Most are sessile, some slide slowly, and others crawl actively across surfaces they inhabit.
3. Mostly they are colony builders; each member is less than 0.5 mm.
4. Colony members are called zooids.
5. Zooids feed by extending their lophophores into surrounding water to collect tiny particles.
6. Zooids secrete an exoskeleton which they live in (Figure 15.4).
7. The exoskeleton (zooecium) may be gelatinous, chitinous, or stiffened with calcium and possibly
impregnated with sand.
8. Shape may be boxlike, vaselike, oval, or tubular.
9. Are about 4500 living species.
10. Live in both shallow freshwater and marine habitats.
11. Some colonies form limy encrustations on seaweed, shells, and rocks; others form fuzzy or
shrubby growths or erect branching colonies (Figure 15.5).
12. Freshwater colonies may form mosslike colonies on stems of plants or o n rocks.
13. The term “bryozoan” once referred to both entoprocts and ectoprocts; it is still used to refer to
ectoprocts.
B. Form and Function
1. Each member lives in a tiny chamber called the zoecium.
2. Each zooid consists of a feeding polypide and a case-forming cystid.
a. A polypide includes the lophophore, digestive tract, muscles, and nerve centers.
b. A cystid includes the body wall of an animal, together with its secreted exoskeleton.
3. Polypides pop up to feed but withdraw quickly at the slightest disturbance.
a. To extend its tentacular crown, muscles contract; this increases hydrostatic pressure within the
body cavity and pushes the lophophore out.
b. Other muscles contract to withdraw the crown to safety.
4. To feed, the lophophore is extended and the tentacles are spread out into a funnel.
a. Cilia on the tentacles draw water into the funnel.
b. Food particles caught by cilia in the funnel are drawn into the mouth by the pumping action of
the muscular pharynx and by the action of the cilia.
5. Digestion begins extracellularly in the stomach and is completed intracellularly within the
intestine; the gut is complete.
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Chapter 15 – Polyzoa and Kryptrochozoa
6.
15.5.
15.6.
15.7.
Respiratory, vascular, and excretory organs are absent.
a. Gas exchange is through body surface.
b. In some, pores between adjoining zooids permit materials exchange through the colony.
c. In others a funicular system transports lipids and other nutrients through the colony.
7. They possess a ganglionic mass and nerve ring around the pharynx, but no sense organs are
present.
8. Most colonies contain only feeding individuals.
a. Some colonies have heterozooids, zooids incapable of feeding.
b. One modified zooid, an avicularium, resembles a bird beak to snap at invaders.
c. One modified zooid, a vibraculum, has a long bristle to sweep away particles.
9. Most ectoprocts are hermaphroditic.
a. Some species shed eggs into seawater, but most brood their eggs.
b. Brooding occurs within the coelom or in an external chamber called an ovicell.
c. Sometimes embryos proliferate asexually from the initial embryo in a process called
polyembryony.
d. Cleavage is radial but mosaic.
e. Larva of nonbrooding species have a functional gut and swim for a few months before
settling.
f. Larva of brooding species do not feed and settle after a brief free-swimming existence.
g. They attach to the substratum by secretions from an adhesive sac, then metamorphose to the
adult form.
h. New colonies begin from this single metamorphosed primary zooid, which is called an
ancestrula.
i. The ancestrula undergoes asexual budding to produce many zooids of a colony.
j. Freshwater ectoprocts undergo budding that produces statoblasts which are hard, resistant
capsules containing a mass of germinative cells (Figure 15.7).
Clade Trochozoa
A. The taxon Trochozoa consists of animals with a trochophore larva.
B. Trochozoa are a group of thin marine worms called nemerteans and two other taxa that form Clade
Brachiozoa. Molluscs and annelids are also trochozoans.
Clade Brachiozoa
A. The clade Brachiozoa unites brachiopods with phoronids.
B. Both possess a lophophore, though one group is shelled and one is wormlike.
Phylum Brachiopoda
A. Brachiopoda, also called lamp shells, are an ancient group (Figure 15.8).
1. Presently there are about 325 living species, but the fossil record indicates there were some 12,000
species living during the Paleozoic and Mesozoic seas.
2. Brachiopods are attached, bottom-dwelling, marine forms that prefer shallow water, but inhabit all
depths.
B. Externally they resemble bivalved molluscs.
1. They have two calcareous shell valves.
2. Unlike molluscs, they have dorsal and ventral valves (not right and left).
C. They attach to a substrate either directly or by a fleshy stalk called a pedicel.
D. Shell valves distinguish the two classes of brachiopods.
1. Articulata have a connecting hinge with an interlocking tooth-and-socket arrangement.
2. Inarticulata are held together by muscles.
E. Their body occupies only the posterior space between the valves.
1. Extensions of the body wall form mantle lobes that line and secrete the shell.
2. They have a large horseshoe-shaped lophophore in the anterior mantle.
3. Ciliated tentacles are involved in feeding and respiration.
4. Food sources include organic detritus and some algae.
F. There are three coelomic cavities; the protocoel, mesocoel, and metacoel.
1. The viscera are in the metacoel.
2. Nephridia open into the coelom and empty to the mantle cavity.
3. Coelomocytes, which ingest waste particles, are expelled by nephridia.
G. There is an open circulatory system with a contractile heart.
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Chapter 15 – Polyzoa and Kryptrochozoa
15.8
15.9.
H. Most have separate sexes; fertilization is mostly external, though a few species brood their young.
I. Cleavage is radial; coelom and mesoderm formation in some is enterocoelic.
1. The blastopore closes, but its relationship to the mouth is unclear.
2. Articulate larvae metamorphose after attaching by a pedicel.
3. Inarticulate larvae resemble small adults; they do not metamorphose.
Phylum Phoronida
A. Phylum Phoronida contains ~20 species of small, wormlike animals.
1. Most live on the substrate of shallow coastal waters, especially in temperate seas.
2. Range in length from few millimeters to 30 cm.
3. Each worm secretes a leathery or chitinous tube in which it lies free, but never leaves.
a. The tubes may be anchored singly or in a tangled mass on rocks, shells, or pilings or buried in
sand.
b. They thrust out the tentacles on the lophophore for feeding but when disturbed withdraw into
the tube.
B. Feeding Characteristics of Phoronida
1. A lophophore has two parallel ridges curved in a horseshoe shape (Figure 15.10).
2. Cilia in the tentacles direct water currents toward a groove between two ridges on the lophophore.
3. Plankton and detritus caught in this current become entangled in mucus and are carried by cilia to
the mouth.
4. The anus lies dorsal to the mouth.
5. Cilia in the stomach of the U-shaped gut aid in food movement.
C. Form and Function
1. The body wall consists of cuticle, epidermis, and both longitudinal and circular muscles.
a. The protocoel is present as a small cavity in the epistome.
b. A septum separates the mesocoel and the metacoel.
2. Phoronids have an extensive system of contractile blood vessels in a functionally closed
circulatory system period
a. They have no heart.
b. Blood contains hemoglobin within nucleated cells.
3. They have a pair of metanephridia.
4. A nerve ring sends nerves to tentacles and the body wall; the system is diffuse and lacks distinct
ganglion.
D. Reproduction
1. There are both monoecious and dioecious species.
2. At least two species reproduce asexually.
3. Fertilization may be internal or external.
4. Cleavage is radial, and the coelom forms via modified enterocoely.
5. A free-swimming larvae, the actinotroch, metamorphoses into a sessile adult and secretes a tube.
Phylum Nemertea (Rhynchocoela)
A. Characteristics
1. Nemerteans, thread- or ribbon-shaped worms, are often called ribbon worms (Figure 15.11).
2. They have a long muscular tube, the proboscis.
3. There are over 1000 species; most are less than 20 cm long.
4. Their general body plan is similar to that of turbellarians.
a. The epidermis is ciliated and has many gland cells.
b. The excretory system has flames cells; several species have rhabdites.
5. They are mostly dioecious.
6. The ciliated larva of some resembles the trochophore larva of annelids and molluscs.
7. They have derived features that are absent in flatworms.
a. They have an eversible sheathed proboscis.
b. The adult has an anus, producing a complete digestive system that is more efficient.
c. They are the simplest animals with a closed circulatory system.
8. A few are found in moist soil and freshwater; however most are marine.
9. A few are commensals or parasites, but most are active predators of small invertebrates.
B. Form and Function
1. Slender and fragile, longer ones are difficult to study in the laboratory.
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Chapter 15 – Polyzoa and Kryptrochozoa
2.
3.
4.
5.
6.
7.
15.10.
Amphiporus is a common example; it is 20–100 mm long (Figure 15.12).
The body wall is ciliated epidermis with layers of circular & longitudinal muscles (Figure 15.14).
Locomotion is gliding over a slime track; larger species can swim with undulations.
There is a complete digestive tract, from anterior ventral mouth to anus.
Digestion is mostly extracellular, with cilia moving food through the intestine.
Most nemerteans feed on annelids and other small invertebrates.
a. Some species appear to be able to track their prey.
b. Prey is seized with the proboscis that protrudes from the rhynchocoel.
c. The proboscis is everted by fluid pressure and retracted by muscles; it often has a sharppointed stylet at the tip for toxin injection.
8. Nemerteans have a true circulatory system.
a. There is no heart; blood is moved by muscular walls of blood vessels and by body
movements.
b. Blood flow is irregular and may reverse direction.
9. Flame-bulb protonephridia are associated with the circulatory system for excretion.
10. There is a pair of nerve ganglia and longitudinal nerve cords.
11. Some species reproduce asexually, through fragmentation and regeneration.
12. Most sexually reproducing species are dioecious, with external fertilization.
a. Some species are hermaphroditic.
b. Some species have internal fertilization.
c. Some species display ovoviviparity.
C. Phylogeny of Nemertea
1. Nemerteans exhibit spiral cleavage, similar to other lophotrochozoans.
2. They produce a variety of larval forms.
a. There are some similarities to a trochophore, but they lack a prototroch.
b. There is some evidence that a trochophore larvae was present in ancestral nemerteans.
3. There is controversy over whether nemerteans are acoelomate or coelomate.
a. The rhynchocoel is an internal cavity, lined with mesoderm formed via schizocoely.
b. The rhynchocoel differs from a typical coelom in both position and function.
D. Classification of Phylum Nemertea
Class Enopla
Class Anopla
Phylogeny
A. Lophotrochozoans
1. Spiral cleavage and associated developmental characters are presumed ancestral for the
lophotrochozoans.
2. Some of these ancestral characters have therefore been lost in taxa.
a. Spiral cleavage does not occur in rotifers, gastrotrichs, bryozoans, phoronids, or brachiopods.
b. Mesoderm does not come from the typical 4d cell in most of these taxa.
c. It is not yet possible to determine if these characters have been lost, or if they never existed in
these taxa.
3. Placement of Nemertea is contentious as there are some similarities with Platyhelminthes but
nemerteans have a complete digestive tract and a unique coelomic cavity, the rhynchocoel.
4. A recent study that attempted to account for systematic errors in phylogenetic methods found the
following:
a. Strong support for clades Trochozoa, Platyzoa, and Brachiozoa
b. Some support for clade polyzoa and Nemertea was still not well understood.
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Chapter 15 – Polyzoa and Kryptrochozoa
Lecture Enrichment
1.
2.
3.
4.
Provide either specimens or slides to compare size, structure, habitats and relative complexity of the different
invertebrates.
Try to keep the major themes running through the coverage of these nine phyla: syncytial tissues, cuticle,
longitudinal muscles, etc. in the forefront. Compare the body structure of the flatworm and the ribbon worm,
with emphasis on body layers and digestive tract.
The Morbidity and Mortality Weekly Report from the CDC will provide up-to-date data and cases of helminth
infections.
The phylum Cycliophora provides a “current” event view of the process of gaining acceptance for a major new
taxon. Consider how this would be different if it was merely the addition of a new species, a new genus, a new
family, etc. The 1995 Nature 378: 711–714 article contains a large line drawing that can easily illustrate these
organisms for students.
Commentary/Lesson Plan
Background: Even students who live near or visit the coast and with diving experience will not recognize these
remote animals. Preserved specimens or visuals will be important.
Misconceptions: Terms can be misleading (i.e., the term “proboscis” needs to be carefully defined across these
groups as it is uniquely different from a butterfly proboscis, etc.).
Schedule:
HOUR 1
Some Evolutionary Experiments
Lophotrochozoan Protostomes
15.1.
Clade Polyzoa
15.2.
Phylum Cycliophora
15.3.
Phylum Entoprocta
A. Characteristics
B. Form and Function
15.4.
Phylum Ectoprocta (Bryozoa)
A. Characteristics
B. Form and Function
HOUR 2
15.5.
Clade Trochozoa
15.6.
Clade Brachiozoa
15.7.
Phylum Brachiopoda
15.8.
Phylum Phoronida
15.9.
Phylum Nemertea (Rhynchocoela)
A. Characteristics
B. Form and Function
C. Phylogeny of Nemertea
15.10.
Phylogeny
A. Lophotrochozoans
ADVANCED CLASS QUESTIONS:
1. A “pseudo-” something is a false something, but the cavity is actually a cavity. Therefore, why is it called a
pseudocoelom?
2. A set number of cells and cell divisions is an unusual feature in the animal kingdom but extends across several
of the phyla discussed here. Why would an organism find this to be a positive adaptation? Must all features be
explained in adaptation terms?
3. If most of these “minor” phyla are found worldwide, why were many not discovered until the last century?
4. Some of the phyla covered here have protonephridia with flame cells whereas others lack these excretory cells.
How does this diversity argue for classifying them together or splitting the phyla apart?
5. There is a wide range of reproductive modes both within and among the phyla in this chapter, from monoecious
to dioecious to protandrous. How does this affect our view of reproductive modes as a classification trait?
6. Just look at how morphologically similar the entoprocts are to Obelia, etc. Why are they not grouped with the
cnidaria too?
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