Animals: Porifera, Cnidaria, Lophotrochozoa

Question 1

Phylum Porifera (Sponges)

General Information

Answer 1

Phylum Porifera are basal animals that lack true tissues.
- Phylum Porifera (por = pore, fer = to bear) are informally known as sponges.
- Sponges include ~5,500 extant species, predominantly found in marine environments, ranging from a few millimetres to several meters in height.
- Sponges are sessile (attached to one spot), except for a motile larval stage.

Question 2

Sponge characteristics

Answer 2

Sponges lack true tissues.
- e.g. no internal organs, muscles, nervous system, or circulatory system.
- Sponges only have a few specialized cell types.

Structural support provided by:
- Spicules, tiny needles or rods of silica or calcium carbonate that maintain the shape and structure of the sponge.
- Spongin, a fibrous collagen-protein network for support.

Question 3

What are the four simple cell types of sponges

Answer 3

Choanocytes (collar cells) - line the inner cavity of the sponge (spongocoel). They generate water currents by beating their flagella and capturing suspended food particles. They also deliver oxygen and nutrients to the sponge while also removing waste and carbon dioxide.

Porocytes - tubular cells that make up the pores of a sponge, allowing water to flow through.

Epidermal cells - form the outer layer of tightly packed cells

Mobile amoebocytes - found in the mesohyl, where they transport nutrients to other cells of the sponge body, produce materials for skeletal fibres (spicules), and can differentiate into other cell types as needed.

Question 4

Sponge suspension feeding

Answer 4

Sponges are suspension feeders, capturing food particles suspended in water passing through their bodies.
- Water is drawn by beating choanocyte flagella through pores (ostia) into a cavity called spongocoel and out through an opening (osculum).
- Collar cells extract food particles (protist plankton) using mucus-covered microvilli, which are then engulfed through phagocytosis and digested (endocellular digestion) or transferred to amoebocytes.
- Amoebocytes transport nutrients to other cells or utilize them for structural materials like spicules.

Question 5

Sexual reproduction of sponges

Answer 5

Most sponges are hermaphrodites, possessing both male and female reproductive functions.
- Sponges do not have gonads (reproductive organs).
- Sperm develop from modified choanocytes.
- Eggs develop from modified amoebocytes.
- Most sponges are sequential hermaphrodites, which means they cannot self-fertilize.
- Sponge fertilization can be external (rare) or internal (common), with zygotes developing into motile ciliated larvae within the mesohyl.
- Sponge embryonic development is highly variable.

Sponges can reproduce asexually by fragmentation and by budding.

Question 6

Sponge ecology and importance

Answer 6

Sponges contribute to marine ecosystems by cleaning water through suspension feeding.
- A single sponge filters thousands of kilograms of water to grow 1 kg of mass.

Many sponges form symbiotic mutualisms with unicellular algae.
- Limits the distribution of these sponges to the photic zone, the zone to which light can penetrate the water column.

Most sponges produce toxic chemicals, making them unpalatable to predators.
- Many organisms live inside sponges.

Soft sponges, particularly those with spongin, are harvested for bath and art sponges

Question 7

Clade Eumetazoa

Answer 7

Clade Eumetazoa includes all animals, excluding sponges and a few other groups, characterized by the presence of true tissues.

The most basal Eumetazoan phyla are Ctenophora and Cnidaria.
- Despite sharing similar body plans, these phyla are not closely related.
- Most basal Eumetazoans exhibit radial symmetry and are diploblastic, comprising ectoderm and endoderm layers.
- Basal eumetazoans develop muscle and nerve tissues but lack centralized nervous systems or cephalization.

Question 8

Phylum Cnidaria

Answer 8

Phylum Cnidaria is one of the earliest lineages of Eumetazoa.

Phylum Cnidaria includes a diverse range of both sessile and motile forms, including jellyfish, sea anemones, and corals.
− ~10,000 extant species, the majority of which inhabit marine environments.

Cnidarians have a simple body plan that is radially symmetrical and diploblastic

Question 9

General Cnidarian body plan

These include jellyfish

Answer 9

Cnidarians have a sac-like body plan with a central gastrovascular cavity (digestive compartment).
- A single opening to the gastrovascular cavity functions as a mouth and anus.
- Tentacles equipped with stinging cells, known as cnidocytes, surround this opening for prey capture and defence.
- The body comprises an outer epidermis derived from the ectoderm and an inner gastrodermis derived from the endoderm that lines the gastrovascular cavity.
- The epidermis and gastrodermis are separated by the mesoglea (‘middle jelly’), a gelatinous, non-cellular matrix.
- Gas exchange occurs via diffusion across the epidermis.

Question 10

What are the 2 variations of the Cnidarian body plan?

Answer 10

The sessile polyp, which attaches to a substrate with its oral end upwards, such as a sea anemone
- Sea anemone
- Coral

The bell-shaped, motile medusa that moves freely through the water using a hydrostatic skeleton, with its oral end downwards, like a jellyfish
- Jellyfish

Question 11

Cnidarian feeding

Answer 11

Cnidarians are predators that use tentacles to capture prey.
- Tentacles are armed with cnidocytes, explosive cells that function in defence and capture of prey

Cnidocytes contain complex organelles called nematocysts that deliver stinging toxins to immobilize prey.
- The nematocyst consists of a bulb- shaped capsule containing a coiled tubular thread.
- When an external hair-like “trigger” (a mechano- and chemo-receptor) is activated, the tubular thread is ejected forcefully into the target organism, and toxins are injected to immobilize the prey.
- Tentacles move the immobilized prey to the gastrovascular cavity for digestion.

Question 12

What are the 2 major clades of phylum Cnidaria

Answer 12

Cnidarians diverged into two major clades early in their evolutionary history.

Clade Medusozoa includes all cnidarians that produce a medusa stage in their life cycle.
- Hydrozoans
- Scyphozoans (jellyfish)
- Cubozoans (box jellyfish)

Clade Anthozoa includes cnidarians that occur only as polyps, such as corals and sea anemones.
- Solitary or colonial form

Question 13

Hydrozoans

Clade Medusozoa

Answer 13

Most hydrozoans alternate between polyp and medusa forms.
- The polyp is sessile and often colonial, reproducing asexually by budding.
- e.g.fresh water hydrozoan Hydra exists solely in polyp form (only reproduces asexually).

Medusae are produced asexually through budding from polyps, but they reproduce sexually.
- Motile larvae settle and transform into polyps.
- The life cycle alternates between sexual and asexual forms, both of which are diploid.

Question 14

Scyphozoans/cubozoans

Clade Medusozoa

Answer 14

Medusa is the predominant stage in the life cycle of most scyphozoans (“true jellyfish”) and cubozoans (box jellyfish).
- Coastal scyphozoans may have a brief polyp stage, whereas oceanic species generally have no polyp stage.
- Jellyfish (scyphozoans) use their hydrostatic skeleton to move through contraction-pulsation of the bell-shaped body.
- Some species actively swim, while others float passively.
- Most of the medusa’s mass consists of gelatinous ‘middle jelly’ (mesoglea), which is mostly water.
- Box jellyfish (cubozoans) often have highly toxic cnidocytes.
- e.g. the sting of various cubozoan species found in Australian tropical waters can lead to respiratory failure, cardiac arrest, and death within minutes.

Question 15

Clade Anthozoa

Phylum Cnidaria

Answer 15

Anthozoans include corals and sea anemones (antho = flower).

Anthozoans occur only as polyps; they lack a medusa stage.
- Some live as single individuals, such as most sea anemones.
- Others are colonial, like most corals.
- Colonies form asexually through budding or fission.
- Many corals gain nutrition from symbiotic algae.
- Corals that secrete a hard exoskeleton (external skeleton) of calcium carbonate are crucial reef-building organisms.

Question 16

Cnidarian ecology and importance

Answer 16

Reefs constructed by corals provide shelter and food for numerous marine species.
- Coral reefs are economically important (tourism, fishing). − Runoff from agriculture and climate change are two major threats to coral reefs.
- Coral bleaching due to the loss of symbiotic algae is symptomatic of warming oceans.

Jellyfish blooms indicate ecosystem stress.
- Blooms are often caused by overfishing of jellyfish predators and eutrophication (nutrient addition), which increase zooplankton abundance and decrease [O2].

Toxins produced by cubozoan species, like box jellyfish, can be dangerous to humans and are sometimes fatal.

Question 17

True or False

The majority of animals exhibit bilateral symmetry

Answer 17

True

The majority of animals exhibit bilateral symmetry, belonging to the clade Bilateria.
- Triploblastic (ecto-, endo-, and mesoderm).
- Muscle tissues and most internal organs originate from the mesoderm.
- Bilateria includes protostomes and deuterostomes.
- Most bilaterians possess a coelom (body cavity) and a digestive tract with both a mouth and an anus.

Question 18

Clade Bilateria

Answer 18

Bilaterians are strongly differentiated along the anterior- posterior axis.
- This differentiation facilitates directional motility.
- Sensory and feeding structures are concentrated in the anterior region as a head region (cephalization).
- Cephalization involves the concentration of neural ganglia, forming a rudimentary brain.
- Sedentary bilaterians, such as sessile shellfish, exhibit reduced cephalization.
- Digestive and reproductive structures typically discharge posteriorly.
- Hox genes regulate anterior-posterior differentiation during embryonic development.

Question 19

Phylum Acoela

Basal bilaterians

Answer 19

Molecular phylogenetic analyses position phylum Acoela as basal to other bilaterians.
- Acoela diverged before the emergence of the three primary bilaterian clades.
- The majority of Acoela species are marine ‘worms’ (~400 species) that are predators or scavengers, feeding on small organisms or organic detritus.

Characteristics of Acoela:
- Small, flattened body with minimal cephalization and a simple nerve net (no brain)
- No body cavity (no coelom or hemocoel) or complex organ systems.
- Simple digestive system with a mouth, but no gut cavity or anus.
- Use endocellular digestion (digestion occurs within individual cells).

Question 20

What are the three major clades in clade Bilateria?

Answer 20

• Lophotrochozoa
• Ecdysozoa
• Deuterostomia

Question 21

Clade Lophotrochozoa

Animals

Answer 21

Includes nearly half of all animal phyla (17 phyla).
- Lophotrochozoa is defined by molecular phylogeny; however, relationships among lophotrochozoan phyla remain unclear.

Exhibits the widest range of animal body plans:
- Very simple (e.g. flatworms) to morphologically and behaviorally very complex (e.g. octopuses).

No single unifying characteristic.
- Some lophotrochozoans develop a lophophore for feeding, others pass through a trochophore larval stage, and a few have neither!

Lophotrochozoa includes flatworms, rotifers (syndermata), ectoprocts, brachiopods, molluscs, and annelids.

Question 22

Phylum Platyhelminthes

Answer 22

Flatworms (~20k species)
- Inhabit marine, freshwater, and damp terrestrial habitats.
- platy = flat, helminth = worm

Simple body plan:
- Triploblastic development but lack fluid-filled body cavities.
- Incomplete digestive tract.
- Have a mouth and a gut cavity, but no anus.
- Some parasitic flatworms lack a mouth and gut cavity.
- No circulatory or gas exchange systems.
- Gas exchange occurs across the body surface, aided by a dorsoventrally flattened shape maximizing surface area.

Question 23

What are the two lineages of flatworms

Answer 23

Catenulida - Low diversity

Rhabditophora is more diverse (~20k species), including free-living and parasitic species.

Question 24

Free-living Rhabditophora

Answer 24

Well-known free-living Rhabditophora are planarians.
- Planarians inhabit freshwater and prey on smaller animals.
- Exhibits anterior cephalization, with light-sensitive eyespots and a pair of ganglia (dense clusters of nerve cells) that extend to a pair of ventral nerve cords (form a centralized nerve net).
- Gastrovascular cavity with one opening.
- No anal opening; undigested food ejected from the mouth.
- Planarians are hermaphrodites and can reproduce sexually or asexually (fission).

Question 25

Parasitic Rhabditophora

Answer 25

Parasitic rhabditophorans live in or on other animals. - Common characteristics of parasitic Rhabditophora: - Suckers or hooks for host attachment. - Tough outer covering to protect the parasite within their hosts. **Complex life cycles** involving two or more hosts: - **Intermediate host**, where asexual reproduction occurs. - **Definitive host**, where the parasite undergoes sexual reproduction. Two important groups are **trematodes** and **tapeworms**

Question 26

Trematodes

Answer 26

Trematodes parasitize a wide range of hosts and often have complex life cycles involving **alternating sexual and asexual stages**. - e.g. blood flukes, affecting ~200 million people worldwide, can cause symptoms like pain, anemia, and diarrhea.

Question 27

Mechanisms of parasitism | Tapeworms

Answer 27

Tapeworms are parasites of vertebrates. Tapeworms are entirely **endoparasitic**, living within the **digestive systems of their hosts**. - Tapeworms do not have a digestive system; instead, they absorb nutrients directly from the host’s intestine - The anterior scolex has suckers and hooks that allow the tapeworm to attach to the host. - Posterior to the **scolex** is a long chain of segments (proglottids) that contain male and female sex organs (hermaphrodites). - New proglottids form at the base of the scolex, while older proglottids are located toward the rear. - Mature proglottids, which are filled with fertilized eggs, exit the host’s body in feces.

Question 28

Life Cycle of Parasitic Rhabditophora

Answer 28

Tapeworms have complex life cycles, requiring two or more hosts to complete: - Fertilized eggs are consumed by an intermediate host. - Develop into larvae that encyst in tissues of intermediate host. - The intermediate host is eaten by the definitive host. - Larvae develop into adult tapeworms in the gut of the definitive host. - Fertilized eggs produced by sexual reproduction leave the definitive host’s body in feces

Question 29

Parasitic Rhabditophora – Human Infections

Answer 29

Tapeworms infect humans: - Pork/beef tapeworms (Taenia spp.) from undercooked meat. - Fish tapeworm (Diphyllobothrium spp.). - Fish tapeworm infection from sushi made from non-marine fish, including salmon! - Dog and cat tapeworms also occasionally infect humans through the ingestion of fleas, their intermediate hosts

Question 30

What are the two groups under Phylum Syndermata

Answer 30

Phylum Syndermata unites two groups previously classified as separate phyla - **Rotifers** (~1,800 species): free-living, microscopic animals. - **Acanthocephalans** (~1,100 species): specialized parasites of vertebrates.

Question 31

Rotifers

Answer 31

Rotifers are tiny animals that inhabit freshwater, marine environments, and damp soil. Although microscopic, rotifers are **multicellular with distinct organ systems**. - Rotifers have an **alimentary canal**, a digestive tract with both a mouth and an anus, surrounded by a fluid-filled hemocoel. Many rotifers **reproduce asexually** through **parthenogenesis**, where females produce offspring from unfertilized eggs. Rotifers are crucial in freshwater ecosystems; they help **decompose organic material** and **filter water** by consuming small waste particles and algae.

Question 32

Acanthocephalans

Answer 32

Acanthocephalans, also known as “spiny-headed worms”, are sexually reproducing **parasites** of vertebrate hosts. These parasites have an **anterior specialized proboscis with curved hooks**, allowing them to attach securely to host tissues. - They **lack a complete digestive tract** and absorb nutrients directly from their hosts. Acanthocephalans have **complex life cycles that typically involve two or more hosts**, allowing them to develop through different stages in various environments.

Question 33

Lophophorates | Ectoprocts and Brachiopods

Answer 33

Lophophorates have a unique feeding structure, a **lophophore**, which is a fan of ciliated tentacles surrounding the mouth. - Lophophorates have a **coelom**. Lophophorates include two phyla of aquatic organisms: Ectoprocta and Brachiopoda. - **Both are sessile**, but both have **motile, planktonic larval stages** after sexual reproduction.

Question 34

Phylum Ectoprocta

Answer 34

Ectoprocts (also called bryozoans) are sessile colonial animals that superficially resemble coral. - More diverse than brachiopods (~4,500 extant spp.); **mostly marine**. - Abundant throughout the Paleozoic era (>15k spp.). Most marine ectoproct colonies are encased in a **hard exoskeleton**. - Colony members secrete CaCO3 tubes. - Exoskeletons of freshwater ectoprocts are gelatinous or chitinous. Ectoprocts are capable of both sexual (most hermaphrodites) and asexual reproduction (budding to form colonies). Ectoprocts are **suspension feeders** using a **retractable lophophore**. - The gut is U-shaped, with the anal opening located outside of the lophophore. - ecto = outside, proct = anus

Question 35

Phylum Brachiopoda

Answer 35

Brachiopods (lamp shells) superficially resemble bivalve molluscs like clams. - Brachiopods are marine (~300 spp.) and most attach to the seafloor by a stalk (brach = arm, pod = foot). Brachiopods are **suspension feeders** using **paired lophophores**. - Have a complete gut ± anal opening. Brachiopods were highly diverse (>12k spp.) during the Paleozoic. - Dominant reef-building animals of the Paleozoic era.

Question 36

What are the major clades in Phylum Lophotrochozoa? | Name all six

Answer 36

- Platyhelminthes - Flatworms - Rotifera - Rotifers - Ectoprocta - Brachiopoda - looks like mulluscs - Mollusca - Annelida

Question 37

What phylums are in Trochozoans? What are the traits of the clade?

Answer 37

Trochozoa includes phylum Annelida, the segmented worms, and phylum Mollusca (snails, squid, octopods, clams, etc). - Annelids and molluscs do not resemble each other as adults. Trochozoans share patterns of early embryonic development, including microscopic **motile trochophore larvae**. - Trochophores are **planktonic larvae** characterized by **two bands of cilia that facilitate movement and guide food toward the mouth**. - Above the cilia bands is a sensory plate, including a **simple eye spot**.

Question 38

Phylum Mollusca

Answer 38

Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids. - Very diverse: >100k extant species. - While predominantly marine, molluscs also inhabit freshwater habitats, and some snails and slugs are terrestrial. Molluscs are **soft-bodied animals**, but most are protected by a **hard calcareous (i.e. calcium carbonate) shell**. - The shell is lost or reduced in many taxa. - The name “Mollusca” originally stemmed from the soft bodies of octopuses and cuttlefish (moll = soft).

Question 39

What are the three primary parts of a molluscs body plan

Answer 39

All molluscs have an unsegmented body plan with three primary parts: The **muscular ventral foot** is used for locomotion. A **visceral mass** above the foot containing internal organs. - Includes a **reduced coelom** and a **larger hemocoel** enclosing most internal organs The **mantle** is a thin tissue layer that secretes the shell. - The mantle is often involved in respiration and excretion by forming a water-filled **mantle cavity** in many molluscs

Question 40

Characteristics of Mollusca

Answer 40

The mollusc body plan typically has **gills within the mantle cavity** for gas exchange and sometimes feeding (e.g. bivalve scallops). Most molluscs feed using a rasp-like radula to scrape or cut food. Most molluscs have an **open circulatory system**. - The heart pumps hemolymph through arteries into the open hemocoel, bathing the organs. Most molluscs have **separate sexes**, with gonads located in the visceral mass; however, many snails are hermaphrodites. Most aquatic molluscs undergo **indirect development** through a ciliated **trochophore larval stage**.

Question 41

What are the four major classes of molluscs

Answer 41

- Polyplacophora (chitons). - Gastropoda (snails and slugs). - Bivalvia (clams, oysters, and other bivalves). - Cephalopoda (squids, octopuses, cuttlefish, and chambered nautiluses).

Question 42

Class Polyplacophora (chitons) | Molluscs

Answer 42

Class Polyplacophora (chitons) are oval-shaped marine molluscs (~1000 spp.) with a protective shell consisting of eight dorsal plates. - Use their foot like a suction cup to adhere to rocks and their radula to scrape algae off surfaces.

Question 43

Traits of Class Gastropoda | Molluscs

Answer 43

Gastropods are a diverse group that includes **slugs** and **snails**. - ~70k species (~75% of extant molluscs). **Most gastropods are marine**, but many occupy freshwater and terrestrial habitats. - Most aquatic gastropods have trochophore larvae. - Terrestrial gastropods have direct development of juveniles without a trochophore larval stage. Most gastropods have a **single, coiled, tubular shell** secreted by the mantle. - The shell protects from injury, dehydration, and predation. - Many lack shells entirely; ‘slugs’ are those that have lost shells. Terrestrial snails and slugs have lost their gills. - The mantle cavity evolved into a simple lung Gastropods move using muscular motions of the ventral foot or cilia action. - gastro = stomach, pod = foot Gastropods exhibit **strong cephalization**, featuring a well- developed head region with a mouth, neural ganglia (brain), antennae, and eyes. Most gastropods are herbivores, scraping algae or plants with the radula. - Some are predatory, e.g. cone snail

Question 44

Which animals are in Class Bivalvia? What does their shell structure look like?

Answer 44

Bivalves are aquatic, mostly marine (~20k species). - Includes many species of clams, oysters, mussels, and scallops. - bi = two, valv = door Bivalves have a **shell** divided into t**wo halves** drawn together by adductor muscles. - Some have eyes and sensory tentacles along the edge of their mantle.

Question 45

Traits of Class Bivalvia | Molluscs

Answer 45

**Hard Shells** - two part **Most bivalves are sedentary**, but others exhibit limited mobility. - e.g. clams dig with their foot; scallops can swim by clapping their valves: - Some are sessile, e.g. marine mussels Most bivalves are suspension feeders, utilizing gills in the mantle cavity for feeding and gas exchange. - **Lack cephalization: no head, no radula**. Bivalves are **economically important**. - Many marine species are eaten in whole (e.g. oysters) or in part (e.g. scallop adductor muscles). Some bivalves have become **invasive pests**. - e.g. zebra mussels have disrupted North American freshwater ecosystems

Question 46

What animals are in clade Cephalopoda | Molluscs

Answer 46

Cephalopods include squids, octopuses, cuttlefish, and chambered nautiluses (~900 species). − cephalo = head, pod = foot

Question 47

# True or false Cepholopods have shells

Answer 47

**False** (Mostly) No external shell. - Shell lost during cephalopod evolution, except in nautiluses. - Some have a gas-filled internal shell used for buoyancy control, e.g. the cuttlebone of cuttlefish.

Question 48

Cephalopod mobility

Answer 48

Cephalopods have long tentacles arranged around the mouth. - Tentacles evolved from the primitive molluscan foot. - Tentacles are used for feeding, grasping, and movement. - Tentacles have suckers and/or hooks. Squids and some octopus use their **siphon** (fused tube of the mantle) to fire a jet of water, which allows them to swim very quickly. - Most octopuses creep along the seafloor in search of prey

Question 49

# True or False Cephalopods are the only molluscs with **closed circulatory systems**

Answer 49

**True** Supports a very active lifestyle. - Muscle contractions used for jet propulsion might otherwise squeeze blood out of areas where it is needed.

Question 50

Cephalopod ecological role and feeding

Answer 50

Cephalopods are **predatory**. - Radula is modified as a biting beak. - Immobilize prey with poisonous saliva.

Question 51

Cephalopod brain and behaviour | Remember this is the octopus group

Answer 51

Cephalopods have well-developed sense organs and complex brains. - Cephalopods have excellent vision, rivalling that of vertebrates. - Cephalopods show **complex behaviour**. - Communicate by colour and posture - Learn visual patterns and solve problems to capture prey - Many octopuses show **maternal care**, protecting fertilized eggs until the young hatch.

Question 52

Cephalopod development

Answer 52

Unlike other aquatic molluscs, cephalopods hatch as **direct-developing juveniles**, resembling miniature adults from birth. - Cephalopods do not develop through trochophore larvae.

Question 53

Phylum Annelida

Answer 53

Phylum Annelida are **segmented worms**. - Their bodies are composed of a series of fused rings called annuli (annulus = ring). Phylum Annelida includes a wide array of mostly marine and freshwater species (>22k species). - Earlier taxonomic classifications recognized three main classes: Polychaeta (polychaetes), Oligochaeta (oligochaetes), and Hirudinea (leeches). - These classes are now recognized as paraphyletic. Molecular phylogenetic analyses indicate that the annelids can be divided into two major clades: - Clade Errantia - Clade Sedentaria Named after the predominant lifestyle of their members

Question 54

What are the two major clades under Phylum Annelida?

Answer 54

Molecular phylogenetic analyses indicate that the annelids can be divided into two major clades: - Clade Errantia - Clade Sedentaria Named after the predominant lifestyle of their members

Question 55

Characteristics of Annelida | Name all seven

Answer 55

Annelids have **segmented bodies**, each segment containing similar internal and external anatomy, separated by internal partitions called **septa**. - The digestive tract is unsegmented. Annelids have a **coelom**, a fluid-filled body cavity lined with mesodermally derived tissues, which functions as a **hydrostatic skeleton**. Annelids exhibit **cephalization**, featuring a well-developed mouth and neural ganglia (brain), and possess a **complete digestive system** with both mouth and anus. Annelids have a **closed circulatory system**, where blood circulates within vessels

Question 56

Sedentarians | Phylum Annelida

Answer 56

Sedentarians have a **more sessile, sedentary lifestyle than errantians**. - Sedentarians are typically burrowers, feeding by ingesting substrate, or they are tube dwellers, living in protective tubes. - Tube-dwelling sedentarians often have elaborate gills or tentacles used for suspension feeding (filter food particles out of the water, e.g. giant tube worms: Many sedentarians **lack parapodia and have reduced cephalization**. Some sedentarians undergo direct development without a trochophore larval stage. Clade Sedentaria includes **leeches** and **earthworms**

Question 57

Sedentarians – Leeches | Phylum Annelida

Answer 57

Leeches are **predominantly found in freshwater habitats** but can also be marine or terrestrial. - **Predators** of invertebrates: - **Parasites** that feed on the blood of vertebrates. - Parasitic leeches secrete anesthetics and anticoagulants. - Historically, leeches were utilized for bloodletting as a general cure for many ailments, and they are now being reconsidered for medicinal purposes. Leeches **lack parapodia and chaetae**. Leeches are **simultaneous hermaphrodites** but typically reproduce through reciprocal fertilization, exhibiting direct development without a larval stage.

Question 58

Sedentarians – Earthworms | Ecological roll

Answer 58

Earthworms, though less morphologically diverse than errantians, play crucial roles in terrestrial and freshwater ecosystems. Earthworms are important for soil formation and maintenance of soil fertility. - Earthworms ingest soil, digesting the organic matter.

Question 59

Sedentarians – Earthworms | Structure

Answer 59

Earthworms **lack parapodia** but possess **four pairs of chaetae per body segment**. Earthworms (and leeches) are hermaphrodites but cross-fertilize by **reciprocal fertilization**. - Earthworms do not free-spawn. - Some reproduce asexually by fragmentation. - Earthworms undergo direct development in gelatinous cocoons (no larval stage).

Question 60

Errantians | Phylum Annelida

Answer 60

Most errantians are mobile marine organisms (often predatory). - Errantians are active swimmers or crawlers. Many errantians have a pair of paddle-like or ridge-like structures called **parapodia** (“beside feet”) on each body segment. - Each parapodium has numerous **chaetae**, bristles made of chitin. - External gills are often associated with parapodia. - Parapodia are not unique to this clade. Errantians typically have a **differentiated head** with sensory antennae and eyes. Errantians typically reproduce sexually by free-spawning. - Eggs and sperm are released into the water. - Fertilization occurs externally, and fertilized eggs typically hatch into trochophore larvae.