Flatworms, Mesozoans, and Ribbon Worms (Chapter 14)

A comprehensive set of question-and-answer flashcards covering key concepts from Chapter 14 on flatworms, mesozoans, and ribbon worms.

What distinguishes triploblastic animals from diploblastic animals?

Triploblastic animals have three tissue layers (endoderm, mesoderm, ectoderm), whereas diploblasts have only two.

What type of symmetry do most triploblastic animals exhibit?

Bilateral symmetry.

In Protostomia, what structure does the blastopore become?

The mouth.

In Deuterostomia, what does the blastopore form?

The anus.

What are the three major body-cavity conditions found in triploblastic animals?

Acoelomate, pseudocoelomate, and coelomate conditions.

Which phylum contains small, flat marine worms once confused with flatworms?

Phylum Acoelomorpha.

What unique feature is found at the base of Acoelomorpha epidermal cilia?

Rootlets that interconnect to form a network.

Do members of Phylum Acoelomorpha possess a complete gut?

No; their gut is absent or incomplete and saclike.

By what asexual method do many acoelomorphs reproduce?

Fragmentation.

What is the literal meaning of the name "Platyhelminthes"?

Flat worm (Greek platys = flat, helmins = worm).

What type of epidermis characterizes turbellarian flatworms?

A cellular, ciliated epidermis.

What are rhabdites and what is their function in turbellarians?

Rod-shaped secretions that swell with water to form protective mucus.

Which dual gland system enables turbellarians to stick and then release from surfaces?

Adhesive glands for attachment and releasing glands for detachment.

What replaces the epidermis in parasitic flatworms?

A syncytial tegument containing many nuclei within one continuous membrane.

Why is the syncytial tegument advantageous to parasitic flatworms?

It resists the host’s immune defenses and facilitates nutrient absorption.

How do parasitic flatworms without a digestive tract obtain nutrients?

They absorb pre-digested nutrients directly across the tegument from the host.

What osmoregulatory/excretory organ is characteristic of flatworms?

Protonephridia containing flame cells.

How do flame cells function in flatworms?

Flagella beat within a cup (weir) to draw fluid in, filtering wastes and regulating water balance.

What photosensitive structures are present in many flatworms?

Ocelli (eyespots).

Which sensory organs help some flatworms detect equilibrium and water currents?

Statocysts for balance and rheoreceptors for water flow detection.

Name two asexual reproductive methods commonly seen in flatworms.

Fission and fragmentation.

Are most flatworms monoecious or dioecious?

Monoecious (having both sexes in one individual), though they usually cross-fertilize.

How do turbellarians primarily move?

By creeping using muscular waves aided by cilia and mucus.

All adult flukes in Class Trematoda (Subclass Digenea) parasitize which group of hosts?

Vertebrates.

What two host types typically occur in a digenean trematode life cycle?

An intermediate mollusc (often a snail) and a definitive vertebrate host.

What is the first free-swimming larval stage that hatches from a trematode egg?

The ciliated miracidium.

Inside a snail host, which larval stage of Clonorchis develops before producing rediae?

The sporocyst.

Which stage of Clonorchis encysts in freshwater fish and is infective to humans?

The metacercaria.

How do Schistosoma cercariae infect humans?

They actively penetrate human skin while swimming in contaminated water.

Which larval stage present in many digeneans is absent in Schistosoma’s snail phase?

Rediae.

What is "swimmer’s itch" and what causes it?

An itchy rash caused when bird schistosome cercariae mistakenly penetrate human skin and then die.

Which trematode is commonly called the lung fluke?

Paragonimus westermani.

How do humans usually acquire Paragonimus infection?

By eating undercooked or pickled crustaceans containing metacercariae.

On what body part of fish do monogeneans typically live?

The gills.

What ciliated larval stage characterizes Monogeneans?

The oncomiracidium.

What posterior attachment structure anchors monogeneans to their host?

The opisthaptor.

For what purpose is the scolex of a tapeworm specialized?

Attachment to the host’s intestinal wall via hooks and/or suckers.

What are proglottids in tapeworms?

Repetitive reproductive segments that form the strobila.

How do tapeworms maximize surface area for nutrient absorption?

Through fine projections called microtriches covering their tegument.

Which larval stage of Taenia saginata develops within cattle muscle tissue?

The cysticercus (plural cysticerci).

What is the infective stage for humans in the beef and pork tapeworm life cycles?

Ingestion of cysticerci in undercooked beef (T. saginata) or pork (T. solium).

Which evolutionary hypothesis proposes a shared planuloid ancestor for Cnidaria and Platyhelminthes?

The planuloid ancestor hypothesis.

Organisms in Phylum Mesozoa typically contain about how many cells arranged in how many layers?

Approximately 20–30 cells arranged in two layers.

Where are rhombozoans commonly found as parasites?

In the kidneys of cephalopod molluscs.

What asexual stage characterizes orthonectid mesozoans?

A multinucleate plasmodium that later divides into separate male and female individuals.

What unique eversible organ is diagnostic of Phylum Nemertea?

An eversible proboscis housed in a fluid-filled rhynchocoel.

Do nemerteans have a complete or incomplete digestive tract?

A complete digestive tract with both mouth and anus.

What larval form is typical of many nemerteans?

The pilidium larva.

How is blood circulated within nemerteans?

By a true closed vascular system driven by body movements and muscular contractions.

How do nemerteans capture and subdue their prey?

They rapidly evert the proboscis to seize prey and retract it into the rhynchocoel.