Chapter 18 – Smaller Ecdysozoans

Fifty question-and-answer flashcards covering taxonomy, anatomy, life cycles, and ecological importance of smaller ecdysozoan phyla: Nematoda, Nematomorpha, Kinorhyncha, Priapulida, Loricifera, Onychophora, and Tardigrada.

Approximately how many described species are in Phylum Nematoda, and where are they especially abundant?

About 25,000 described species (with many more undiscovered), and they are especially abundant in topsoil.

Which nematode species is a model organism whose entire cell lineage has been traced?

Caenorhabditis elegans.

What is "eutely" in nematodes?

A fixed, species-specific number of somatic cells in the body.

Name two key structural features of nematodes related to their body wall and movement.

A thick, flexible, nonliving collagenous cuticle and only longitudinal muscles (no circular muscles).

What type of body cavity do nematodes possess?

A fluid-filled pseudocoelom.

Which muscle type found in many worms is absent in nematodes?

Circular muscles.

How is waste expelled from a nematode’s intestine?

Muscles pull the anus open while high hydrostatic pressure forces the waste out.

Describe the nematode excretory system.

Gland cells empty into an excretory pore; there are no protonephridia.

From the nerve ring around the pharynx, which two longitudinal nerve cords arise in nematodes?

A dorsal nerve cord and a ventral nerve cord.

What are amphids (and phasmids in parasites) in nematodes?

Paired pit-like chemosensory organs with cilia located on the head (phasmids are analogous structures near the tail in many parasitic forms).

How many juvenile stages do nematodes have, and what separates each stage?

Four juvenile stages, each separated by a molt of the cuticle.

What infection rate of Ascaris lumbricoides has been recorded in some southeastern U.S. counties?

About 25 % of the human population.

Trace the migration path of Ascaris larvae after a host swallows infective eggs.

Hatch in intestine → burrow into veins/lymph → carried to lungs → break into air spaces → crawl up trachea → are swallowed → mature in the intestine.

Approximately how many eggs can a female Ascaris produce each day?

Roughly 200,000 eggs per day.

How do hookworm juveniles normally enter a human host?

They leave the soil and actively burrow through skin that comes in contact with the ground.

What serious health effect can heavy hookworm infection produce in children?

Retarded physical and mental growth due to chronic blood loss and nutrient depletion.

By what route do Trichina juveniles reach and encyst in human muscle tissue?

After birth in the intestine, they enter the bloodstream, disperse throughout the body, and encyst intracellularly in skeletal muscle.

Consumption of what under-cooked food most commonly transmits trichinosis?

Undercooked pork (or other meat containing encysted larvae).

In filarial worms, what are microfilariae?

Live, tiny, sheathed first-stage larvae released into the host’s blood or lymph.

Name two human diseases caused by filarial worms.

Lymphatic filariasis (elephantiasis) and river blindness (onchocerciasis).

Which common pet ailment in the United States is caused by a filarial worm?

Dog heartworm disease.

What is the common name for members of Phylum Nematomorpha?

Horsehair worms.

In what type of hosts are juvenile Nematomorpha parasites?

Arthropods (often insects or crustaceans).

Which three organ systems are entirely lacking in Nematomorpha?

Circulatory, respiratory, and excretory systems.

How are Nematomorpha eggs released into the environment?

Adults discharge eggs into water as long strings from a cloaca.

How many trunk segments characterise the body of a Kinorhynch?

Eleven trunk segments.

What are scalids in Kinorhyncha, and give two of their functions.

Spines on the head (introvert) used for locomotion, chemoreception, and mechanoreception.

Describe the excretory organ of Kinorhyncha.

A pair of multinucleated solenocyte protonephridia.

How many juvenile stages occur before the non-molting adult stage in Kinorhyncha?

Six juvenile stages.

What general habitat do priapulid worms occupy?

Marine sediments from intertidal zones to deep-sea mud.

List the three principal body regions of a priapulid.

Introvert (proboscis), trunk, and caudal appendage(s).

Which iron-containing respiratory pigment may be found in priapulid amebocytes?

Hemerythrin.

What distinctive protective structure defines members of Phylum Loricifera?

A rigid, corset-like external case called a lorica.

What is the specialized larval stage of Loricifera called?

A Higgins larva.

How many circlets of scalids surround the introvert of Loricifera?

Nine circlets of scalids.

Besides Arthropoda, which two phyla form Clade Panarthropoda?

Onychophora and Tardigrada.

In Panarthropods, what cavity largely replaces the coelom as the main body cavity?

The hemocoel (derived from a fusion of coelom and blastocoel).

What is the common name for animals in Phylum Onychophora?

Velvet worms (or walking worms).

How do Onychophorans capture prey and deter predators?

They eject streams of sticky slime from oral papillae.

Why are Onychophorans restricted to moist habitats?

Their spiracles (openings of the tracheal system) cannot close, leading to high water loss.

What notable social trait has been documented in some Onychophoran species?

Social behavior including group hunting and hierarchical feeding.

Within what numerical range can the number of leg pairs fall in Onychophorans?

Between 14 and 34 pairs of unjointed legs.

What is the common name for phylum Tardigrada members?

Water bears.

What is cryptobiosis in tardigrades?

A state of suspended animation with negligible metabolism that enables survival of extreme conditions.

How do tardigrades obtain nutrients from plant or animal cells?

They use stylets to pierce cells and suck out their contents via a muscular pharynx.

How many claws can each tardigrade leg bear?

Four to eight claws per leg.

Which body space serves the function of a circulatory system in tardigrades?

A hemocoel filled with fluid; tardigrades lack a dedicated circulatory system.

How many molts do tardigrades typically undergo in their life cycle?

At least four molts.

Give two different ways tardigrade eggs may be released or fertilized.

Eggs can be cemented to substrates, left inside the shed cuticle, fertilized internally in a seminal receptacle, or fertilized indirectly when a male deposits sperm into the shed cuticle containing eggs.

List three extreme conditions that tardigrades can survive while in cryptobiosis.

Extreme temperatures (very high or near absolute zero), severe dehydration, high radiation levels, vacuum of space, or exposure to strong chemicals (any three).