Gnathostomulida

Question 1

characteristics (5)

Answer 1

Small, bilaterally symmetrical, acoelomate worms.

Jaw apparatus made of cuticular structures, unique among microscopic animals.

Ciliated epidermis, but unlike other microscopic animals, cilia are arranged in a
single row per cell.

Lack a circulatory, respiratory, and excretory system.

Can survive in low-oxygen environments by relying on diffusion for gas
exchange.

Question 2

adult body form

Answer 2

Tiny, elongated, soft-bodied worms (0.5–4 mm in length).

Question 3

body wall and support

Answer 3

Body Wall: Covered by a monociliated epidermis (each epidermal cell has a
single cilium, unlike most ciliated animals).
-
Support: No rigid skeletal system; body maintains shape through muscular
structure.

Question 4

Locomotion and body cavity

Answer 4

Locomotion: Moves using cilia along the ventral surface and muscular
contractions.
-
Body Cavities: Acoelomate, meaning they have no body cavity.

Question 5

feeding/digestion (3) and circulation

Answer 5

Nutrition (Feeding/Digestion):
- Feed on bacteria and detritus in marine sediments.
- Use a pharyngeal jaw apparatus to scrape food particles from surfaces.
- Digestion occurs intracellularly within gut cells.
-
Circulation: No circulatory system; diffusion is the primary mode of transport for
nutrients and gases.

Question 6

Excretion and gas exchange

Answer 6

Excretion (Osmoregulation): No specialized excretory structures; waste is
eliminated by diffusion.
-
Gas Exchange (Respiration): Oxygen is absorbed directly through the body
surface via diffusion.

Question 7

nervous system (2)

Answer 7

Simple nervous system with a small brain-like cluster of nerve cells.
●
Some species have specialized sensory cilia for detecting environmental
changes.

Question 8

Reproduction and development (3)

Answer 8

Hermaphroditic, with both male and female reproductive organs in the
same individual.
●
Internal fertilization, often involving a complex copulatory system.
●
Direct development—there is no larval stage.

Question 9

Habitat (2)

Answer 9

Found in marine environments, especially in interstitial spaces between sand
grains.
-
Thrive in anoxic (low-oxygen) and sulfide-rich environments, where few other
animals can survive.

Question 10

What makes them unique?

Answer 10

Despite their microscopic size, they have complex jaws, which make them
unique among soft-bodied microscopic animals.

Question 11

When were they discovered?

Answer 11

First discovered in the 1950s, making them a relatively “new” phylum in terms of
classification.

Question 12

What can they tolerate?

Answer 12

low-oxygen conditions