Dicyemida

Question 1

Synapomorphies (5)

Answer 1

Tiny, parasitic, and highly simplified marine invertebrates.

Lack true tissues, organs, circulatory, respiratory, and digestive systems.

Consist of only 20–40 cells arranged in a simple body plan.

Live exclusively in the kidneys of cephalopods (squid, cuttlefish, and octopuses).

Have a unique, highly specialized reproductive cycle involving both sexual and
asexual phases.

Question 2

general adult body form

Answer 2

Small (usually 0.5–3 mm in length) and elongated, resembling a tiny worm.

Question 3

Body wall and support

Answer 3

Body Wall: Composed of a single layer of ciliated cells, which assist in
movement within the host’s kidney tubules.
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Support: No skeleton; body structure is maintained by cellular organization.

Question 4

Locomotion and Body cavity

Answer 4

Locomotion: Moves via ciliary gliding within the host’s kidney tubules.

Body Cavities: No coelom or defined internal compartments.

Question 5

Feeding/digestion and circulation

Answer 5

Nutrition (Feeding/Digestion): Absorbs nutrients directly from the urine of the
host cephalopod.
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Circulation: Lacks a circulatory system; diffusion is the primary mode of nutrient
and gas exchange.

Question 6

Excretion and gas exchange

Answer 6

Excretion (Osmoregulation): Waste elimination occurs through diffusion.

Gas Exchange (Respiration): Oxygen exchange occurs passively through the
body surface.

Question 7

Nervous system

Answer 7

Nervous System (& Senses): No nervous system or sensory organs.

Question 8

Reproduction and development (4)

Answer 8

Asexual Phase (Nematogen Stage): Produces vermiform larvae within
the host, allowing for population expansion without leaving the host.
●
Sexual Phase (Rhombogen Stage): Produces gonads (infusorigen), which
create sexually produced larvae (infusiform larvae) that eventually leave
the host to infect a new cephalopod.
●
The life cycle ensures survival and transmission between generations of
cephalopods.
●
They have eutely, which is where each adult of a given species has the
same number of cells

Question 9

Habitat (3)

Answer 9

Exclusively found in the renal appendages (kidneys) of cephalopods.

Most commonly infect benthic (bottom-dwelling) species of squid, cuttlefish, and
octopuses.

Highly host-specific, often restricted to a particular cephalopod species.

Question 10

4 fun facts

Answer 10

One of the simplest multicellular organisms, making them a subject of interest in
evolutionary biology.
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Originally thought to be protists due to their extreme simplicity but later classified
as metazoans.
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Despite their parasitic nature, they do not appear to cause harm to their
cephalopod hosts.
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Their exact evolutionary relationships remain debated, with some scientists
considering them related to flatworms.