Chapter 13 – Cnidarians and Ctenophorans

Fifty Question-and-Answer flashcards summarizing the major concepts, anatomy, life cycles, ecological roles, and terminology for cnidarians and ctenophorans as outlined in the lecture notes.

What specialized organelle do cnidarians use as a "dart gun" to inject toxins?

The nematocyst (a type of cnidae housed in a cnidocyte).

Which unique cell of the phylum Cnidaria contains nematocysts?

Cnidocytes.

How old is the earliest known fossil record of cnidarians?

Roughly 700 million years.

What two main body symmetries are found in cnidarians?

Radial and biradial symmetry.

Name the two body forms in the cnidarian life cycle.

Polyp (sessile) and medusa (motile).

Which life stage of a cnidarian is generally sessile and reproduces asexually?

The polyp stage.

List three asexual reproduction methods used by polyps.

Budding, fission, and pedal laceration.

Which cnidarian stage shows tetramerous symmetry and has statocysts and ocelli?

The medusa.

Define mutualism, commensalism, and parasitism.

Mutualism: both benefit; Commensalism: one benefits, the other unaffected; Parasitism: one benefits, the other harmed.

Why are coral reefs economically important?

They support fisheries and tourism and host over 50 % of marine species in <1 % of ocean area.

Cnidarians are diploblastic with what gelatinous layer between ectoderm and endoderm?

The mesoglea.

Do cnidarians possess excretory, respiratory, or coelomic systems?

No, they lack all three.

What lid covers a nematocyst capsule, and what mechanoreceptor triggers discharge?

Operculum (lid) and cnidocil (trigger).

Where does extracellular digestion occur in cnidarians?

Inside the gastrovascular cavity.

What causes coral bleaching and what is the consequence if heat persists?

Warm water expels zooxanthellae; prolonged heat starves and kills the coral.

How is the cnidarian nerve net unique compared to most animal nervous systems?

It conducts impulses both ways across synapses and lacks myelin or a true center.

What sensory structures, located between lappets, are present in scyphozoans and cubozoans?

Rhopalia.

In hydrozoan colonies, what is the root-like base and the protective covering on the stalk called?

Hydrorhiza (base) and perisarc (covering).

What is the difference between thecate and athecate polyps?

Thecate polyps are covered by a protective cup; athecate polyps are naked.

During the Obelia life cycle, which form reproduces sexually to release gametes?

The free-swimming medusa.

What conical structure on Hydra bears the mouth?

The hypostome.

Name the flotation structure and fishing tentacles of Physalia (Portuguese Man-o-War).

Pneumatophore (float) and dactylozooids (fishing tentacles).

Which class of cnidarians lacks a velum and possesses scalloped umbrella margins with lappets?

Class Scyphozoa.

What is strobilation in Aurelia, and what does it produce?

A transverse budding process that releases ephyrae, immature medusae.

Which upside-down jellyfish has frilly oral arms that filter feed and houses zooxanthellae?

Cassiopeia.

Which cnidarian class has no medusa stage and features a stalked polyp with eight arms?

Class Staurozoa.

List two key features that distinguish cubozoan medusae.

Pedalia at tentacle bases and a velarium (in-turned umbrella edge).

Which anthozoan subclass includes sea anemones and hard corals?

Hexacorallia (Zoantharia).

What ciliated grooves create water flow into a sea anemone’s pharynx?

Siphonoglyphs.

How do many anemones reproduce asexually?

By pedal laceration.

What kind of skeletal structure do hexacorallian corals secrete, and where?

An external calcareous cup (exoskeleton) secreted beneath the living tissue.

What symmetry characterizes octocorallian corals?

Strict eight-part symmetry with eight pinnate tentacles and eight septa.

Give four contributions of zooxanthellae to reef-building corals.

Photosynthesis, carbon fixation, nitrogen/phosphorus recycling, enhanced CaCO₃ deposition.

Differentiate among fringing, barrier, and atoll reefs.

Fringing: close to shore, little or no lagoon; Barrier: parallel to shore, wide lagoon; Atoll: ring reef around a lagoon without an island.

Name the three main zones of a coral reef profile.

Reef front (fore-reef), reef crest, and reef flat (inner reef).

What locomotory structures give Ctenophora their common name "comb jellies"?

Eight rows of fused ciliary plates called combs or ctenes.

Which adhesive cells on ctenophore tentacles capture planktonic prey?

Colloblasts.

What balance organ do ctenophores use for orientation?

A statocyst located at the aboral pole.

Are most ctenophores monoecious or dioecious?

Monoecious (possessing both gonads).

What visual phenomenon makes some ctenophores spectacular at night?

Bioluminescence.

What mechanical property does the cnidarian mesoglea provide?

Hydrostatic support and elastic rebound (the “jelly” of a jellyfish).

Which hydrozoan-specific developmental layer forms medusan muscles?

The entocodon.

Medusae typically show which type of radial partitioning of organs?

Tetramerous symmetry (in fours).

Explain the concept of phylogenetic constraint in polyp/medusa variation.

Some groups (e.g., Anthozoa) branched off before medusae evolved, limiting later body-form options.

Define the term strobila in scyphozoan development.

A polyp stage undergoing transverse segmentation to form ephyrae.

What inward projection of the hydrozoan medusa margin distinguishes it from scyphozoans?

A velum.

How does zooxanthellae loss affect coral calcification?

It reduces photosynthetic energy and carbon fixation, slowing CaCO₃ deposition and weakening reefs.

What class name means "flower animals" and lacks a medusa stage entirely?

Class Anthozoa.

Which ctenophore order contains classic "sea walnuts" and is currently considered paraphyletic?

Order Cydippida.

Why is the nematocyst considered the fastest animal movement recorded?

It can discharge its harpoon-like thread in microseconds with extreme acceleration.