Chapter 17 - Annelids and Allies

Fifty question-and-answer flashcards covering the key concepts, structures, functions, and representative taxa of Annelida, Echiura, and Sipuncula from Chapter 17.

What type of body cavity do annelids possess and how is it formed?

They are coelomate protostomes whose coelom forms by schizocoely.

What kind of embryonic cleavage characterizes annelids?

Spiral mosaic cleavage.

Which three phyla are covered in Chapter 17?

Annelida, Echiura, and Sipuncula.

What are the external rings marking annelid segments called?

Annuli.

What name is given to the chitinous bristles found on many annelids?

Setae.

Define metamerism in annelids.

The division of the body into serially repeated segments called metameres.

How is the circulatory system of most annelids organized?

They have a closed circulatory system (though many leeches use coelomic sinuses instead).

Name the two regions that compose the annelid head.

Prostomium and peristomium.

What is the terminal segment of an annelid body called?

The pygidium.

In which class of annelids are parapodia considered an ancestral trait?

Class Polychaeta.

Where are polychaetes primarily found?

Mostly in marine habitats.

Into what two lifestyle groups are polychaetes commonly divided?

Sedentary forms and mobile forms.

What is the main function of parapodia in polychaetes?

Locomotion and gas exchange; they usually bear setae or gills.

What larval form is typical of most polychaetes?

The trochophore larva.

Which feeding strategy is typical of mobile polychaetes such as Nereis?

They are predators or scavengers.

What defensive adaptation is unique to fireworms?

Hollow, brittle setae that break off and inject venom.

Which polychaetes use radioles for filter feeding?

Featherduster tubeworms.

What special organ in beardworms houses chemoautotrophic bacteria?

The trophosome.

How do beardworms obtain nutrition despite lacking a digestive tract?

They absorb dissolved nutrients via symbiotic bacteria housed in the trophosome.

What reproductive structure defines the clade Clitellata?

The clitellum, a ring of secretory epidermal cells.

Are clitellate annelids generally monoecious or dioecious?

Monoecious (simultaneous hermaphrodites).

How do the setae of oligochaetes compare with those of polychaetes?

Oligochaetes have fewer, generally shorter setae (aquatic forms may have longer ones).

Describe peristaltic movement in earthworms.

Alternating contraction of circular and longitudinal muscles while anchoring parts of the body for forward motion.

Which structure stores food before it reaches the gizzard in earthworms?

The crop.

What internal fold increases intestinal surface area in earthworms?

The typhlosole.

Which tissue in earthworms synthesizes glycogen and fat?

Chloragogen tissue.

How many major blood vessels run through an earthworm, and which serves as the main pumping vessel?

Five trunks; the dorsal vessel (aided by aortic arches) acts as the main pump.

Where does gas exchange take place in earthworms?

Across the moist skin (cutaneous respiration).

What is the chief excretory organ in earthworms?

Metanephridia, one pair per segment.

How do aquatic and terrestrial annelids differ in nitrogenous waste excretion?

Aquatic species excrete mainly ammonia; terrestrial species often excrete mostly urea (or a mix of both).

What large nerve fibers provide rapid conduction in earthworms?

Giant axons in the ventral nerve cord.

During reproduction, what secretes the cocoon around earthworm eggs and sperm?

The clitellum.

How many body segments do “true leeches” (Order Hirudinea) have?

Thirty-four segments.

What distinctive feature do true leeches lack that many other annelids possess?

They lack setae.

Describe typical leech locomotion.

They creep by looping with anterior and posterior suckers or swim by undulating the body.

In many leeches, what replaces a true circulatory system?

Coelomic sinuses that function as a blood-vascular system.

How is sperm commonly transferred during leech mating?

By a penis or by hypodermic impregnation (spermatophore injection).

How many separate "brains" do leeches possess?

Two—an anterior brain with six pairs of ganglia and a posterior brain with seven pairs.

What is the typical habitat of echiurans?

Marine sediments, burrows, or crevices worldwide.

What morphological trait gives “spoon worms” (echiurans) their common name?

A flattened, extensible proboscis with a ciliated feeding groove.

Which part of the echiuran gut is specialized for gas exchange?

The hindgut, through cloacal irrigation.

What larval stage is common to echiurans, polychaetes, and many other lophotrochozoans?

The trochophore larva.

Why are sipunculans nicknamed “peanut worms”?

Because they can contract into a shape resembling a peanut.

Do sipunculans have segmentation or setae?

No, they lack both segmentation and setae.

Describe the configuration of the sipunculan digestive tract.

It is complete and U-shaped.

Through what structures are sipunculan gametes typically released?

The nephridia.

What major benefit does a hydrostatic coelom provide annelids?

It acts as a fluid skeleton, allowing effective locomotion and shape maintenance.

What is a leading explanation for the evolutionary advantage of metamerism?

It provides redundancy and permits segment specialization, enhancing mobility and complexity; evidence suggests segmentation evolved multiple times.

What sensory pits on polychaete heads function as chemoreceptors?

Nuchal organs.

Do polychaetes possess permanent sex organs?

No; gonads are temporary swellings that appear only during breeding seasons.