LECTURE 08 - Spiralia: Annelida

Question 1

What is a trochophore larva?

Answer 1

A trochophore is a small, translucent, free-swimming larva characteristic of marine annelids and most groups of mollusks. Trochophores are spherical or pear-shaped and are girdled by a ring of cilia, the prototroch, that enables them to swim.

Question 2

Describe the trochophore larva development.

Answer 2

• Embryogenesis promptly forms a microscopic feeding trochophore larva
• Nourishment ensures the growth and transformation of larval tusses
• The trochoblasts are the first larval cells that differentiate, and will be discarded during metamorphosis
• Differentiated endodermal, ectodermal and mesodermal cells of the larva are “recycled” and contribute to adult transformation
• Ectoderm and mesoderm in the posterior growth zone (PGZ) form the segmented portion of the worm, which largely corresponds to the reproductive side of the animal

Question 3

Describe the anatomy of annelids.

Answer 3

• Annelids have a pear- to ball-shaped trochophore larva with a through-gut
• The mouth is ventral and the anus is terminal
• Trochophore refers to the pro- and metam=troch, rings of small hairs encircling the body both beneath and above the mouth (respectively)
• There may also be a telotroch near the anus
• The body cavity has a pair of protonephridia
• During metamorphosis, the top part of the body becomes the prostomium and the bottom part of the pygidium

Question 4

What does an indirect life cycle evolve?

Answer 4

• An indirect life cycle evolves when the adult body plan is inviable as a minute self-sustaining animal
• It involves the transition from a minute ciliated planktonic individual to a large muscular benthic individual

Question 5

What is the polychaete annelid?

Answer 5

• A large hollow segmented worm

Question 6

What are the large problems of large worms?

Answer 6

• Large worms, constructed on the same plan as small worms, can neither swim nor breathe
• They cannot swim because they cannot overcome inertial forces with cilia, and they cannot breathe because they are too thick for gas exchange by diffusion
• Large worms require body plans different from those of small worms
• In particular, they must incorporate respiratory and excretory systems, and they need an effective means of propulsion

Question 7

Why are large worms hollow?

Answer 7

• In Cnidarians, gas exchange can occur through the endoderm of the spacious gastric cavity, which contains seawater continually renewed through the mouth
• A large solid-bodied worm with a through-gut cannot do this; narrow gut cannot be irrigated sufficiently to support respiration
• Solid-bodied worms have to be very small or very thin or very flat or diffusion to do the job
• A large worm therefore must be hollow, with a secondary body cavity containing seawater (or an equivalent secreted fluid) for respiration and excretion
• This secondary cavity, between gut and body wall, is the coelom

Question 8

In order to function properly, what must the coelom be supplied with?

Answer 8

• In order to function properly, the coelom must be supplied frequently with fresh seawater
• In most cases the fluid is recycled rather than renewed

Question 9

What are the two activities necessary to renew coelomic fluid continually?

Answer 9

• Oxygen supply and carbon dioxide (waste, more generally) removal
• To supply oxygen requires vascularization of the lining of the coelom; this may be restricted to a particular structure, the gill
• Removing wastes likewise requires a specialized filtration apparatus, the nephridium

Question 10

The gill and nephridium function efficiently if what?

Answer 10

• Gill and nephridium function efficiently if the coelomic fluid is continually circulated; this can be done by a contractile vesicle permitting one-way flow through a valve, the heart

Question 11

What is the importance of modules?

Answer 11

• A modular body plan allows each module (segment) to be modified independently of other modules, without requiring that the entire body plan be modified

Question 12

What does a segment possess?

Answer 12

body wall muscles
appendage
blood vascular system
nerves
excretory system

Question 13

Describe the segmental organization.

Answer 13

• The coelom and segmentation have contrary outcomes
• The subdivision of the body of the worm by septa (in order to facilitate local muscular control) makes it difficult to supply each semi-isolated unit through coelom-wide systems (because the septa obstruct a general circulation)
• The compromise is to decentralize the primary functions of the coelom, circulation and excretion, to local subsystems that may or may not communicate with a central system
• Each unit thus develops as a suite of structures: muscle groups, lateral blood vessels, ganglia with lateral branches, and nephridia
• This is the segmental body plan

Question 14

What does the head bear?

Answer 14

The head bears specialized appendages for sensing the environment and handling prey

Question 15

Do annelids have jaws?

Answer 15

Yes

Question 16

Describe segmental differentiation.

Answer 16

• The segmental body plan of annelid worms is akin to a linear series of zooids, although segments are not as autonomous as hydrozoan zooids and isolated segments are not viable
• Like zooids, segments are modules each of which can be modified without directly affecting the development of any of the others
• This facilitates the evolution of a complex body through regional specialization for different functions
• Cephalized
  
  • jaws
• Parapodia
  
  • The trunk segments each bar a pair of parapodia
  • The parapodium consists of one or two fleshy lobes each bearing a group of setae

Question 17

How does undulatory locomotion work?

Answer 17

• Worms can swim through coordinated contraction of longitudinal muscles deforming the body into a wave passing from head to tail
• The muscles on either side of the body are re-extended by contraction of the muscles on the other side, or by the turgor pressure of a fluid-filled body cavity, or by the elasticity of the body wall
• Note that the alternate contraction of a single muscle strand on either side would not produce locomotion; contraction must be locally limited and controlled

Question 18

What is oared swimming and crawling?

Answer 18

• Many polychaetes are capable of rapid locomotion in water or on the substrate surface
• This is made possibly by the parapodis: outgrowhts of the body wall on the trunk segments that act as oars to propel the worm by reaction

Question 19

What are the dynamics of the coelom?

Answer 19

• Once a secondary body cavity has evolved to support respiration and excretion, it can be recruited for other functions
• In particular, it can be used as an incompressible hydrostatic skeleton that opposes circular and longitudinal muscles to produce changes in body shape
• Changes in shaped can be harnessed to produce
  
  • eversion of proboscis
  • forcible burrowing in sediment
• This translates a new body plan into a new way of life

Question 20

How do large hollow muscular worm circulate?

Answer 20

• Ciliary locomotion is effective only at low Re and will not propel large organisms at a velocity approaching 1 body length per second
• This limit can be extended by providing a viscous medium for ciliary action (flatworms) or by forming grouped cilia into oars (ctenophores), but it cannot be transcended by such makeshifts
• Beyond Re > 1, locomotion requires muscle fibres as the basic contractile element
• Undulatory locomotion works even in solid-bodied worms, but only if they are very thin
• Large worms require a secondary body cavity
• Muscle contraction alone cannot cause change of place; it only causes changes of shape
• Directional change of place - locomotion - results when change of shape is achieved with respect to a fixed reference point

Question 21

How does locomotion in coelomate worms work?

Answer 21

• Worms with an undivided (or essentially undivided) coelom can use it as a hydrostatic skeleton that enables the circular and longitudinal muscles of the body wall to oppose one another
• This produces changes in shape that can be used to burrow into the sediment
• In peristaltic burrowing, the circular muscles antagonize the longitudinal muscles in phased contractions that travel along the body
• Unlike pedal locomotion, in which locomotory forces are generated only by the ventral longitudinal muscles, peristaltic locomotion utilizes all the musculature of the body wall, generating more powerful thrust

Question 22

How do segmented worms burrow?

Answer 22

• In worms with an undivided coelom, contraction of the body wall muscles acts equally on all regions of the body
• Peristaltic burrowing is much more effective when contraction of the circular or longitudinal muscles in one section of the body extends the antagonistic muscles in that section alone
• Subdivision of the coelom by septa isolates pressure changes in segments where muscles are contracting and thereby enable waves of peristalsis to travel smoothly and repeatedly the length of the body
• Segmented worms are capable of continued forceful burrowing through viscous substrates such as soil

Question 23

Describe tube dwellers.

Answer 23

• Burrows are often lined with mucus and material collected while feeding
• It is a short step to secreting tubes that transcend or replace the burrow
• These serve to concentrate water flow and provide protection

Question 24

What is the annelid radiation?

Answer 24

• The deeply-branching groups are segmented; e.g., Oweniidae are tubiculous filter-feeders with smooth segments lacking parapodia
• Chaetopteridae have pronounced segmental specialization
• By contrast, unsegmented Echiura and Sipuncula are derived, presumably by secondary loss of segmentation

Question 25

Describe errant polychaetes.

Answer 25

• Elongate segmented worms; body wall with circular and longitudinal muscles, lined with peritoneum; peritoneum forms septa separating the segments
• Mouth with jaws on peristomium; eversible pharynx; simple intestine with terminal anus
• External gills borne on parapodia
• Contractile blood vessels push blood forward in dorsal vessel, returning in ventral vessel
• Excretory system of segmental nephridia
• Brain in head; ventral nerve cord runs length of body, with segmental ganglia
• Usually dioecious; gonads shed gametes into body cavity; external fertilization, with or without pairing
• Radial cleavage and development via a trochophore larva

Question 26

What are Eunicidae?

Answer 26

• Predatory errant polychaetes
• Carnivores with powerful jaws; body up to 2m in length (most 5-10cm)
• Palalo worm swarms in phase with lunar cycle in tropical Pacific: hind end of body detaches, swims to surface and releases gametes, while head end remains on seafloor and regenerates a new hind end

Question 27

What are Saellidae?

Answer 27

• Tubicolous polychaetes which construct a tough permanent tube of sand grains cemented with mucus
• Head bears pair of semi-circular crowns each with paired side-branches forming a double-sided comb for filtering edible particles from the water

Question 28

What are Spionidae?

Answer 28

• Selective deposit-feeders using two long palps to locate prey
• Mostly small and thin, usually 10-30 mm in length
• Ubiquitous burrowers in shallow sandy areas, constructing a sand tube or crawling freely

Question 29

What are Archiannelida?

Answer 29

• There are several groups of minute annelids that form part of the “interstitial fauna” of aquatic sands and sediments
• Other members of this fauna include equally minute animals such as gastrotrichs and kinorhynchs
• These annelids were formerly assigned to a separate group, the Archiannelida, but it is now recognized that their similarity and simple body plan has arisen through convergent evolution to interstitial life
• Protodrilidae is a group of small, superficially simple-looking annelids, often unsegmented and lacking chaetae and appendages, except for two prostomial palps
• Gliding locomotion by a ventral ciliary band
• Protodrilids are found worldwide in the interstices of intertidal and subtidal marine sediments

Question 30

What are Arenicolidae?

Answer 30

• Large worms that live in a U-shaped burrow
• By piston movements of the anterior region of the body, they liquefy the sand in the head region of the burrow, swallow it and extract edible particles
• Faecal pellets ejected to surface from tail region of burrow
• The middle region of the body bears conspicuous paired gills

Question 31

What are the two derived groups of Annelida?

Answer 31

Echiurids and Sipunculids

Question 32

What are Sipuncula?

Answer 32

• Proboscis extension
• Sipunculans have a spacious undivided fluid-filled coelom in two parts: a trunk region and proboscis
• Contraction of circular body-wall muscles in the trunk extends the proboscis because the coelomic fluid is incompressible
• The proboscis can be retracted by contraction of the longitudinal retractor muscles
• This permits the body of the worm to shelter in a burrow or cavity while its proboscis gathers food from the surface of the surround substrate
• Food is gathered by branched ciliated tentacles and passed in a mucus stream to the mouth

Question 33

What are Echiura?

Answer 33

• Open-plan worms
• Echiurans have a similar life-style
• They also are worms with a spacious undivided coelom, stout trunk and extensible proboscis
• Simple closed circulatory system; respiration through anal sac
• Dioecious with external fertilization
• Equal radial cleavage leading to trochophore larva
• Ciliary-mucus feeders: trunk buried in sediment; proboscis protrudes from burrow, lies on surface and collects food particles that are transported along ventral gutter to mouth

Question 34

What are Clitellate?

Answer 34

• A large monophyletic group that includes earthworms and leeches
• Thee have successfully radiated in freshwater and terrestrial habitats; polychaetes (all other annelids) are almost exclusively marine
• The clitellum is a thickened, non-segmented, glandular area in mid-body that secretes the egg cocoon
• There are also less familiar minute oligochaetes lacking a clitellum that inhabit freshwater and often reproduce bi fission

Question 35

What is the role of earthworms in soil fertility maintenance through production of biogenic structures?

Answer 35

• The soil biota benefit soil productivity and contributes to the sustainable function of all ecosystems
• The cycling of nutrients is a critical function that is essential to life on earth
• Earthworms (EWs) are a major component of soil fauna communities in most ecosystems and comprise a large proportion of macrofauna biomass
• Their activity is beneficial because it can enhance soil nutrient cycling through the rapid incorporation of detritus into mineral soils

Question 36

What are Hirudinea?

Answer 36

• LEeches are freshwater (usually), marine and terrestrial annelids with strongly modified jaws, adapted to pierce the skin or membrane of vertebrates
• Mostly ectoparasites, some predators
• Limnatis enters through the mouth of animals (or people) drinking from rivers and ponds, and clings to the wall of the pharync
• It may remain there, feeding, for some time
• Its jaws are relatively blunt and weak; it cannot penetrate skin
• Hirudo is a large leech that pierces the skin of frogs (as a juvenile) or mammals (as an adult)
• It has very sharp triradiate jaws
• It injects a local anaesthetic to prevent detection and an anticoagulant to keep the blood flowing (and itself from clotting)
• It detaches itself after feeding
• This is the medicinal leech used to remove blood from patients

Question 37

What are Myzostomida?

Answer 37

• Parasitic annelids
• Myzostomids spend their adult lives living on or inside crinoid echinoderms (a few live on ophiuroids and asteroids)
• Myzostomids occur in shallow subtidal to abyssal depths throughout all of the world’s oceans, but are mainly found in shallow-water tropical reefs where crinoid diversity is greatest

Question 38

Describe the lifestyle of Myzostomida.

Answer 38

• Myzostomids possess a variety of body plans and lifestyles in which they steal food from or directly consume the host
• Those that live on the surface of the animal are mainly disk-shaped or elongated; they use their chaetae to hold onto the host while inserting their proboscis into the host’s food groove to steal food
• These external types may “mimic” the host by possessing similar colours and/pr having appendages that resembles parts of the crinoid, for example, extensions that look like pinnules
• Other myzostomids reside in galls (hard) or cysts (soft) along the host’s food grooves, or within the host’s mouth, digestive system, coelom, or gonads
• Those living on the outside of the animal and within galls, cysts, the mouth, and digestive track are presumed to be stealing the host’s food, while those within the coelom and gonads are believed to be eating the host directly

Question 39

Describe Pogogonophora.

Answer 39

• The strange bearded animals
• Solitary, tubicolous, extremely elongate worms
• Inhabit permanent chitinous tubes
• Length 10-35 cm, width generally less than 1mm
• Body consists of a short anterior section and a very long trunk
• Posterior region segmented, with chaetae
• No trace of mouth, anus, gut, or gills
• The “beard” is a bunch of tentacles borne on the protostome (first compartment of a two-region coelom)
• Interior of body occupied by trophosome, which replaces the embryonic gut and is packed with bacteria
• Metabolism is based on chemoautotrophic bacteria

Question 40

Describe the external anatomy of Nemertea

Answer 40

• Nemerteans range in length from a few millimetres to 50 metres (stretched) in length (most commonly 20 cm or less) and may be drab or very brightly coloured
• They are unsegmented and usually flattened
• They have very thickly muscled body walls covered by a mesenchyme, and a ciliated epidermis comprised of columnar epithelial cells, as well as some sensory and mucous gland cells
• The muscle wall is typically made up of longitudinal and circular muscles in two or three layers
• The interior of the body wall is coated with mesenchyme, which includes a gel matrix, and sometimes dorsoventrally oriented musculature

Question 41

Describe the internal anatomy of Nemertea

Answer 41

• Nemerteans have a simple circulatory system consisting of vessels and thin walled lacunae
• Although this system can vary in complexity depending on species, the most basic arrangement is a pair of longitudinal vessels that extend the entire length of the body and connect to cephalic and anal lacunae
• The blood is colourless fluid in which cells such as corpuscles (some of which may contain hemoglobin), lymphocytes and leukocytes can be found; the blood does not circulate in any particular direction
• This system is mainly involved with transporting nutrients, gases, secretions, and wastes through the animal’s body and may aid in regulating hydrostatic pressure
• Oxygen and carbon dioxide diffuse across the animal’s body surface
• Nemerteans have a central nervous system consisting of a complex cerebral ganglion with four connected lobes, giving rise to a pair of longitudinal, gangliated nerve cords from the ventral lobes
• The nerve cords connect to each other at points throughout the worm’s body and give rise to peripheral sensory and motor nerves

Question 42

Describe the nemertean proboscis.

Answer 42

• The foregut, stomach and intestine run a little below the midline of the body, the anus is at the tip of the tail, and the mouth is under the front
• A little above the gut is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body
• All species have a proboscis which lies in the rhynchocoel when inactive but everts (turns inside-out) to emerge just above the mouth and capture the animal’s prey with venom
• A highly contractile muscle attached to the rear of the rhynchocoel pulls the proboscis in when an attack ends

Question 43

How to Nemerteans capture their prey?

Answer 43

• Nemerteans are hunters and scavengers
• Hunting methods vary between species and depend on prey type
• Sticky, sometimes toxic substances are produces by the proboscis in order to trap and immobilize prey
• In some cases, the proboscis is everted when the worm comes into contact with prey; it then coils around the prey, which is swallowed whole
• In other instances, prey may be stabbed by a styley, with toxins delivered through the resulting wound
• The prey is then either swallowed, using peristaltic action of the body wall as well as ciliary currents within the gut, or injected with digestive enzymes and consumed suctorially

Question 44

Describe the fertilization of Nemerteans

Answer 44

• Nemertean fertilization may be external or internal
• Egg cases or capsules are formed, in which development occurs
• Some terrestrial, freshwater and deep sea nemerteans are ovoviviparous
• Cleavage is holoblastic and spiral
• Some species undergo gradual direct development; these develop within egg cases, nourished by a yolk until they hatch, undergoing no abrupt metamorphosis
• Other nemertean species undergo different types of indirect development, beginning life as pilidium (free swimming larvae)

Question 45

Describe Nemertean metamorphosis

Answer 45

• The ‘juvenile rudiment’ consumes the rest of the larva (itself) to achieve metamorphosis into the final form of the species

Question 46

What are the relationships of Nemertea

Answer 46

• Nemertea was formerly considered close to Platyhelminthes as a second acoelomate phylum
• It is now thought that the rhynchocoel (and perhaps the blood vessels) has a coelomic lining and arises by schizocoely
• Molecular studies currently place Nemertea within Trochozoa, with Annelida, Mollusca and Lophophorata