Annelids Flashcards
What are the characteristics of the Annelida?
Coelomate protostomes
Usually vermiform
Metameric segmentation (adaptation for burrowing)
Segments usually have chaetae
Fibrous cuticle (protection)
Gut typically a straight tube with anterior mouth and posterior anus
Well developed blood vascular system
Nervous system of anterior brain and single or paired ventral nerve cords
What is the typical body plan of the Annelida?
Segmented
Prostomium as the first segment and Pygidium last
What does segmentation of the Annelida result in?
Segmentation results in the partitioning of the coelom into compartments separated by septa
This reduces muscle activity needed for movement
Branches of the nervous system, blood vessels and excretory system also have a segmental arrangement
How can the shape of each segment in the Annelida be altered?
Shape of each segment can be altered independently by muscles acting against the fluid pressure of the coelom
What type of segmentation evolved in the Annelida and why?
Metameric segmentation evolved as an adaptation for peristaltic burrowing in sediments
The hydraulic pressure generates a powerful force and paired chaetae grip the burrow wall
What classes exist in the Annelida?
- Polychaetes (>12,000)- the bristle worms e.g. ragworms, lugworms etc- almost all are marine, very few freshwater
- Clitellata
Sub classes: - Oligochaeta (10,000 spp)- earthworms etc- Most are freshwater and terrestrial, 200 marine species
- Hirudinea (>500 spp)- the leeches, marine, freshwater and terrestrial
- Echiura (140 spp, unsegmented, formerly a separate phylum)
- Siphuncula (320 spp, unsegmented, formerlu a separate phylum)
What are some general points for the class Polychaeta?
Macrofaunal
High level structure, ecological and taxonomic diversity
Within the 1mm to 10cm size range, some are meiofaunal and a few are very large
Divided into errant and sedentary groups
Probably evolved from burrowing annelids by developing adaptations to epibenthic crawling
How are the Polychaeta usually divided?
Errant- free-moving and active
e.g Nereis spp.
Sedentary- mostly immobile
Burrows (e.g. Arenicola marina)
Tubes (Serpula vermicularis)
What is the typical structure of the Errant polychaetes?
Typical structure- Head has well developed sensory structures, parapodia are relatively large and structurally similar along the length of the body
What is the typical structure of the sedentary polychaetes?
Head lacks well developed sensory structures (but may have feeding structures), parapodia are often reduced and are often structurally different on anterior and posterior parts of the body
How is the head of the Polychaeta structured?
Prostomium often has well developed sensory structures (eyes, antennae and palps)
Immediately posterior to the prostomium is the peristomium (the first true segment)
The mouth is ventral and lies between the prostomium and the peristomium
The peristomium may have sensory structure (tentacular cirri) or feeding appendages (tentacular palps)
What are the parapodia?
They are a major distinguishing feature of the polychaetes
They are paired lateral appendages formed by an extension of the body wall
What is the structure of the parapodia?
Basically biramous with an upper notopodium and a lower neuropodium
Internally supported by one or more chitinous rods (Aciculum)
There is a tentacle-like cirrus on the dorsal base of the notopodium and in the ventral base of the neuropodium
Chaetae project from the parapodium
Why are parapodia important?
There is considerable variation and specialisation of parapodia structure
e.g. the dorsal cirri in scaleworms are modified to form a cover of overlapping dorsal scales
How does the structure of parapodia effect the body of the polychaete?
Structure of the parapodia can vary along the length of the body (especially in tube dwellers and burrowers) effectively dividing the body into a thoracic region and abdominal region
What are the chaetae?
Typically bundles of chaetae emerge from the parapodia
Each individual chaetae originates from a single cell and the chaetae are continuously replaced as older ones are lost
Why are the chaetae important and what types are there?
There is considerable variation in cheatae shape and a species may have more than one type of chaetae
They are of taxonomic importance
There are 4 main types:
- Capillary chaetae
- Compound chaetae
- Crotchets (hooks)
- Uncini
What do fire worms have?
They have poisonous chaetae as a predator defence
What type of burrowing do some polychaetes use?
Some use peristaltic burrowing- typically have elongate bodies with reduced parapodia and head appendages tend to resemble earthworms in form
How else can polychaetes move?
Some use an eversible pharynx to punch forward and pull themselves along (e.g. Arenicola)
Some can swim through loose sand by rapid undulations of the body (e.g. Nephtidae)
Some interstitial meiofaunal polychaetes use cilia for locomotion
What type of crawling do polychaetes possess?
Epibenthic crawling
Some have well developed parapodia and chaetae for crawling on the surface of the substrate- body segments tend to all be similar and prostomium has well developed sense organs
What are the types of epibenthic crawling?
Slow crawling- parapodia act as legs- extend and move forward on recovery stroke- alternate effective and recovery stroke either side of body
Rapid crawling- Rapid body undulations accompany movement of parapodia- coordinated so that effective stroke occurs on crest of undulatory wave
Also effective for swimming either on an occasional basis
How are the sedentary burrowing polychaetes structured?
Simple vertical or U-shaped burrows in sediment
Prostomial sensory appendages generally absent but may have specialised feeding on head end
Parapodia reduced- sometimes replaced with hook-like uncini to grip burrow walls
What does tube dwelling allow Polychaetes to do?
May provide protection from predators, opportunity to ambush prey, means of colonising hard substrates, means of raising animal above substrate to filter feed
What do the tubes from tube dwelling polychaetes consist of?
Tubes may consist of secreted material (calcareous, membraneous or parchment), cemented sand grains or combination of the two
What is the structure of the tube dwelling polychaetes?
Some are active predators that emerge to ambush prey- structurally similar to the active crawlers
Most are sedentary and specialised- prostomial sensory appendages poorly developed but specialised feeding structures common
Parapodia reduced with uncini for gripping walls
Segments often differentiated between thorax and abodomen
What is an example of a tube dwelling Polychaete?
Sabellaridae
Build tubes of sand grains
Chaetae on head end specialised to act as an operculum to block the tube
What are some other examples of tube dwelling polychaetes?
Fan worms- Sabellidae, Serpulidae and Spirobidae
Peristomium adapted into a collar that is responsible for tube building
What are some other lifestyles of the other polychaetes?
Shell borers- Some bore into living or dead shell
Commensalism- Many burrowers and tube dwellers have commensal species associated
Parasites- There are a few parsites
Where is raptorial feeding seen?
Typical of the active epibenthic species, but also found in pelagic species and active species inhabiting tubes and burrows
How does raptorial feeding work?
Eversible pharynx with jaws of tanned protein- coelomic pressure used to rapidly evert pharynx
Some of these species may be omnivorous or scavengers as well as predators
What is non selective deposit feeding as seen in polychaetes?
Typically evert pharynx and shallow sediment. Includes active burrowers, sedentary burrowers and tube dwellers
How are selective deposit feeding polychaetes structured?
Normally have specialised head structures for selectively gathering food particles from sediment surface
What is suspension feeding in the polychaetes?
Many tub dwellers and sedentary burrowers have specialised head appendages for filtering the water e.g. Sabellidae, Serpulidae and Spirobidae- Funnel shaped crown of pinnate radioles- cilia set up feeding currents and feeding is selective
What is the feeding mechanism of the Chaetopterus?
It is a suspension feeder that uses a rather different process- it has highly differentiated segments and builds a parchment tube- it generates a water current and uses a mucus bad to filter the water
What are the sense organs in the ploychaetes?
Eyes, mechanoreceptors, chemoreceptors and statocysts
How are the eyes structured in the polychaetes?
Best developed in active epibenthic species- in most cases probably only detect light intensity and direction- but the large eyes if the predatory pelagic Aciopidae can form images
What do some Sabellids and Serpulids have in regards to sensory organs?
They have photoreceptors in radioles- detect shadows and worm rapidly withdraws into the tube- speed of reaction due to giant axons in ventral nerve cord- important to avoid cropping by predators
How do the polychaetes reproduce?
Most reproduce sexually and have separate sexes- copulation is rare
Asexual reproduction by budding recorded in some species- most have good powers of regeneration- some can regenerate entire body from single segment
How does sexual reproduction in polychaetes work?
Eggs may be shed directly into plankton, deposited in gelatinous egg masses attached to the substrate, brooded on body or within tube and few brood eggs internally in the coelom
Trochophore larva hatches and enters plankton- may feed in plankton or live off yolk
Trochophore metamorphoses to juvenile polychaete which may settle immediately but often remain in the plankton for some time
What is Epitoky?
Production of a pelagic reproductive individual- an epitoke
Occurs in many polychaetes- morphological changes occur to facilitate swimming- eyes enlarged and parapodia and chaetae adapted for swimming
What does Epitoky result in?
Depending on the species the entire individual may change or epitokes may be produced by budding
The adaptation of the body is often divided into two distinct regions
There is usually synchronised swarming of epitokes to maximise reproductive success
How are the sub-class Oligochaeta described?
Most are freshwater and terrestrial, relatively few marine species
Generally simpler structure than polychaetes- prostomium is simple and lacks appendages, no parapodia, chaetae are simple rods and there are relatively few
Most are peristaltic burrowers in sediments
Terrestrial species feed on dead organic matter
Mostly hermaphrodites
Where are the Oligochaeta found in marine environments?
They are most often found in organically enriched intertidal sediments but are present in other environments including the deep sea
How are the subclass Hirudinea described?
Most are freshwater, a few terrestrial and marine species
The most specialised of the annelids- no parapodia, chaetae or head appendages- lack external segmentation and septa
About 75% of species are ectoparasites
Anterior and posterior suckers are characteristic
How are the class Sipuncula described?
Coelomate unsegmented worms
320 species, all marine mostly shallow water
Macrofaunal- most are <10cm
All benthic
Generally non-selective deposit or suspension feeders
How are the Sipuncula structured?
Body composed of swollen posterior trunk and retractable anterior introvert
Mouth at tip of introvert and surrounded by ciliated lobes or tentacles
Surface of introvert commonly covered by spines or other cuticular projections
How are the class Echiura described?
Coelomate unsegmented worms
140 species, all marine mostly shallow water
Macrofaunal- range from 1cm>50cm
All benthic- burrowers in sediment
Most are deposit feeders
How are the Echiura structured?
Body composed of cylindrical trunk and highly mobile and extendable prostomium (proboscis)
Prostomium is anterior to mouth, ribbon-like, glandular and ventrally ciliated
Trunk with pair of anterior hooked chitinous setae used for burrowing
