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