Module 13 Flashcards
Three main characteristics of Annelids
Repeated segments
Specialised segments
Connections
Repeated segments
Series of ringlike structures running the length of thee body
Segments are divided internally from one another by partitions called septa
In each cylindrical segment, the excretory and locomotor organs are repeated
The body fluid within the coelom of each segment creates a hydrostatic skeleton that provides rigidity to the segment
Muscles within each segment pull against the fluid in the coelom → because each segment is separate, each can expand or contract independently
When an earthworm crawls on a flat surface, it lengthens some parts of its body while shortening others
Specialised segments
The anterior segments of annelids contain the sensory organs → elaborate eyes with lenses have developed in some annelids
One anterior segment contains a well-developed cerebral ganglion, or brain
Connections
Circulatory system carries blood between segments
The nerve cords connect the nerve centres in each segment with each other and the brain
The brain can then coordinate the worm’s activities
Septa
Segments are divided internally from one another by partitions
Groups that exhibit segmentation in their body plan
Annelids
Arthropoda
Chordata
Metamerism
Basal segmentation
Includes serial repeats of the same body segment along the longitudinal axis of the animal
Key organs are typically located towards the anterior of the animal
Advantages of segmentation
Independent hydrostatic compartments with fixed fluid volume
- Implications for locomotion and other physiological functions
From an evolutionary perspective, segmentation is responsible for the huge radiation of segmented taxa
- Animals could modify specific segments for different functions
Can cope with injuries more effectively, especially when they occur in non-essential segments
Muscles of annelids
Circular
Longitudinal
Diagonal
Annelid locomotion in errant annelids
Each segment includes chaetae, bristle like structures that allow for increasing traction as they move through dirt
Movement involves extending the body, anchoring it to a surface with setae, and contracting body muscles
When the worm begins to move forward, circular muscles at the anterior end contract, extending the head forward. At the same time, the anterior end lifts from thee surface to facilitate forward movement
When the wave of contraction nears the mid-region of the body, longitudinal muscles contract, shortening the region
A wave of contraction follows, and the cycle repeats
There are a lot of different types of chaetae attached to the parapodia
Oligochaetes
Earthworms that tunnel through soil
Errant annelids
Chaetae
Bristle like structures that allow for increasing traction as they move through dirt
Locomotion in errant marine worms
Additional structures to support locomotion
Extensions on either side of the segment called Parapodia
Body undulations produced by the contraction of longitudinal muscles in addition to independent over their parapodia
Locomotion is sedentary annelids
Movement is limited
Typically burrowing annelids that establish themselves into substrate and do not move from their established location
Parapodia is reduced or absent
Specialised setae, the uncini, function in much the same way as do parapodia in free-moving forms
Sexual reproduction occurs internally of the animal
Parapodia
Extensions on either side of the segment
Chaetopterus – Sedentary Annelind
Parchment worm that is a genus of marine polychaete worm that lives in a tube it constructs in sediments or attaches to a rocky or coral reef substrate
Adults are filter feeders and spend their entire lives in their tubes
- Planktonic in their juvenile form
The tube the worm lives in is either attached to rocks or buried in sandy bottomless in shallow waters
The worm has spines along its body segments that are modified for tunnelling into sandy substrates to create the u-shaped tube which it lives
Feeding in errant annelids
Most earth feed on organic matter in the soil – detritivores
- perform a vital ecosystem service in improving soil quality
Buccal cavity structure
- Feeding on decaying matter does not require fancy mouthparts
- Simply need a strong, flexible and elastic mouth that can engulf copious amounts of soil
- Mouths are strong and muscular
- Muscular pharynx found inside the mouth
- When food is caught, it passes through the throat and is covered in saliva
- It is then pushed down the esophagus and stored in the crop
Feeding in sedentary annelids
Exploit the abundance of planktonic organisms and detritus drifting through the water as a stable food source
Filter-feeders
Tube-dwellers utilise their large feather-like tentacles to filter small food particles from the water
Feeding in parasitic annelids
Attach to a host and feed on its blood
Parasitic leeches locate a suitable host by sensing body heat and attach to the host by the anterior sucker which surrounds the mouth
Some leeches have sharp jaws which they make a three-way incision with into the flesh of their victim
Other leeches that lack sharp jaws instead secrete powerful enzymes that digests a hole in the skin of their host, giving them access to its blood
The digestive tract is complex and occupies a larger portion of the coelomic cavity
This is facilitated by the loss of internal partitions between segments in leeches
Contrast structural features that influence feeding in polychaetes and oligochaetes
Polychaetes:
Distinct head
Tentacular crown
Filter feeders
Large parapodia
Cylilary action
Can eat and spit back out
Oligochaetes:
Carnivorous or detritivores
Metamerism
Contract structural features that influence locomotion in polychaetes and oligochaetes
Polychaetes:
Hook claws to latch on top of burrow and slip down; also used to move along substrate
Oligochaetes:
Reduced parapodia for continuous burrowing
Move around by contracting body – circular muscles to move forward then the longitudinal muscles pull the rest of the body forward
Earthworms and leeches
Peristalsis locomotion
Have microscopic chetae
Earthworms have no parapodia
Bullet shaped head where all organs are with a streamlined body
Distinguish between Annelids and Nematodes
Annelids and nematodes can be easily distinguished from their locomotion
- The combination of longitudinal, circular, and diagonal musculature in annelids facilitates peristaltic movement
Nematodes possess only longitudinal muscles and lack circular muscle
- Also have a strong cuticle made of chitin and collagen
- In addition to a pressurised coelom which acts as a strut, the longitudinal muscles can only contract one side at a time – this antagonises the cuticle so they appear to thrash from side to side during movement
Nephridipores
Part of the nephridium, an excretory organ
Polychaetes typically release their gametes into the water column using nephridipores
