BIOL10005 Lecture Notes - Lecture 18: Parapodium, Polychaete, Annelid
LECTURE 18
ANIMAL DIVERSITY - FROM WORMS TO ARTHROPODS
CHAPTER 39 - ANNELIDS, MOLLUSCS, NEMATODES &
ARTHROPODS
COELOMATE PROTOSTOMES
•Annelids, molluscs, arthropods and nematodes are all protostomes, characterised by the
blastopore becoming the mouth during development.
•Annelids, molluscs and arthropods are called coelomate protostomes because their internal organs
lie within a coelom, a fluid filled cavity between inner and outer layers of mesoderm.
•However, nematodes lack a true coelom, and their body cavity, called instead a pseudocoelom,
develops differently.
•The two phyla Annelida and Mollusca are related on a branch of the tree called the
Lophotrochozoa, while the phylum Arthropoda is on a separate branch called cuticulata, related
to onychophorans and distantly to phylum Nematoda.
SEGMENTED WORMS: PHYLUM ANNELIDA
•Annelids are soft bodied, segmented worms of soils and marine and freshwater habitats.
•Eg. Marine bristle worms, earthworms, leeches.
THE ANNELID BODY
•They are bilaterally symmetrical, triploblastic, coelomate (unlike flatworms) and are considered
protostomes.
•Their body is segmented with serial repetition of functional units (compare with flatworms). This
kind of segmentation is known a metameric segmentation.
•The annelid body has 3 parts; the prostomium at the anterior end, then the soma behind it, then
the pygidium. The soma is also segmented, the first of which is the peristomium, and the pygidium
includes the anus.
•The coelom acts as a fluid-filled (hydrostatic) skeleton.
•Closed vascular system - transport vital gases, food and excretory products (greater potential for
increased size). A key issue for animals is getting rid of waste products, and this creates a
mechanism for them to do that.
•Ventral nervous system; ventral nerve cord. Doesn’t change for a long time in the animal
kingdom. Gives a capacity for far more movement than available in other phyla in both water and
on land.
•Movement is muscular; similar to our own. Earthworms have antagonistic circular &
longitudinal muscles (helped by parapodia or chaetae).
MARINE BRISTLE WORMS: CLASS POLYCHAETA
•Most polychaetes are marine worms but some are
freshwater and they can be divided loosely into either free
moving (errant) or sedentary forms.
•They usually have lateral, leg like
appendages, parapodia, on each body
segment, which can move independently
from the others.
•The name ‘polychaete’ refers to the
cations bristles, chaetae, associated with
the parapodia.
Lecture 18 - Friday 2 September 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
•Polychaetes are mostly dioecious, with separate males and females but some are hermaphroditic.
•Their method of reproduction is mostly sexual via spawning or hypodermic impregnation, in
which a bundle of sperm (a spermatophore) is injected directly across the body wall; basically
insemination by a penis. Moves through an abnormal reproductive passageway through body
cavities.
•Most polychaetes have a free swimming ciliated larvae, the trochophore.
ANNELIDS WITH A CLITELLUM: CLASS CLITELLATA
•Earthworms and leeches have a clitellum, a thickening of the epidermis that secretes the cocoon in
which eggs are deposited and embryos are protected during development.
EARTHWORMS AND RELATIVES: SUB CLASS OLIGOCHAETA
•An example of oligochaeta are earth
worms.
•They are mostly terrestrial and as such
usually live in the soil and feed on organic
matter. They can grow very long (meters
long).
•They are hermaphrodites but usually have
sexual reproduction and cross fertilisation
seems to be the rule. Asexual reproduction
is common in aquatic oligochaetes, with
the parent worm simply dividing in two.
•Oligochaetes are mostly burrowing
animals. The name oligochaete means ‘few
chaete’ (bristles) and there are usually only
4 pairs of simple chaete per segment.
•They react to vibrations made by sounds,
not the sounds themselves.
LEECHES: SUBCLASS HIRUDINEA
•Leeches (hirudineans) are related to oligochaetes and also have a
clitellum. They lack both parapodia and chaetae.
•They have a more specialised diet than polychaetes and oligochaetes.
•Many are external parasites feeding on host blood, and a few are
carnivores.
•They are freshwater and terrestrial animals, which mostly feed on
vertebrate blood, and usually live in humid conditions.
•Their saliva contains anti-coagulant proteins and they have thermo-
sensors to detect prey.
•Their coelom is usually reduced in size and segmentation in this
species is found in the nervous and excretory systems only.
•Their reproductive method is internal fertilisation, and a few use
hypodermic impregnation.
•They attach to their hosts by posterior and anterior suckers which
are used for locomotion by ‘looping’, the side to side movements of
most polychaetes. Like a squiggly worm.
•Many have jaws and many blood sucking leeches secrete an
anticoagulant protein, hirudin, which prevents blood clotting.
•Leeches are highly muscular but don’t use their muscles to move in the same way as worms.
•Intestine – reduced segmentation
•Coelom - reduced (allows more space for muscles & intestine)
•Suckers - feeding, anchoring and movement
Lecture 18 - Friday 2 September 2016
BIOL10005 - GENETICS & THE EVOLUTION OF LIFE
earthworm
Document Summary
Lophotrochozoa, while the phylum arthropoda is on a separate branch called cuticulata, related to onychophorans and distantly to phylum nematoda. Segmented worms: phylum annelida: annelids are soft bodied, segmented worms of soils and marine and freshwater habitats, eg. The annelid body: they are bilaterally symmetrical, triploblastic, coelomate (unlike flatworms) and are considered protostomes, their body is segmented with serial repetition of functional units (compare with flatworms). This kind of segmentation is known a metameric segmentation: the annelid body has 3 parts; the prostomium at the anterior end, then the soma behind it, then the pygidium. A key issue for animals is getting rid of waste products, and this creates a mechanism for them to do that: ventral nervous system; ventral nerve cord. Doesn(cid:282)t change for a long time in the animal kingdom. Gives a capacity for far more movement than available in other phyla in both water and on land: movement is muscular; similar to our own.
