M N I S I N M L
Phylum: Annelida
Segmented Worms
STUDY OBJECTIVES
The student should be able to understand the following concepts:
1. Segmentation / annuli / metamerism / setae 2. Annelid characteristics
3. Form & function of annelid morphology 4. Annelid classes & their characteristics 5. Significance of metamerism
CHARACTERISTICS
Metameric body
Body wall has: circular & longitudinal muscle layers Cuticle – secreted by epithelium
Chitinous setae; absent in leeches Coelom – divided by septa
Coelomic fluid ssupplies turgidity (hydrostatic skeleton) Parapodia
Blood system is closed & arranged segmentally
Amebocytes & respiratory pigments in blood plasma Digestive system is complete
Oxygen & carbon dioxide exchanged through skin, gills or parapodia Excretory system – pair of metanephridia
Nervous system – double ventral nerve cord & pair of ganglia, lateral nerves in each memebrane; brain: pair of dorsal cerebral ganglia with connectives to nerve cord Sensory system: tactile organ, taste buds, statocysts, photoreceptor cells & eyes Asexual reproduction: budding
CLASSIFICATION
1.
Class:
Polychaeta
(marine worms)
A.
Subclass:
Errantia
: free-moving pelagic forms, active
burrowers & crawlers (
clam worms
)
B.
Subclass:
Sedentaria
: worms expose from their burrows in
which they live (
tube worms
)
2.
Class:
Oligochaeta
(earthworms)
3.
Class:
Hirudinea
(leeches)
M A R I N E W O R M S
FORM AND FUNCTION
Prostomium (head)
Bears eyes, antennae &
sensory palps
First segment
(peristomium) surrounds
mouth
Bear
setae
,
palps
or
chitinous jaws
Ciliary feeders may bear
CHARACTERISTICS of POLYCHAETS
Parapodia
- may have
lobes, cirri, setae
Used for crawling, swimming or anchoring Serve as chief respiratory organs
DIGESTIVE TRACT
Foregut (stomodeum, pharynx, esophagus), midgut & hindgut
CIRCULATION
Dorsal longitudinal vessel – takes blood anteriorly Ventral longitudinal vessel – takes blood posteriorly
Blood flows between 2 vessels via networks in parapodia, septa &
around intestine
Coelomic fluid functions as circulation agent
Respiratory pigments: hemoglobin, chlorocruorin & hemerythrin
GASEOUS EXCHANGE
CHARACTERISTICS of POLYCHAETS
EXCRETION & OSMOREGULATION
By
metanephridia
Each
nephridium
collects
fluid from coelom
through
ciliated
funnels
(
nephrostome
), processes it in
nephridial tubule
&
empties urine to outside by a pore (
nephridiopore
).
NERVOUS & SENSORY SYSTEMS
Pair of
cerebral ganglia
joined to
double ventral nerve cord
Neurosecretory cells
– for growth, reproduction &
regeneration
Sense organs
:
Eyes with cornea, lens, retina
POLYCHAETA SUBCLASSES
Subclass: Errantia
Free-moving active burrowers / crawlers
E.g. Clam worm – Nereis Live in mucus-lined burrows Body with 200 somites; 30-40
cm long
Prostomium – head
Has sensory palps (sensitive to
touch & taste)
Tentacles
2 pairs of light-sensitive eyes
Peristomium
Ventral mouth
Pair of chitinous jaws
4 pairs of sensory tentacles Each somite bears a pair of
parapodia
Subclass: Sedentaria
Tube worms
E.g. Parchment worms –
Chaetopterus spp.
“U”
shaped
parchment
tube
Fans
(
modified parapodia
)
(segments 14-16) pumps
water through tube
Parapodia secrete a
Parapodium
2 lobes:
dorsal notopodium &
ventral neuropodium
Each lobe supported by chitinous spines
(acicula)
Parapodia bear setae & supplied with blood
vessels
Function: used for
E A R T H W O R M S
CHARACTERISTICS
Have
no parapodia
Few setae for movement
Includes earthworms
Bodies may have over 100 metameres
Internal partitions called
septa
Prostomium
reduced to a small lobe
4 pairs of setae / segments
Movement:
via
peristaltic contractions
assist by
NUTRITION & DIGESTION
Mostly scavengers – on decayed organic matter & vegetation Calcium in soil – food tends to produce high blood calcium levels Calciferous glands;
along esophagus secrete calcium ions into gut & Reduce calcium ion concentration of blood Also regulate acid-base balance of body fluids
Food move – mouth – pharynx – esophagus Temporarily stored in crop
Passed to thick muscular gizzard Digestion & absorption – intestines
1 side of wall of intestine bears a typhlosole = increases absorption &
digestive surface
Digestive system secrete various enzymes
Pepsin – which acts on proteins Amalyse – act on polysaccharides Cellulase – act on cellulose
Lipase – act on fat
Surrounding the intestine & dorsal vessel is a layer of chloragogen tissue.
Synthesis gycogen & fat
Digestive System
Intestine Crop
Gizzard
Pharynx
Buccal cavity
Mouth
CIRCULATORY SYSTEM
Double transport system
Coelomic fluid
&
circulatory system
Food, waste & respiratory gases
carried by both
coelomic fluid
& blood.
Closed blood system of blood vessels & capillaries.
With pumping
dorsal vessel
aided by
5 pairs of aortic arches
(“hearts”)
Aortic arches
maintain steady
pressure of blood into ventral
blood vessel.
Ventral vessel serve as
aorta
– it receives blood from vessels of
body wall & digestive tract and pumps it to brain & rest of body
EXCRETION
Paired
metanephridia
in most segments in
coelomic cavity of segment just anterior to it.
(a tubule lays in one segment with its ciliated
collecting funnel =
nephrostome
)
Nephridium
empties through
nephridiopore
in
NERVOUS SYSTEM
Pair of
cerebral ganglia
above pharynx
Joined to ventral nerve cord
by pair of
connectives
Nerve cord has pair of ganglia in each somite
Neurosecretory cells
in brain & ganglia
Secrete endocrine & neurohormones
MALE REPR. SYSTEM
FEMALE REPR. SYSTEM
2 pairs of
testes
2 pairs of
sperm funnels
surrounded by
3 pairs
of
large seminal
vesicles.
Sperm matures in
seminal
vesicle
, pass onto
sperm
funnels
, down the
sperm
duct
, to male genital pores
(in somite 15).
Expelled during copulation
Eggs discharged by
pair of
ovaries
,
Into coelomic cavity
Picked up by
ciliated
funnels,
Carried by oviducts to
outside through female
genital pore (somite
14)
Reproductive System
Seminal receptacle Sperm duct Seminal vesicle
Ovary Testes
L E E C H E S
CHARACTERISTICS
•
Most live in
fresh bodies of water
, but some live among
moist vegetation
.
•
Suckers
found on
both ends
•
Unlike other annelids, its segments are not separated
internally
LEECHES
COELOM
Lack distinct coelomic compartments – septa have disappeared
Coelomic cavity filled with connective tissue & a system of spaces
called lacunae
Coelomic lacunae form a system of channels filled with coelomic
fluid
NUTRITION
Parasitic
Active predators or scavengers equipped with proboscis
Most are fluid feeders – tissues fluids & blood
Oral suckers around mouth
They secrete anticoagulants, hirudin, to keep blood from clotting
and an anesthetic that prevents the host from feeling their presence.
LEECHES
LOCOMOTION
Creep by movements of body & suckers
Aquatic leeches can swim gracefully
RESPIRATION & EXCRETION
Gas exchange is through
skin
(fish leeches through gills)
LEECHES
NERVOUS & SENSORY SYSTEMS
2 “BRAINS” – 1 head. 1 tail (made up of 6 – 7 pairs of fused ganglia)
Additional ganglia (21 pairs) segmentally arranged along double nerve cord.
Free sensory nerve endings & photoreceptor cells in epidermis
Row of sense organs (sensillae) in central annulus of each segment
and a number of pigment cup ocelli.
REPRODUCTION
Hermaphroditic
Cross-fertilization during copulation
Sperm transferred by penis or hypodermic impregnation
After copulation clitellum secretes coccon, receive eggs & sperm,
