bio 205 chapter 13: the annelids

phylum annelida

segmented worms
first to exhibit segmentation
equal segmentation internally and externally
12,500 species
bilateral symmetry and some level of cephalization
complete digestive tract
coelomates
protostomes

segmentation

the division of an animal body either equally or unequally into repetitive parts

metamerism

serial repetition of segments and organ systems

metameres

visible external segments of an annelid worm, evidence of external segmentation

chaetae (setae)

pairs of chitinous bristles used for movement, swimming, or traction while burrowing

septa

walls that internally separate one metamere from the next metamere

nervous system

2 cerebral (cephalic) ganglia, ventral nerve cord, segmental ganglia

circulatory system

closed, consists of a dorsal and ventral blood vessel that are connected by ring-like blood vessels within each metamere and aortic arches

aortic arches

the first 5 rings of blood vessels that constitute a pumping heart

(meta)nephridia

true excretory organs that filter waste out of the blood and coelomic cavity and filter waste from the metamere anterior to the one it sits in
blood vessels surrounding tubules allow for reabsorption of ions, salts, sugars, and amino acids

nephrostome

the opening into the nephridium

bladder of the nephridium

carries the true waste to the nephridiopore for excretion (ammonia, urea or uric acid)

digestive system

mouth → pharynx → esophagus → crop → gizzard → intestine → blood vessels → anus

mouth

allows for the ingestion of food and leads to the pharynx

pharynx

serves as a storage chamber, moves food into the esophagus

esophagus

a narrow tube surrounded by aortic arches, carries food to the crop

calciferous glands

out-pockets of the esophagus wall that remove calcium and ingested carbon dioxide from the digestive tract and release it into the bloodstream to help regulate blood pH by producing bicarbonate ions (carbon dioxide ingested is produced by soil bacteria)

crop

muscular chamber for temporary storage, moves food into the gizzard

gizzard

responsible for mechanical and enzymatic digestion of food, moves food to the intestine

intestine

surrounded by numerous thin blood vessels

blood vessels of the digestive system

absorb the digested nutrients from the intestine which allows them to be circulated through the body

peristalsis

an alternating contraction and relaxation of muscle tissue creating waves of motion

earthworm movement

as circular muscles contract and longitudinal muscles relax, metameres elongate extending the worm body
circular muscles relax and longitudinal muscles contract to return metameres to normal state

trochophore larva

the larval stage for all annelid worms, “diamond-shaped” with apical tufts of sensory cilia at the top and bottom and a belt of cilia around the middle and consists of 3 bands

prototroch

the anterior end of the larva

metatroch

the middle section of the larva

telotroch

the posterior end of the larva

acron (prostomium)

develops from the prototroch, the anterior end of the worm but is not considered a true segment or metamere

pygidium

develops from the telotroch, the posterior end of the worm but is not considered a true segment or metamere

peristomium

develops from the metatroch and becomes the worm’s first true metamere which possesses the mouth

growth zone

new segments are added anterior to the pygidium and after the peristomium

class hirudinea

leeches
ectoparasitic
anterior (breaks skin) and posterior sucker
release an anesthetic and anticoagulant

anesthetic

a numbing agent

anticoagulant

a blood thinner, called hirudin in leeches

class polychaeta

marine bristle worms and tube worms
largest class in phylum annelida
parapodia
most cephalized annelids
protrusible pharynx with jaw
some possess external gills (lugworms)

parapodia

flap-like extensions of the body wall from with the chaetae (setae) extend

epitoky

a fragmentation where the body is designed to break apart at predetermined spots along the body, a type of polychaete asexual reproduction

atokes

represents the original anterior portion of the worm, must regenerate the posterior portion of the worm

epitokes

represent the fragments that become new worms

polychaete sexual reproduction

dioecious
1) males swim around females (swarming)
2) males create a cyclonic movement of water around females worm
3) both worms release gametes into the water and external fertilization and development occur
4) development gives rise to a trochophore larva

class oligochaeta

earthworms
lack parapodia
chaetae (setae)
lack obvious cephalization
monoecious
clitellum

clitellum

a region of fused metameres #32-#37, used during copulation and cocoon formation and secretes mucus

copulation

when 2 worms connect and exchange sperm

cocoon formation

when a worm deposits fertilized eggs in a sac

earthworm reproduction (male)

metamere #15 - pore of sperm duct (male gonopore)
metameres #9-10 - spermathecal openings (pores of seminal receptacles)
1) sperm get released from male gonopores during copulation
2) released sperm swim through sperm (seminal) grooves to spermathecal openings on partner worm
3) testes in metameres #10-11 produce sperm through meiosis
4) sperm move into seminal vesicles to store sperm (metameres #9-12)
5) sperm moved into seminal funnels (metameres #10-11) into ducts that run to male gonopore
6) sperm enter partner worm through spermathecal openings and stored within seminal receptacles

earthworm reproduction (female)

metamere #14 - pore of oviduct (female gonopore)
1) ovaries in metamere #13 produce eggs through meiosis
2) eggs released into ovarian funnels sitting in metamere #13 and passed into oviducts that run to female gonopores
3) eggs released as mucus from clitellum slides anteriorly during cocoon formation
4) worm returns to soil once it collects sperm during copulation
5) clitellum is activated and begins secreting mucus that slides anteriorly
6) mucus slides past female gonopores to release eggs
7) mucus slides past spermathecal openings to release sperm from seminal receptacles
8) fertilization is external within cocoon
9) cocoon slides off anterior end of the worm and deposited in the soil

earthworm cocoons

can contain hundreds of fertilized eggs, eggs can hatch in soil and undergo standard embryonic development resulting in a trochophore larval stage