Invertebrate Zoology
Lecture 15: Phylum Annelida, Part 2
Lecture outline
 Phylum Annelida, Part 2
Circulation/gas exchange
Osmoregulation/excretion
Nervous system/Sensory structures
 Class Polychaeta
 Class Clitellata next time…
Circulation/gas exchange:
Overview
 Why do members of Phylum Annelida
need a circulatory system?
Circulation/gas exchange:
Class Polychaeta
 Closed circulatory system
 Key gas exchange sites
Parapodia
Additional structures
(some)
 Anterior gills (Terebellidae)
 Tentacular crown (Sabellidae)
Body surface
 If no other structures
Circulation/gas exchange:
Class Polychaeta
 Two major vessels
Dorsal: blood  anteriorly
Ventral: blood  posteriorly
 Extensive capillary beds in
Parapodia
Other gas exchange structures
Anterior and posterior
Intestine
 Blood moves between main vessels via
Capillary beds
Segmental vessels (=ring vessel)
Circulation/gas exchange:
Class Polychaeta
Circulation/gas exchange:
Class Polychaeta

No hearts
 Movement of blood via vessel contraction &
body wall contraction

Respiratory pigments
 Primarily dissolved within plasma
 Primarily hemoglobin
 Two forms of hemoglobin for polychaetes in
potentially low-oxygen environments

How do these two forms differ? Adaptive value?
Circulation/gas exchange:
Class Polychaeta
 Terrebellidae circulatory system
Circulation/gas exchange:
Class Clitellata, Subclass Oligochaeta



Closed circulatory system
Organization similar to Polychaeta
Key differences include
 Subneural vessel (so 3 main vessels)
 More highly-developed pumping structures




Large, muscular dorsal vessel
2-5 pairs of enlarged circumesophageal vessels
One-way valves prevent backflow
Key respiratory surface is the epidermis
 Must stay moist. Why? How?
Circulation/gas exchange:
Class Clitellata, Subclass Oligochaeta
Circulation/gas exchange:
Class Clitellata, Subclass Hirudinoidea

Combination of the “ancestral annelid”
circulatory system & reduced coelomic spaces.
 Sometimes reduced coelomic passages only

Blood movement via vessel & body wall
contraction
Osmoregulation/Excretion
Class Polychaeta


Protonephridia or
metanephridia (many)
Often associated with
coelomoducts
 What is it?
 Function?


Nephridia and
coelomoduct linked in
different ways
Osmoregulatory &
excretory
 Fluid entry  selective
reabsorption along tube
Osmoregulation/Excretion:
Class Clitellata, Subclass Oligochaeta

Metanephridia
 Fluid entry


Via coelom
 Nephrostome
Via circulatory
system
 Capillaries
 Specialization of
regions

Such as..?
Osmoregulation/Excretion:
Class Clitellata, Subclass Hirudinoidea


Metanephridia
Nephrostome
connected to a capsule
with amoeboid cells
 Function?

Intracellular duct
system
 Temporary
 Formed by coalescing
intracellular vacuoles.
Nervous System:
Class Polychaeta

Dorsal brain within
prostomium
 What happens if
brain is removed?

Anterior sensory
structures connect
with brain
 Often a variety of
tentacles, etc…

Circumenteric
connectives to
ventral nerve cord
Nervous System:
Class Polychaeta


Paired, fused
ventral nerve
cords
One ganglion per
segment
 Lateral nerves 
pedal ganglia

Giant axons
 Why giant?
Function?
Nervous System: sensory organs
Class Polychaeta

A variety of tactile
and chemoreceptors
 Anterior nuchal
organs


Ciliated pits
Chemosensory
 Dorsal cirri of
parapodia


Tactile, chemosensory
Also for gas exchange
Photos: www.tolweb.org
Nervous System: sensory organs
Class Polychaeta

Photoreceptors
 Simple pigment cup (Chaetopteridae)
 Pigment cup with lens (Nereidae)
 Complex eye with cornea, lens and retina