Invertebrate Zoology

Lecture 15: Phylum Annelida, Part 2

Lecture outline Phylum Annelida, Part 2

Circulation/gas exchangeOsmoregulation/excretionNervous 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

ParapodiaAdditional structures

(some) Anterior gills (Terebellidae) Tentacular crown (Sabellidae)

Body surface If no other structures

Circulation/gas exchange: Class Polychaeta

Two major vesselsDorsal: blood anteriorlyVentral: blood posteriorly

Extensive capillary beds inParapodiaOther gas exchange structuresAnterior and posteriorIntestine

Blood moves between main vessels viaCapillary bedsSegmental 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