Phylum Annelida – Phylum and Class Characteristics

Phylum Characteristics: coelomate body divided into segments (metameres) via septa; repeated arrangements of organs and structures within segments (excl. digestive system); setae (bristles) and hydrostatic skeleton (excl. leeches); closed circulatory system; paired, ventral nerve cords and cerebral ganglia; paired nephridia in each segment (excretion)

Diversity and Overview of Class Characteristics Class Polychaeta: most marine (“sea worms”); parapodia

(paddle-like appendages); well-developed sensory organsClass Oligochaeta: most terrestrial (earthworms); secrete

cocoon from clitellum (reproduction); hermaphroditicClass Hirudinida: most aquatic (leeches); dorso-ventrally

flattened body; suckers for attachment; suck blood/body fluids; clitellum forms only during reproductive season

Fig. 17.1

Class PolychaetaDiversity: approx. 10,000 species, many create burrows

Clam worms: most predatory with proboscis and chitinous jaws; include Nereis and Eunice spp. (ex. bobbit worm)

Scale worms: flat bodies covered with platesFireworms: toxins secreted through hollow setaeLugworms: deposit feeders (ingest organics from sediment)Tubeworms: sedentary; composition of tubes vary (ex.,

CaCO3, leathery, chitinous) Featherduster worms: filter feed with extended radioles (fan);

include Christmas-tree worms Spaghetti worms: capture food particles with extended tentacles Parchment worm (Chaetopterus): captures food with mucous net

within tube; fan-like parapodia create water flow Pogonophorans (beardworms): discovered around deep-sea vents;

no digestive tract; symbiosis with chemosynthetic bacteria (located in trophosome); chitinous tubes

Class Polychaeta (“Sea Worms”)

The Bobbit Worm (Eunice sp.)

Fig. 17.2

Fig. 17.6

Fig. 17.11

Fig. 17.5

Fig. 17.12

Fig. 17.15

Fig. 17.14

Earthworms and LeechesClass Oligochaeta: approx. 3000 species; earthworms

burrow soil turnover and aeration; loss of parapodia and pronounced heads/sense organsSetae extend from pores in body wall (muscular control)Digestive system compartmentalized (incl. crop, gizzard)Dorsal blood vessel pumps blood; aortic arches stabilize

blood pressure; some with hemoglobin; gas exchange through moist skin

Class Hirudinea: fixed number of segments with superficial annuli; lack setae; possess anterior and posterior suckers for attachment to hostMedicinal leeches (Hirudo medicinalis): used to promote

growth of capillaries and reduce congestion in veins; once used for bloodletting (fevers thought to result from excess of blood)

Fig. 17.16

Fig. 17.16c

Fig. 17.17

Fig. 17.18

Fig. 17.19

Fig. 17.20

Fig. 17.21

Fig. 17.23

Fig. 17.24

Fig. 17.25

Annelid/Arthropod Allied Phyla Phylum Echiura (spoon worms): approx. 140 species, all

are marine burrowing worms, widespread deposit feeders; spoon- shaped proboscis; lack segmentation, but molecular evidence suggests derived from annelids

Phylum Sipuncula (peanut worms): all marine burrowing worms; eversible introvert at anterior end bearing tentacles

Phylum Onychophora (velvet worms): most are predators on insects in rain forests; so-called “living fossils” with little change in morphology since Cambrian Period; some with segmented bodies (ex. Peripatus); unsegmented, paired legs with claws; nephridia and ventral nerve cord

Phylum Tardigrada (water bears): minute (< 1mm) cylindrical bodies; live in film surrounding mosses or in damp soils; cuticle molted; buccal tube adapted for sucking; separate sexes; terrestrial forms can undergo cryptobiosis (state of suspended animation)

Fig. 17.26

Fig. 17.29

Fig. 18.17

Fig. 18.19

My Squishy Friend the Tardigrade