Phylum: Annelida

I. Characteristics

A. Segmented body with bilateral symmetry.

1. Segments arranged in linear series and externally marked by circular grooves called annuli.

2. Body segmentation is known as metamerism, and is internal as well as external.

B. Body wall with outer circular and inner longitudinal muscle layers; outer transparent moist cuticle secreted by epithelium.

C. Chitinous setae often present; exception = leeches.

D. A true coelom which is divided by the septa; exception = leeches.

E. Closed circulatory system and segmentally arranged; respiratory pigments such as hemoglobin, hemerythrin, and chlorocruorin often present; amoebocytes in blood plasma.

F. Complete digestive system which is not metamerically arranged.

G. Respiration through skin, gills, or parapodia. Parapodia are paired lateral processes on each side of most segments in polychaete annelids; variously modified for locomotion, respiration, or feeding.

H. Excretory system typically a pair of nephridia for each metamere. Nephridia are segmentally arranged, paired excretory tubules of many invertebrates, notably the annelids. In a broad sense, any tubule specialized for excretion and/or osmoregulation; with an external opening and with or without an internal opening.

I. Nervous system with a double ventral nerve cord and a pair of ganglia with lateral nerves in each metamere; brain a pair of dorsal cerebral ganglia with connectives to cord.

J. Sensory system of tactile organs, taste buds, statocysts (in some), photoreceptor cells, and eyes with lenses (in some).

K. Hermaphroditic; larvae, if present, are trochophore type; asexual reproduction by budding in some; spiral and mosaic cleavage.


II. Body Plan

A. Prostomium: anterior "segment" which houses the head.

B. Pygidium: posterior terminal "segment" holding the anus.

C. Coelom develops embryonically as a split in the mesoderm on each side of the gut (schizocoel).

1. This forms a pair of coelomic compartments in each segment.

2. Each compartment is surrounded with a peritoneum (layer of medodermal epithelium).

a) This peritoneum forms dorsal and ventral mesenteries.

b) Septa are formed where the peritonea of adjacent segments meet.

3. Coelom is filled with fluid and serves as a hydrostatic skeleton.


III. Classification

A) Class Polychaeta = "many long hairs"

1) Largest and most primitive: more that 5300 species, mostly marine.

2) Size: 5 to 10 cm, although some less than 1 mm and long as 3m.

3) Live under rocks, in coral crevices, abandoned shells, or burrow into the sand or mud; some adopt tubes of other animals; some pelagic.

4) Form and Function

a) Different from other annelids in well-differentiated head with specialized sense organs, paired parapodia, and no clitellum.

b) Many setae, usually arranged in bundles on parapodia.

c) No permanent sex organs, hermaphroditic, metamorphosis involving trochophore larva.

d) Tube dwelling polychaetes secrete many types of tubes

1) Parchmentlike (14-5 pg. 300)

2) Calcareous (14-3a pg. 299)

3) Sand, seaweed, etc. cemented by mucous secretions.

4) Line burrows with mucous (14-7 pg. 301)

e) Has a true head, or prostomium.

f) Sense organs are highly developed and include eyes, nuchal organs, and statocysts.

g) Reproduction

1) Gonads appear as temporary swellings of the peritoneum and shed their gametes into the coelom.

2) They are carried outside through gonoducts, through nephridia, or by rupture of the body wall.

3) Fertilization is external.

5) Genus: Nereis - the clam worms

a) errant polychaetes that live in mucus-lined burrows in or near low tide.

b) grow 30 to 40 cm in length

c) prostomium bears a pair of stubby palps (touch and taste), a pair of short sensory tentacles, and two pairs of small dorsal eyes that are light sensitive

d) peristomium bears the ventral mouth, pair of chitinous jaws, and four pairs of sensory tentacles.

e) parapodium formed of two lobes: a dorsal notopodium and a ventral neuropodium.

1) these lobes are supported by one or more chitinous spines (acicula).

f) feeds on small animals, other worms, larval forms


B. Class Oligochaeta = "few long hairs"

1) Setae may be long/short, straight/curved, blunt/needlelike, single/in bundles although less setae than in Polychaeta (hence the name)

2) Earthworms ("night crawlers")

a) "typical earthworm" is 12 to 30 cm long although giant tropical earthworms may have from 150 to 250 or more segments and grow to as much as 4m in length.

b) Form and Function

1) mouth is overhung by a fleshy prostomium at the anterior end

2) in most, each segment bears four pairs of chitinous setae though some may have up to 100 or more per segment

3) seta is a bristlelike rod set in a sac within the body wall and moved by tiny muscles. they project through small pores in the cuticle.

4) peristaltic movement: contractions of circular muscles in the anterior end lengthen the body, pushing the anterior end forward where it is anchored by setae; contractions of longitudinal muscles then shorten the body, pulling the posterior end forward.

c) Nutrition

1) scavengers feeding mainly on decayed organic matter

2) food is moistened by secretions from the mouth, then drawn in by sucking action of the muscular pharynx

3) calciferous glands along the esophagus secrete calcium ions into the gut and so reduce the calcium ion concentration formed by the calcium in the soil - they also regulate pH of the body

4) after esophagus, food is stored temporarily in the thin-walled crop before being passed on into the gizzard, which grinds the food into small pieces

5) digestion and absorption take place in the intestine with the aid of intestinal folds known as typhlosoles

6) chlorogogue tissue surrounds the intestine and dorsal vessel and serves as a center for the synthesis of glycogen and fat (equivalent of liver cells)

7) when ripe, the chlorogogue cells are released into the coelom where they float free as cells called eleocytes which transport materials to body tissues

8) chlorogogue cells can pass through segments, and are often found in places of damage

d) Circulation and Respiration

1) double transport system: coelomic fluid and circulatory system carry food, wastes, and respiratory gases

2) blood is carried in a closed system of blood vessels, including capillary systems in the tissues and five main blood trunks (aortic arches) running lengthwise through the body

e) Excretion

1) a pair of metanephridia, the organs of excretion, is found in each somite except the first three and the last one

2) each nephridium occupies parts of two successive somites

3) nephrostome, a ciliated funnel, lies just anterior to intersegmental septum and leads by a small ciliated tubule through the septum into the somite behind, where it connects with the main part of nephridium

4) this part of nephridium is made up of complex loops of increasing size, terminating in bladderlike structure leading to nephridiopore which opens to the outside to release the waste

f) Nervous system and sense organs

1) CNS made of pair of cerebral ganglia(brain) above the pharynx and a pair of connectives passing around the pharynx connecting the brain with the first pair of ganglia in the nervecord

2) each pair of fused ganglia gives off nerves to the body structures, which contain both sensory and motor fibers

3) neurosecretory cells for endocrine function are found in the brain and ganglia

4) for rapid escape movements most annelids are provided with several giant axons or giant fibers located in the ventral nerve cord - this large diameter increases the rate of conduction and makes possible simultaneous contractions of muscles in many segments

5) many small sense organs; no eyes but many lens-shaped photoreceptors in the epidermis; mostly negatively phototactic to strong light and positively phototactic to weak light

g) Reproduction and development

1) monoecious

2) two pairs of small testes and two pairs of sperm funnels are surrounded by three pairs of large seminal vesicles

3) eggs discharged by ovaries in coelomic cavity where they are carried by oviducts to the outside through female genital pores

h) primary representative is Lumbricus sp., which is used for study in most classrooms

3. Freshwater oligochaetes

a) usually smaller and have more conspicuous setae than earthworms

b) more mobile and better developed sense organs

c) generally benthic forms that creep about on bottom or burrow in the soft mud


C. Class Hirudinea = "characterized by leech"

1) predominately freshwater although some marine and few terrestrial

2) most between 2 and 6 cm, but the largest is 30 cm

3) hermaphroditic

4) lost setae in exchange for suckers for attachment while sucking blood

5) gut specialized for storage of large quantities of blood

6) Form and Function

a) fixed # of somites (usually 34 although 17 or 31 in some groups) although they appear to have more because each somite is marked by transverse grooves to form from two to 16 superficial rings or annuli.

b) leeches lack distinct coelomic compartments and coelom is filled with connective tissue and system of spaces called lacunae.

c) movement by looping movements of body, by attaching first one sucker and then the other and pulling up the body

d) aquatic leaches swim with graceful undulatory movement

7) Nutrition

a) predaceous

b) fluid feeders which prefer to feed on tissue fluids and blood pumped from wounds already open

c) some have cutting plates or "jaws"

8) Respiration and excretion

a) gas exchange through skin except in some fish leeches which have gills

b) 10 to 17 pairs of nephridia for excretion

9) Nervous and sensory systems

a) 2 brains - one in head and composed of six pairs of fused ganglia forming a ring around the pharynx, and the other in the tail composed of seven pairs of fused ganglia

10) Reproduction

a) hermaphroditic but practice cross-fertilization in copulation

11) Circulation

a) some retain oligochaetic circulation

b) in others, no traditional vessels but the coelomic sinuses form the system is which blood is propelled by contractions of certain longitudinal channels