Annelids

The segmented worms

Major Divisions Protostomes Deuterostomes determinate spiral cleavage indeterminate radial

cleavage

mesoderm from a single cell mesoderm from endoderm

coelom is a schizocoel coelom from pouches

mouth from blastopore mouth from new opening, anus from or near blastopore

Lophotrochozoans & Ecdyso- zoans

Phylum Annelida Consists of the segmented worms. Large phylum, approx. 9000 species. Earthworms (oligochaetes) and leeches

(hirudineans) are most familiar. Marine polychaetes not so familiar, but

= 2/3 of the phylum. Worldwide distribution. Marine and brackish waters, fresh water, and terrestrial soils.

Some are beautiful animals

Classes (general description)

Polychaetes chiefly marine. Most benthic, some live free in open sea. Usually divided into two groups: sedentary polychaetes (=Sedentaria), and free-moving (=Errantia). The errantia are now considered to be the most primitive polychaetes.

Errant, or free- moving poly. have various habitats; some are strictly pelagic and others live in crevices or under rocks or shells, never straying far in open water. Many polychaetes are euryhaline (can tolerate wide range of salinity) and live in brackish water. F.w.polychaetes are more diversified in warmer regions than in temperate zones.

The sedentary polychaetes usually live in tubes or permanent burrows. Many, esp. tube dwellers have elaborate feeding and respiration devices. Eat using ciliary currents which pass water between tentacles and carry food entangled in mucus to the mouth.

Classes (general description)

Oligochaetes and Hirudinea (leeches) occur predominantly in fresh water or terrestrial soils. Some f.w. species burrow in bottom mud and sand and others burrow among submerged vegetation. Swimming species usually have long setae (bristles). Earthworms have short setae

Many leeches (class Hirudinea) are predators, and many are specialized for piercing prey and feeding on blood or soft tissues. Few are marine, but most live in f.w. or in damp regions. Suckers are typically found at both ends of body for attachment to substrate or to prey. Some adapted for forcing pharynx or proboscis into soft tissues (e.g. gills of fish). Most specialized leeches have saw-like chitinous jaws with which they can cut through tough skin. Many leeches live as carnivores on small inverts.; some are temporary parasites; and some are permanent parasites, never leaving their host.

Metamerism (segmentation)

A serial repetition of body parts arranged longitudinally down the body.

In annelids it affects all of the organ systems, except the usually tubular digestive tract (even here often find paired series of glands on the gut wall).

Major blood vessels extend the length of the body above and below the digestive tube, but give rise to segmentally arranged lateral branches in each segment.

Each segment contains a pair of nephridia (but, some segments may loose them in adults). Segmental ganglia and segmental nerves give the n.s. a metameric character.

External rings, parapodia and setae give the external surface a metameric character.

How did metamerism evolve?

Metamerism involves the compartmentalization of the wormlike body into segments marked off by septa (membranous structures separating one septum from another).

Newest evolutionary theories suggest that septa evolved in order to allow the development of blood vessels running transversely (sideways).  A highly complex blood vascular system became advantageous with the development of  lateral appendages, the parapodia.  If you have such appendages it becomes necessary to supply them with nutrients.

Metamerism Once metamerism developed it provided

an aid to locomotion in long animals who could burrow. Contrary to the traditional view, the ability to use metamerism for  locomotion is a secondary development.

Can burrow with peristaltic waves if segmented. Coelom and body wall probably became segmented and rest of body parts followed.

Metamerism Use coelom as hydrostatic skel. against which muscles act to

change the body shape. Contraction of circular muscles causes the coelomic fluid to exert a force forwards and backwards and body elongates.

Contraction of longitudinal muscles causes coelomic fluid to exert lateral force and body widens. When divide coelom into compartments by septa can localize coelomic skeleton and body shape changes of widening and elongation can be restricted to certain segments.

Waves of peristaltic contraction pass down length of body, causing lengthening and then shortening of a number of segments encompassed within the wave. Anchor with short fat segments as they push against the substrate. Push forward with long thin segments. as they are extended. Alternation of anchoring and extending is very efficient way of getting around for a worm

Metamerism Metamerism important because can

modify parts of the body for special functions without having to affect the rest. Turn on different genes in different segments and entire course of development can be changed without requiring major mutations. Annelids do this, but Arthropods are the real beneficiaries of the idea.

Outstanding characteristics of phylum Annelida

Bilateral symmetry Metamerism (body segmentally organized) Body wall composed of an outer epidermis

and segmentally arranged outer circular and inner longitudinal muscle layers. A cuticle, secreted by the epidermis is usually present.

Metamerically arranged, chitinous setae usually present, secreted by epidermal follicles.

Characteristics  Head of variable complexity, typically formed

of a preoral segment, the prostomium, and a peristomium containing the mouth.

Closed circulatory system typically with a dorsal vessel in which blood flows anteriorly and a ventral vessel in which it flows posteriorly; blood often with a respiratory pigment dissolved in the plasma.

Coelom a schizocoel, generally spacious, and usually divided into segmental compartments.

Primitively with one pair of nephridia in all but the first and last somites.

Characteristics Circumenteric nerve ring, containing dorsal

cerebral ganglia, subpharyngeal ganglion, and connectives and a ventral, double nerve cord with paired segmental ganglia and commissural connections.

Brain function in flatworms chiefly a sensory relay center for receiving stimuli from the sense organs and then sending impulses down the nerve cord.

In Annelids similar. Can move in coordinated way without brain, in fact moves more than usual. Will meet obstacle and instead of backing off and changing dir. will continue to plow ahead. Maladaptive, but shows that brain has new function i.e. inhibition.

Characteristics Spiral, determinate cleavage and

primitively, a trochophore larva.

Polychaetes If the original adaptive value of

metamerism was to divide the body with septa in order to provide lateral blood vessels to the parapodia, the original annelids were probably pelagic swimmers and crawlers over the mud . 

Parapodia and a well developed head would be adaptive to exploit such an environment Then get radiation into other habitats and the assumption of other life styles. Get tube-dwellers, hard bottom worms, burrowers, etc.

Polychaetes Internal transport provided by a more or less

closed blood-vascular system, by coelomic fluid, or by a combination of the two.

Respiratory pigments, especially hemoglobin, common.

In blood, Hb. is found in fluid and is large molecule; in coelom it is found in cells and is small molecule (why?).

Hb. can load and unload at very small oxygen pressures in some worms so that they can tolerate very low oxygen in envir. Can also live for 20 days or more anaerobically. Important for intertidal burrower at low tide.

Polychaetes Most polychaetes have paired

segmental metanephridia, with nephrostome opening into coelomic compartment anterior to one possessing the tubule. Some polychaetes have protonephridia (may be what ancestor had, = primitive state for the phylum).

Polychaetes Most dioecious. Gametes are

produced by the peritoneum, mature in coelom, and exit by coelomoducts, coelomoducts joined to the nephridia, nephridia alone, or rupture of the body wall.

Primitively gametes are produced by the peritoneum of most segments.

Copulation is rare and synchronous emission of sperm and eggs is important. Epitoky and swarming bring a dispersed benthic population together for a brief pelagic existence, when gametes are shed and the likelihood of fertilization is increased.

Epitoky A reproductive individual, an epitoke, is formed from a

worm and leaves bottom burrows, tubes and other habitations and becomes pelagic.

Epitokal modifications include changes in the formation of the head, the structure of the parapodia and setae, the size of the segments and the segmental musculature, among others.

Often only gamete-bearing segments are affected, so worm looks like two different worms in one. Can get epitoke from non- epitokus worm by direct transformation of entire individual or by transformation and separation of the posterior end from the atoke (anterior end remains behind and regenerates).

Epitoke segments are crammed with gametes and often branch so that even more gametes can be produced.

Epitoke swarming Usually epitokous polychaetes swim to the

surface during the shedding of the eggs and sperm.

Congregates sexes and increases probability of fertilization.

Female produces pheromone to attract male and stimulates shedding of the sperm. The sperm, in turn, stimulate shedding of eggs.

To be effective, swarming must be synchronized. Not fully understood how such precise synchronization occurs. Changes in light intensity in some. Lunar cycles in others.

Palolo worm South Pacific Palolo worm swarms at dawn one

week after the November full moon. "The sexual pieces rise to the surface in countless millions, and the appearance of the water at this time has been compared to vermicelli soup. Later it appears milky from the eggs and sperms that are discharged. The anterior part of the worm, which remains hidden in some crevice in the coral rock when the posterior piece breaks off, regenerates the missing parts. On the corresponding day of the next year, the regenerated posterior end, laden with sex cells, breaks away.

The natives of the Samoan and other islands are familiar with the habits of the palolos. They consider them a great delicacy and look forward to their breeding season. When the day arrives, they scoop them up in buckets and prepare a great feast, gorging themselves just as we do on Thanksgiving day, knowing that there will not be another treat like it until exactly the same day of the next year."

Palolo Worms

Oligochaetes  Believed to have evolved independently from the

ancestral annelids. Some early polychaetes invaded fresh waters.

Oligochaetes evolved from these. During Cretaceous (100 million years ago) some oligochaetes invaded moist terrestrial environments at time when flowering land plants were proliferating. Can trace reliance of modern earthworms on deciduous vegetation to exploitation of this food source by their ancestors.

Others remained aquatic but became adapted for living in loose debris and algae.

Oligochaetes Most have retained a peristaltic mode of

locomotion and have well-developed metamerism and a simple prostomium.

The digestive tract is adapted for a diet of decomposing organic matter, largely plant material.

There are no gills, but cutaneous vascular networks are well developed in larger forms, especially earthworms.

Excretory organs like polychaetes.

Oligochaetes Most of the earthworm's adaptations for

terrestrial life are behavioral. Oligochaetes are monoecious, with well-

developed reproductive systems limited to a few segments. There is copulation and reciprocal sperm transfer. Fertilization and direct development occur within a cocoon secreted by the clitellum.

Oligochaetes Ecologically important as soil conditioners. 40

tons of soil per acre per year pass through earthworms. Since much undigested plant debris passes through with the ingested soil, the excrement, death and decay of earthworms builds organic constituents of soils.

Burrowing of worms aerates the soil and improves drainage. Shredding of vegetation and incorporation into fecal material increases surface areas of plant matter and hastens its eventual decomposition by bacteria and fungi.

Hirudinea

http://www.biology.ualberta.ca/courses.hp/zool250/Labs/Lab09/Leech.GIF

Hirudinea No parapodia and head appendages.

Dorsoventrally flattened and taper anteriorly. Have 34 segments, but don't look like they do because they have become secondarily divided into rings or annuli so each true segment looks externally like several rings or small segments.

The anterior and posterior segments usually modified into suckers.

Hirudinea Musculature highly complex, coelom

lost its partitions so method of locomotion different from other classes. Doesn't use independent coelomic compartments. Instead uses single hydrostatic cavity in looping (inchworm- like) locomotion. Can also swim by undulations of body. 

Hirudinea Feed on body fluids or entire body of other

iinvertebrates or on blood of vertebrates. Association between leech and "host" brief

so may be more accurately called predators than parasites. Not species specific as are most parasites. Class specific instead. Leech that preys upon a reptile such as a turtle may also prey upon an alligator, but probably would not prey on an amphibian such as a frog or on a fish.

Hirudinea Mouth opens in middle of anterior sucker. Some

have proboscis which can extrude into prey; others have mouth armed with jaws. While feeding, leech attaches to prey by anterior sucker and either extends proboscis into prey or uses its jaws to slice through host tissues. Salivary glands secrete anticoagulant. Why?

Behind mouth is muscular pharynx that pumps body fluids of prey into leech. Esophagus --> stomach with lateral cecae. Most leeches gorge stomachs and lateral cecae, increasing body mass 2-10 times so can stand periods of fasting that can last for months. Feast or famine. Short intestine --> anus.

Hirudinea Gas exchange across body wall.

Basic annelid circulation design retained in some, but in most, highly modified.

Vessels replaced by coelomic sinuses. Coelomic fluid takes over the function of blood and all but two orders lack respiratory pigments.

Hirudinea Basic annelid nervous system. Variety of

epidermal sense organs; (1) photoreceptor cells in pigment cups along dorsal

surface of anterior segments (usually are neg. phototactic, but some change when seeking prey),

(2) sensory cells with terminal bristles in a row along meddle annulus of each segment = sensory papillae whose fn. is unclear (useful for taxonomic purposes),

(3) chemosensors (many are attracted to prey by extracts of prey tissues) (4) temperature sense. Hirudo, the medicinal leech, has well-developed temperature sense which helps it find mammalian prey.

Hirudinea

Excretory system is 10-17 pairs of metanephridia, one per segment in middle of body. Highly modified and possess a capsule which probably is involved with production of coelomic fluid.

Hirudinea

All leeches are monoecious. No asexual reproduction or regeneration. They have a single pair of ovaries and from four to many testes. Have clitellum, as do oligochaetes, visible in spring when leeches breed. Sperm transfer and egg deposition as with oligochaetes.