Distinct locomotory/feeding parapodia

Mostly marine, some tube-dwelling

Tentacles and eyes present

Subclass Oligochaeta-earthworms and relatives

Few setae

Prostomium

Subclass Hirudinea-leeches

Posterior sucker

Secondary loss of internal metamerism

Most lack setae

Many ectoparasites

Trophosome organ with chemosynthetic bacteria.

Segmented only on rear portion of animal.

Lack a digestive system, mouth, and anus

Not segmented as adults

Anal sacs

Extensible proboscis with ciliated "gutter"

Few or no setae

No respiratory structures

Coelom lining is source of gametes

Although you don’t think of annelids as being advanced animals, compared to the groups we have covered so far, they are very complex and have a number of evolutionary innovations that we ourselves possess. They are fairly diverse, representing approximately 15,000 species. The name annelid is from the greek word annulatus or ringed. You have all seen the various outer rings on an earthworm or other annelid.

1) They are coelomate. That is they have a fluid filled cavity that is completely bound by mesodermal tissue.

2) And, coelomates, are further divided into two main branches, the protostomes and deuterostomes. Annelids are protostomes, which means that the blastopore gives rise to the mouth, they are schizocoelous, which means the mesoderm arises from a split in the archenteron, they show spiral cleavage, and they have determinate cell fate. The protostome condition is shared most notably, with molluscs and arthropods-and to that extent you can consider the fact that most animals in the world are protostomes. The two main lines of development-the protostomes and deuterostomes suggest that the coelom may have evolved twice. We’ll look at this theory in more detail at the end of the course, but for now, we’ll just try to keep track of which animals are protostomes versus deuterostomes

and annelids are clearly protostomes.

3) Annelids have metamerism, or the repetition of body parts within segments. This condition is also known as serial homology. There is repetition of musculature, excretory structures, nervous system, and mesenteries throughout the animal. Annelids also have the coelomic compartment divided up into discrete units within each segment. There are septa that are mesodermal tissue that separate the coelom. This means that each segment is completely enclosed. What does this mean? Each segment functions as an independent hydrostatic skeleton.

The segments are often grouped into functional units particularly in marine sedentary polychaete worms. These functional groups of segments are sometimes called soviets.

4)There is one major drawback to having a compartmentalized coelom. The transport function of the fluid is greatly hampered. As a result, annelids have a true circulatory system. Which means they have channels lined with epithelia-or in other words blood-vessels. They have blood that flows through them, and many, but not all, have an oxygen-carrying pigment. Some have hemoglobin like we do, some have hemerythrin, and some have chlorocruorin, a pigment close to hemoglobin in structure.

These pigments may occur in the blood, in the coelomic cavity or both. Since annelids are large in size relative to most of the other animals you’ve seen so far, and many live in marine sediment, or in soil where oxygen levels are low, they require a means of increasing oxygen levels to body tissue. Hemoglobin alone may account for up to 40% of the worm’s oxygen-carrying capacity. In addition to blood and blood vessels, they also have a pumping mechanism-or a functional heart. The heart is really nothing more than a heavily muscularized area of blood vessels.

5) Also, in line with higher metabolic needs due to body size and anoxic habitats of some annelids, there are specialized respiratory structures in some annelids

6) Annelids, like most pseudocoelomates, have a complete gut that shows even a greater degree of specialization and regionalization throughout.

7) Annelids have a well developed nervous system-with a dorsal cerebral ganglion and a ventral nerve cord-blood vessels are associated with many of these regions since nervous tissue is metabolically expensive to maintain-both in oxygen requirements and nutrients.

8) Annelids possess protonephridia, but many also possess metanephridia. These are the osmoregulatory and excretory organs. What is the difference between the protonephridia and metanephridia? Metanephridia are much more selective than protonephridia in what they filter. Metanephridia are also associated with a blood vascular system.

9) The sexes are either separate, or they are simultaneous hermaphrodites. Most produce a characteristic trochophore larva

10) They are found in marine, freshwater, and terrestrial environments.

There are four main lines in annelids:

Polychaetes

Clitellata

Pogonophora

Echiura

SHOW OVERHEAD of different groups

The polychaetes are considered to be the primitive state.

Oligochaetes and Leeches are more specialized for specific habitats and lifestyles.

Polychaetes are almost entirely marine.

Each segment has a pair of parapodia extensions of the body wall.

SHOW PARAPODIA OVERHEAD

Each parapodia also has setae (or chaetae) extending off the parapodia. These are usually chitinous bristles.

These parapodia have been modified extensively in many polychaets-usually into respiratory and/or feeding structures.

Polychaetes clearly show a compartmentalized coelom resulting in a metameric body. The compartmentalization of the coelom improves the efficiency of locomotion.

The sexes are separate with segmental, paired gonads

This very general body plan has fostered diversity.

Serial repetition of body parts leaves a lot of room for specialization. WHY?

Serial repetition of parts means that there is a certain amount of redundancy to the body plan-if you remove a couple of segments here or there it doesn’t have that much of an influence. Also, if there are septa between segments, means that the segments have alot of independence. Each has its own excretory duct and independent hydrostat. That means a change in a segment up or down, may not greatly affect that one.

Although there is a great deal of modification of the metameres, there has been some question as to why metamerism originally evolved.

The most cogent hypothesis is the Burrowing theory. R.B. Clark first proposed this idea. He suggested that compartmentalization of the coelom allows for more efficient burrowing.

SHOW OVERHEAD PAGE 502 BARNES

Remember that a hydrostat transfers pressure equally throughout. This means that any muscular contraction is transferred throughout the animal. That means to maintain the proper shape requires more energy. Body wall musculature must be used to prevent aneurisms in a worm with an unsegmented hydrostat.

Segmented animals can isolate pressure changes such that only the segment being squeezed needs antagonistic forces acting on it. The rest of the animal has no increase in fluid pressure.

So segmented worms can have more force generated for burrowing and still expend less energy than a similar sized worm that is not segmented.

Parapodia, also are redundant-and prone to evolve different functions. Remember, these are only present in polychaetes-as a result, polychaetes tend to have a greater diversity of forms than oligochaets or hirudineans.

The parapodia in the head region are especially prone to modification.

SHOW OVERHEAD OF HEAD OF POLYCHAETE HEAD AND SOME APPENDAGES

They may be modified into

Sensory tentacles

Palps for manipulating food

Feeding structures

Parapodia use is influenced by feeding habits, differences in habitat and locomotory patterns-all of which are interrelated.

SHOW OVERHEAD BRUSCA PAGE 391

Here are some of the body forms and parapodia forms related to locomotion and

feeding mode.

The parapodia and the setae or bristles that arise from it have a great deal of modifications possible.

Sometimes the parapodia are used as paddles, legs, gills, feeding tentacles or holding sacs for mucous.

SHOW OVERHEAD OF POLYCHAETE MOVEMENT

For errant polychaetes, or those polychaetes that move more readily in the environment, the general pattern of movement allows for the animals to capture mobile prey. Errant polychaetes are found in the open water, benthic habitats in shallow sand, crevices, under rocks and shells, and among other invertebrate constructions. Most have crawling, walking or swimming.

Notice that the segments serve to more localize contractions. These are spaced between segments with normal resting fluid pressure within the coelomic cavity.

This sort of movement pattern is found in nereis and neanthes that you saw in lab. They can also move more quickly by increasing the number of segments involved in a "wavelength" (metachronal wave) of movement. This sort of movement is similar to what you see in snakes-with additional movement caused by the parapodia.

Most of this serpentine motion is caused by longitudinal dorsolateral muscles. Circular muscles are used mostly to maintain proper hydrostatic pressure during locomotion.

Parapodia are extended maximally at the crest of a wave.

The problem with swimming is the wave is generated posteriorly. That means that a current is created with the metachronal wave and inhibits faster swimming. Thus the larger the waves, the less efficient the worm becomes.

The result, early swimming quickly degrades to thrashing about in the water and is used mostly in defense of benthic predators than a form of locomotion.

Some polychaetes have broader parapodia. In general, the flatter and broader they are the more efficient swimmers they are and the less efficient walkers. These polychaetes move with shorter waves instead of longer ones as they swim faster. Most broad, laterally flattened parapodia-bearing polychaetes are pelagic.

Scaleworms like Aphrodite, the sea mouse, use almost exclusively parapodia to move rather than body undulations. These are efficient walkers but cannot swim-since their leg orientation is mostly downward.

The sea mouse has protective platelike scales on its dorsal surface. Overlapping elytra are attached to the body with a small stalk. Under this plate is a ventilating chamber for respiration. These worms barely burrow under the surface with only some setae exposed. This allows active movement in sand while leaving repiratory surfaces exposed.

Most of these errant polychates feed on small crustaceans, molluscs or other annelids.

Many polychaetes burrow in soft sediment.

Interestingly, these animals have secondarily lost their septa, or the septa have become perforated. Also, many of these polychaetes have lost their parapodia. Why would this be so?

This means that individual septa are not of constant volume. A loss of volume in one causes a gain in another.

This fact seems to negate R. Clark’s burrowing hypothesis for why annelids became segmented. Why? There have been studies that show that similar sized worms are different in efficiency due to segmentation. Annelids also have a better developed circulatory system than many non-segmented worms....the debate continues.

Most of these burrowers use peristaltic movement like priapulids to move. They use the inflated end as an anchor.

Arenicola uses this method while burrowing.

Lets consider some of these sedentary worms with reduced or highly specialized parapodia.

SHOW OVERHEAD (BRUSCA 401)

For Arenicola, the lugworm, the parapodia are reduced and specialized as gills. This is common among sediment dwelling worms where the bottom may be anoxic. Not only does Arenicola use peristalsis for burrowing, but it also uses it for feeding.

They create a mucous lined burrow to live in. They then create waves of contractions that generates a current through the burrow. The suction causes the burrow to collapse at one end and the worm feeds on the sand as it collects near its mouth. The animal processes the dirt and backs up and excretes out the back. These are termed castings, or pseudofeces since they actually contain less organics going out than they do going in. This is not the case for true feces.

Remember the slide of lugworm castings in lab-it looked like a moonscape.

Lugworms are termed direct deposit feeders.

There are others that feed as direct deposit feeders.

The ice-creme cone worm, Pectinaria, feeds in a similar manner but is more selective about what it eats. It builds a turreted body covering with sand and mucous and funnels the excurrent out the tube end. It stirs up sand, working it into suspension with its tentacles and removes the organics.

There are also selective deposit feeders. They won’t each just any old dirt.

The main difference between the two is one sorts food prior to ingestion, and the other sorts it by digestion

Amphitrite is an example of this type of feeder. It buries most of its body in the sand and picks up minute organic material and funnels it to the mouth by ciliary action.

Each tentacles curls over on itself and moves food along a track along with mucous balls.

The mouth of amphitrite is also surrounded by respiratory structures. This is common for increasing surface area. Most respiratory structures are modifed parapodia or sensory appendages around the mouth (the prostomial palps is a common one). Some are even modified setae or some other extension of the body wall.

Many of these deposit feeders take in dissolved organics directly through the water.

Many of these worms feed almost constantly given the large amount of sediment that must be filtered from a small area to get enough food.

We have talked about two feeding methods in polychates, direct feeding in errant polychaetes, and deposit feeders-both selective and non-selective.

Another major type of feeding mode is suspension or filter-feeding.

In most cases, these worms live in tubes that they build or secrete.

And they hold tentacles out to capture organic material or small prey.

Remember that filter feeders tend to be sessile-or at least sedentary in their habits.

As a consequence, the animals have a tendency toward a secondary biradial symmetry similar to what you see in an anemone-and they have these non-movable shells that protect them.

Many of the longitudinal and circular muscles are weak. The setae are fairly well developed and grip the walls of the tubes tightly to prevent removal.

They often have well developed retractor muscles to pull the feeding structures in.

SHOW OVERHEAD BRUSCA PAGE 402

There are two general types of filter-feeders.

Sabellids and Serpulid worms are representative of one type.

They possess peristomial tentacles called Radioles. These are bipinnate structures with a long groove in the center that funnels food particles to the mouth. Cilia drive water through the pinnules to the tracts.

This is a hydrodynamically efficient way of filtering food through a water column since there are other structures in other invertebrates that look very similar but evolved independently.-like crinoid echinoderms for instance.

The edges of some of the radioles have knobs on the ends that form plugs when the radioles are withdrawn.

Some polychaetes filter while in a burrow.

This is the case in Chaetopterus "setae-wings".

They secrete chitinous tubes in which they live.

They have highly modified parapodia. Several "fan" parapodia create a feeding current in a tube. These were observed in lab.

The Fans move the current toward the posterior.

Other parapodia are modified as suckers to stick the worm to the wall of the tube.

Segment twelve produces mucous secretions that produce a mucous bag for collecting organic material. This is then wadded up every 15-30 minutes and passed to the mouth for ingestion, then a new one is produced.

The mucous bag collects particles as small as 1 micrometer, even protein molecules are captured due to ionic charge attraction.

These different feeding modes and locomotory patterns contribute to the diversity of forms found in polychaete worms. All of this occurs within the framework of metamerism.

It is important to realize that some worms that are considered errant, are tube-dwelling. They wait for prey to walk near their tubes, then lunge out and grab them.

POLYCHAETE REPRODUCTION

Polychaetes have the most diverse reproductive methods of any annelid.

A few have asexual reproduction mostly in the form of budding and fission. This is most common among tubedwelling worms, often resulting in colonies of clonal worms.

Most polychaetes reproduce sexually and have separate sexes.

In the most primitive polychates, each segment produces gametes.

There is a trend toward reduction and specialization of gonads in specific segments or genital segments.

Some fan worms are hermaphroditic and have eggs produced anteriorly and sperm produced posteriorly.

The gametes are usually shed into the coelom where they mature and then exit the body either through repeating coelomoducts or gonoducts that are found in every pair, or through specialized gonoducts.

Some polychaetes have there body walls rupture to release the eggs or sperm. This usually kills the polychaete.

SHOW EPITOKE TYPES

The most characteristic form of reproduction among polychaetes is called Epitoky.

Many nereids, and other errant polychaetes use this form of reproduction.

This method involves the production of sexual individuals called epitokous individuals. These individuals may arise from the transformation of a nonreproductive one OR through asexual reproduction of epitokous individuals.

When a transformation of a worm occurs, the posterior end becomes swollen with gametes and the parapodia become enlarged too.

The asexually produced epitokes usually don’t have a head and are usually found in clusters along the body.

They bud off and swim to the surface where they release gametes in the millions. Usually fertilization is external. Mating is often at night and is synchronized with the lunar cycles. Some bioluminesce and give an eerie glow to the water.

In some South Pacific Islands, the people collect the ripe epitokes and munch on them.

The larval form of most annelids is a trochophore larvae

DRAW TROCHOPHORE LARVAE

These larval types are also found in molluscs and some other groups like sipunculan and echiuran worms.

They are planktonic larvae.

Walter Garstang explained polychaete development (Phyllodoce) much better than I can and at the same time, demonstrated how completely nerdy invertebrate zoologists can be:

The trochophores are larval tops the polychaetes set spinning

With just a ciliated ring-at least in the beginning-

They feed, and feel an urgent need to grow more like their mothers,

So sprout some segments on behind, first one, and then the others.

And since more weight demands more power, each segment has to bring

Its contribution in an extra locomotive ring:

With these the larva swims with ease, and, adding segments more,

Becomes a Polytrochula instead of trochophore.

Then setose bundles sprout and grow, the sequel can’t be hid:

The larva fails to pull its weight, and sinks---an Annelid.

The translation of all that is that there is a growth zone at the posterior end and new segments are given off anteriorly. So the oldest segments are right behind the head and they get younger as you move posteriorly-the exact opposite of the proglottids of a cestode.

Polychaetes are large.

Errant annelids are both fairly active and large. The sedentary ones are also large and live in sometimes anoxic or low-oxygen water or in constrained areas such as tubes that inhibit oxygen transfer through diffusion.

Combine these habits

With the development of mesoderm, these additional layers of tissue and you can see that annelids were faced with problems of oxygen demand.

Pseudocoelomates mostly overcame this issue by remaining small-although the pseudocoel helped somewhat in oxygen transfer. The few larger ones, like some priapulid worms, have specialized caudal appendages that serve as respiratory structures.

SHOW OVERHEAD -CROSS SECTION OF ANNELID

The coelom, present in all animals that remain to be considered, provides a good hydrostatic skeleton.

Bear in mind the coelom arises as a new space in the mesoderm.

The pseudocoel is leftover space that parenchme doesn’t fill.

In coelomates, as the coelom increases in size, the outer part of the mesoderm becomes associated with the body wall.

The inner mesoderm becomes associated with the gut and viscera.

The lining of the coelom is called peritoneum. Coelomic fluid does not come into direct contact with either body wall or gut-it is separated by peritoneal epithelium

The separation of body wall and viscera has favored their functional separation (freedom from sychronous movement and function results in energy savings)

The coelom is a strategically important space.

Metabolic wastes and absorbed foods are transported in it-much like in the pseudocoel-in fact the coelom is often lined with cilia to move stuff along.

Mesenteries are additional divisions of the peritoneum and are usually sheath-like. These serve to maintain structure and compartmentation.

In animals with a a blood vascular system, or true circulatory system, the heart and major blood vessels develop within this mesentery.

The osmotic properties of coelomic fluid are important in determining the environment of cells in other tissues.

Annelids have both blood and coelomic fluid. What is the difference?

The blood of invertebrates is similar to our own in many ways. There is noncellular plasma containing ions, water, and proteins. Occasionally there are hemocytes or cellular corpuscles in the coelomic fluid as well. Some serve an immune function or transport nutrients or wastes.

Respiratory pigments such as hemoglobin may be carried in the coelomic fluid or in the blood but are generally much more numerous in blood. They may be carried intracellularly or extracellularly-but in the coelomic cavity they are almost always carried intracellularly.

Proteins are more numerous in blood that coelomic fluid.

The osmotic concentration of the blood is higher than that of coelomic fluid.

The osmotic gradient causes water to be taken up by blood-water that has more diffused oxygen in it than the blood. The gradient is maintained by the loss of water to the coelom by ultrafiltration by metanephridia.

For blood to flow through vessels, it has to be pressurized, it also has to be higher pressure than the surrounding coelomic fluid-this makes sense, since the vessels would collapse otherwise.

The pressure is maintained through hearts, or some other pumping structure

So there is always a tendency for blood fluid to leak back into the coelom.

The animal takes advantage of this by having filtration sites that are specialized sites that filter material as it leaks into the coelom. Cells called podocytes do this filtration.

Any filtration system that holds back protein-sized particles is regarded as a ultrafiltration system.

Thus there is ultrafiltration between the blood vessels and coelom. But even coelomic fluid contains water, ions, glucose and other useful stuff along with waste.

The important stuff must be reabsorbed while the wastes are excreted. This reabsorption takes place in the nephridia. Cells do not recognize waste products and do not store them selectively but this takes time. So the membrane in which material is filtered needs to be long to slow down fluid flow.

There is another reason for this. There is a pressure gradient in a hydrostat. So there must be some intermediate membrane and means of slowing down fluid between the outside and inside of the animal-otherwise the entire contents of the coelom may squirt out the first time an animal tries to excrete.

Metanephridia serve this function.

What is the difference between a metanephridium and protonephridium?

A metanephridium is associated with a blood vascular system and has a ciliated tubule. The blood is filtered through the coelom, which in turn, is filtered through the metanephridium and out of the body.

Thus, strictly speaking an animal must have a blood-vascular system and a coelom to have a metanephridial system for excretion.

And the fluid is filtered twice, once through the podocytes, and again through the metanephridia.

Metanephridial structures are more highly developed in oligochates and hirudineans since they are both found in freshwater and terrestrial habitats.

The marine and freshwater oligochaetes and leeches are ammonotelic while terrestrial forms, not surprising are at least partially ureotelic.

Chlorogogen cells of oligochaetes precipitate waste in a solid form, but how or if it is eliminated from the body is unclear.

Leeches have very different metanephridia from other annelids. I won’t go into how in the interest of time-but they do produce dilute urine.

Among terrestrial leeches, they excrete urine onto their suckers where they provide a good suction.

OLIGOCHAETES

Oligochaetes are usually specialized for burrowing, both aquatic and terrestrial.

They have evolved a streamlined external simplicity. They lack parapodia found in polychaetes.

The setae tend to be short and very thick, often with hooks on them.

These worms use the setae to anchor them in their burrows, both in aquatic and terrestrial systems

The gut, particularly of terrestrial oligochaetes, can be very complex and regionalized to extract nutrients out of dirt.

pharynx-esophagus-crop-gizzard-intestine-rectum

In the case of lumbricus and other terrestrial oligochaetes, they may have various accessory glands associated with digestion.

The calciferous glands are used to precipitate out calcium from the soil they ingest. It is formed into calcite and then is dropped back into the digestive system for egestion.

The chlorogogen cells surround the intestine but reside in the coelom. It stores glycogen and lipids and deaminates proteins. In this regard it is really a functional liver.

Oligochaetes also have an obvious structure in the digestive system called the typlosole which is a mid-dorsal groove that results in an invagination of the digestive wall. This increases surface area for digestion since many oligochaetes are dirt eaters, there is alot of processing involved.

DRAW DIFFERENT BURROW TYPES

Terrestrial oligochaetes are extremely important in soil fertility and turnover.

There is an annelid ecologist that did some early work in the 1840s on how earthworms move dirt. The guys name was Charles Darwin, maybe you heard of him-

Charles filled glass jars with combinations of light and dark dirt and observed how long it took worms to homogenize the two types of dirt-it didn’t take long.

Earthworms also aerate the soil and bring up nutrients and increase crop yields over those areas where earthworms are experimentally removed.

Other studies have found that worm castings have high concentrations of nitrogen, phosphorus, and potassium.

Earthworms are also used extensively in composting-since they greatly accelerate the decomposition of organic waste-turning it to soil.

Interestingly, most of the common large earthworms you see are not native to North America. They were brought over in ballasts of ships from England and have since colonized most of North America. Native N. American terrestrial oligochaetes are much smaller.

SHOW OLIGOCHAETE REPRODUCTION

Clitellum-albumin, egg case, mucous are secreted.

Hirudinea

The leeches.

The bloodsuckers with an interesting history.

They are temporary ectoparasites on vertebrates in aquatic systems-but some are terrestrial.

1567-1308 BC the Egyptians believed that an overabundance of body humors was the basic cause of disease.

Leeches could suck out the disease.

The Chinese used them as early as the 10th Century.

In Europe, during the 15th century, leech blood-letting in public baths by barbers, surgeons and bath keepers was common.

During the 19th century, commercial use of the medicinal leech (Hirudu medicinalis) nearly drove them to extinction.

While in many cases, blood letting with leeches cost patients their lives (George Washington), there is a modern role of leeches in medicine and biomedical research.

Leeches have great biomedical and biotechnological importance.

Hirudin and Hementin are two anticoagulants from different species.

Haementeria ghilani is a tropical leech from French Guiana rainforest.

Hementin dissolves clots as they form so is important in heart surgery patiences and stroke patients. It is a blood-thinner.

Biopharm (UK) is a multi-million dollar biotech company that raises leeches commercially for Hementin.

They have isolated the gene for hementin, inserted it into yeast and brew up batches of the stuff.

They have also found substances in leeches that have antibiotic properties.

Also during surgery for limb reattachment, there is often microsurgery involved with attachement of small veins and arteries and transplants. Leeches are often attached to the areas during surgery to keep blood-clots from forming and reoxygenates wounds. This keeps blood flowing into these small blood vessels.

They can ingest 8 or 9 times their body weight over several hours.

They have a simplified digestive system with special adaptations for blood sucking. They release an anticoagulant to prevent blood from clotting as it feeds. They also release an anesthetic which allows leech to feed undetected. A third ingredient is spreading factor which breaks down intercellular matrix incresases diffusion of materials.

Most have jaws for rasping into the flesh. The crop and cecae in the digestive system provide a large surface area and storage for a large meal.

Leeches possess only exopeptidases as digestive enzymes. Other enzymes are lacking. For this reason it may take several months to digest the contents of a single meal.

The coelom in leeches is much reduced, and they largely lost setae.

There body plan internally somewhat resembles an acoelomate condition from this standpoint.

But leeches are very active and rather large-yet the coelom is reduced in function.

It therefore has surface to volume problems, and has a flattened body shape reminescent of flatworms. The gut caeca also aid in transport and digestion.

Unlike in flatworms, the leeches have a circulatory system to accomodate their larger size. They have circulatory vessels and sinuses throughout.

Some have hemoglobin which accounts for up to 50% of the oxygen-carrying capacity.

Leeches have many similarities to oligochaetes

They are usually protandric hermaphrodites, rather than simultaneous hermaphrodites. They have direct development and skip the trochophore larval stage (actually it occurs while in the egg).

They possess a clitellum like oligochaetes, although it isn’t as well defined-and produce a cocoon. Some leeches have hypodermic impregnation where the sperm are injected near the clitellum. In other leeches there is an eversible penis that injects sperm into the gonopore. Since many leeches are protandric, there are cases where they do not mutually exchange sperm.

They have secondarily lost their setae except for some found at the anterior end of some.

And they have evolved suckers that they use in some forms of locomotion.

The segmentation is reduced, the coelom has become packed with cells

In the interest of time, I’ll just briefly compare the traits between the different classes of annelids.

Other annelids--

There are two additional classes of annelids that were previously considered to be separate phyla--

They are the Pogonophora and Echiura.

Pogonophora characteristics

120 species

Discovered this century hundreds to thosands of meters under water.

Some at 20 m depths.

Have a trophosome that is packed with chemosynthetic bacteria.

No mouth, no anus, no digestive system.

Two groups:

Perviata (Perviate worms)-small less than a millimeter wide-6-35 cm long. Surface area large compared to volume.

Absorb dissolved organic matter directly through water column (infaunal where DOM is high) and dissolve through tubes.

Vestimentifera (Vestimentiferan worms)-large-over a meter long. Have fused, lamellated tentacles.-thousands or hundreds of thousands of tentacles.

Opistosomal region is segmented but nowhere else.

Always associated with areas of high hydrogen sulfide content.

This is interesting in a number of ways:

1. hydrogen sulfide is metabolically toxic to just about everything
2. it allows for a food chain that is not ultimately based on sunlight.

Reproduction-

Unknown but trochophore larvae are present and development looks similar to other annelids. Juveniles have a complete gut and anus indicating that the digestive system has been secondarily lost.

Larvae are probably planktonic judging from ability to move from one vent to another.

Vestimentiferans may colonize dead megafauna (i.e. dead whales). These harbor large populations of chemosynthetic bacteria that may serve as an "island" to hop between vents until the resource is exhausted.

Have little evidence of segmentation except early during embryogenesis.

Proboscis with a brain in it that can extend up to 25 times the length of the body.

Proboscis is non-retractable but has a ciliated "gutter" that runs through it. It is a deposit feeder except for one genus that is a suspension feeder.

Suspension feeding occurs through a mucous net crossing the burrow.

Echiurans often also live around hydrogen sulfide rich sediment but detoxify the stuff with an iron-containing pigment in its blood that is subsequently excreted.

Setae are few but do occur anteriorly and posteriorly.

Anal sacs are also unique among Echiurans--these presumably have an excretory function as well.

Yet there is no evidence for their function. They are muscular and are outpockets of the rectum. They have many thousands of ciliated funnels that resemble nephrostomes or nephridia-like structures.

Fluid is not brought into the coelom through the sac however, so it is a one-way exit.

They have no respiratory structures…must be through diffusion of the body surface. For larger animals, that are sedentary, this is usually sufficient. More active annelids require more activity.

ECHIURAN REPRODUCTION

Have typical trochophore larvae

Distinct gonads are lacking, instead the gametes are shed from a specialized coelomic lining. The metanephridia are "converted" to gamete storage (a uterus). They are dioecious with distinct sexes.

Members of one genus, Bonellia, have males and females

The male is much smaller and is a parasite of the female. Males migrate either through the digestive system via the gutter or enter through the anus and migrate to the nephridia. Here they wait until eggs are stored and directed through this structure and then fertilize the eggs. One female may have up to 100 parasitic males.

Sex determination is by touching larvae with the proboscis. If the female touches a larva, it turns male, if not, it becomes a female. To this extent, females have direct behavioral control over the sex ratio of their offspring.

SHOW OVERHEAD OF SUMMARY OF DIFFERENCES BETWEEN THE THREE CLASSES OF ANNELIDS

SHOW EVOLUTIONARY RELATIONSHIPS BETWEEN ANNELIDS

Rotifera-corona, mastax, trophi

Priapulida-coelomocytes

Annelida-trochophore larvae, metamerism, setae