Basic Invertebrate

Zoology (ZLY 201)Delivered by

SHITTU, O.Dept. of Zoology,

University of Ilorin,

Nigeria

HIRUDO

• Hirudinea comprises leeches which occur more in tropical regions than temperate regions.

• Leeches inhabit a wide variety of habitations ranging from freshwater, marine and terrestrial ecosystems.

• About 300 species of Hirudinea abound worldwide.

• They vary in length between 2 and 30cm.• The most specialized Annelids which are

advanced in forms than the Oligochaetes.• They have lost their parapodia (false feet) and

setae.

General Characteristics

General Characteristics

• Swimming by vertical undulations or loops by using its suckers to grip surfaces.

• They exhibit variation in patterns and colours.• They are dorso-ventrally flattened.• Most leeches are ectoparasites

(haematophagus, sanguivorous).• Some are predacious with suckers for

anchorage.• The gut is adapted for storage of large

quantities of blood.• Mouth opens at the base of the anterior sucker,

has 3 half-moon-shaped jaws (Next Slide)

Ventral View of the Anterior Sucker Showing the 3 Jaws

General Characteristics• leeches lack distinct coelomic

compartments.• In some species, the septa have

disappeared, and the coelomic cavity is filled with connective tissue and a system of spaces called lacunae.

• The coelomic lacunae form a regular system of channels filled with coelomic fluid serving as an auxiliary circulatory system.

• Hermaphroditic, possessing temporary clitellum (only during breeding).

• The clitellum secretes cocoon for reception and storage of eggs.

Ecology of Hirudinea

700 species of leeches are currently recognized

100 are marine, 90 terrestrial and the remainder freshwater.

The Indian cattle Leech H. granulosus, lives in ponds, lakes, swamps and slow moving streams.

H. medicinalis lives mainly in stagnant waters of marshes and ponds with a muddy substrate.

They prefer muddy freshwater pools and ditches with large weed growth in the temperate.

Eggs are laid outside water during spring & summer months.

Ecology of Hirudinea

H. medicinalis requires relatively warm

water (19-23ºC) in which to feed and

breed.

Egg cocoons are laid on marginal

plants.

They feed on the blood of vertebrates

and it is thought that mammalian or

possibly avian blood is required to

enable successful breeding.

Taxonomy of HirudineaPhylum Annelida         

               Subphylum Clitellata                   Class Hirudinea                      Subclass Hirudinea Lamarck,

1818 – leeches,                         Infraclass Euhirudinea  Lukin,

1956                            Order Arhynchobdellida

Blanchard, 1894                               Family

Hirudinidae Whitman, 1886

                                 Subfamily

Hirudinariinae Whitman, 1886

                                    Genus

Hirudo Linnaeus, 1758

                                       Species

Hirudo medicinalis Linnaeus, 1758

Morphology

• When the animal is stretched, it appears convex dorsally and flat ventrally.

 Eye position of the leech species Hirudo medicinalis and Haemopis sanguisuga.

Annulus

MorphologyShape and SizeSoft, vermiform, elongated and dorso-

ventrally flattened body Size ranges from 2 to 35cm depending on

diet and species. The skin is always moistened with mucus

and fluid thereby aiding cutaneous respiration.

The body form is narrowest at the anterior and broadened at the posterior end.

They exhibit alternation in shape and proportion by being ribbon shaped when extended and cylindrical when contracted.

External Characters

• A large specimen measures about 12 cm X 1.5 cm when fully extended, although it may contract to less than half this length.

• Variable colour pattern:• May consist of a greenish background with

a pair of longitudinal red stripes and a pattern of irregular black markings near the lateral margins.

• The ventral surface is usually black with white and grey markings.

• Leech body is divided by transverse furrows into 102 annuli

External Characters• Typically, mid-body segment comprises

5 annuli but towards the extremities, the number per segment progressively decreases.

• The distribution of the annuli between the prostomium and body segments is as follows:-

Externals – Anterior Section

• The anterior sucker is a depression on the ventral surface of segment I-IV.

• A small tri-radiate aperture lies at the base of the depression.

• The prostomium forms the anterior border of the sucker, It turns back ventrally & partially closes the oral aperture.

Externals – Anterior Section

• The male pore is conspicuous and lies between annuli 31 & 32, it gives a guide to the female pore,

• The female pore lies five annuli further (36 & 37),

• During the breeding season, the glandular clitellum appears on annuli 26-40.

Externals – Anterior Section

• 3 kinds of principal sense organs abound superficially, viz;– Tactile organs: Every annulus has receptor

organs raised on the papillae, they are annular receptors that serve as tactile organs.

– Segmental receptors/Sensillae (Light sensitive organs): These are not raised on papillae but located on every fifth annulus within white circular areas. (Internal segmentation).

– Larger light sensitive cells: found on segments I-V, 5 pairs of eyes backed by a pigmented cup (black dots)

Externals – Posterior Section• The posterior sucker is a

muscular disc (circular).• It is a more powerful organ

of adhesion than the anterior sucker.

• It is made up of seven fused segments.

• The nephridiopores are found on segment 7 between annuli 14 & 15, and between the 2nd & 3rd annuli of the following 16 segments.

• The anus is a very small aperture located on the mid –dorsal line of the posterior section.

Anus

Oral Sucker• The buccal cavity containing the jaws is

separated from the sucker cavity by low fold velum.

• During feeding the velum retracts backward to allow the jaws to be pushed forward into the sucker cavity to ensure adherence with host skin.

• Each jaw is a muscular ridge shaped like a half circular saw (one median & two ventro-lateral)

• Jaws are covered by cuticle and along the edges forms a row of minute teeth.

• During incision, muscles holding the jaw rock them so that the teeth moves with a sawing action.

• This results in a Y – shaped incision.

Ventral Dissection of the mouth Showing Jaws in Biting Position

Ventral View of the Anterior Section Showing the Oral Anatomy

• Each jaw is a muscular ridge shaped like a half circular saw (one median & two ventro-lateral)

• Jaws are covered by cuticle and along the edges forms a row of minute teeth

Alimentary Canal• The oesophagus, a narrow tube behind the

pharynx conveys blood into the crop.• The crop is the largest part of the leech alimentary

canal and it is adapted for storage of considerable volume of blood.

• To ensure storage, it is lined with eleven pairs of diverticula (one pair each on segments VIII –XVIII.

• The last pair runs back to the hind end of the body.• In segment XVIII, the crop terminates into a

narrower pore leading to the intestine.• The intestine is thin-walled but slightly swollen

(heart-shaped) at segment XIX and runs backward between the last pair of crop diverticula (XXIII) and leads to the rectum.

The Gut

Digestion

• Numerous unicellular salivary glands open into the jaws, they secrete anticoagulin.

• Anticoagulin prevents the clotting of host blood resulting from the wound.

• The blood is sucked into the alimentary canal by the pumping action of the pharynx (5mm oval sac).

• At rest, pharynx wall are deeply folded but dilated by radial muscles during feeding running out to the body wall.

• The space between the radial muscles is completely filled with salivary gland cells.

DigestionNutrition in leeches is of interest for several reasons:-• Typical leeches are blood-sucking ectoparasites,

they are remarkably adept at removing from the host a very considerable quantity of blood without being noticed. This requires sharp, precise cutting equipments and the assistance of a local anaesthetic.

• The blood must be prevented from clotting in the gut, for during locomotion the leech becomes alternately short and thick and long and thin and this would be impossible if the gut contained a mass of clotted blood.

• Finally, a series of investigators failed to identify any proteolytic enzymes in the gut of Hirudo and it appears that the function of digestion has been taken over entirely by symbiotic bacteria.

Digestion• An extract of the head of Hirudo contains a

powerful anticoagulin; hirudin which makes the wound made by a leech bleeds over time.

• However, Lindemann (1939) found that leech head extract also contains a histamine like substance capable of causing the dilatation of capillaries.

• He postulated that this was the substance actually injected into the wound and that the free flow of blood was due to the enlargement of the blood vessels rather than the inhibition of clotting.

• The act of biting and secretion of salivary glands is divisible into two phases

(i) the biting phase, (ii) the sucking phase

Digestion• Hirudin is an hydrolyzing protein product with

empirical formula of C30H60O20N8 and a molecular weight of 852.

• It probably acts by inhibiting the enzyme thrombokinase

• Only 0-8 mg is required to prevent indefinitely the coagulation of 5 ml of rabbit blood.

• Leech takes a meal of blood every six months because digestion is slow.

• Energy consumption is at 15 cal per day at 18°C.

• During starvation the leech utilized the stored carbohydrates and fats thereby dropping its energy consumption to about 7 cal per day.

Digestion• When a meal of blood has been sucked into

the crop it first thickens, water being abstracted and passed out via the nephridia together with considerable quantities of sodium chloride.

• The haemoglobin soon becomes deoxygenated but the erythrocytes remain intact for a very long time (Free from putrefaction).

• Pseudomonas hirudinis is a bacterium that aid blood protein digestion in Leeches, it does this by transforming protein into soluble nitrogenous compounds.

• Pseudomonas hirudinis also prevents the growth of other bacteria thereby retarding putrefaction of the RBC.

Reproductive System (Male)

• Testes in 10 pairs of coelomic sacs (XII – XXI)

• Short vasa deferentia connects testis to the vd of each side.

• At XI, they enlarge and coil to form a storage organ called epididymis/sperm vesicles.

• Followed by thick walled ejaculatory ducts leading to the artrium.

Reproductive System (Male)

• Atrium consists of two parts:-– a basal bulb

covered with several layers of unicellular prostate glands, and

– a penis sheath surrounding an eversible muscular penis.

• Spermatogonia bud off from the walls of the testis sacs & develop into spermatozoa (Coelomic fluid).

Reproductive System (Female)

• Ovaries appear as elongated cords.

• They have club-shaped terminations which lie freely in a single pair of coelomic sacs in segment XI.

• Short ducts run from these to a common oviduct, which is closely invested with a thick layer of unicellular albumen glands.

Reproductive System (Female)

• The oviduct leads to a U-shaped muscular vagina which in turn opens to the exterior.

• Ova are budded off from the cords in the ovisacs,

• When Cross fertilization takes place, zygote is coated with albumen in the oviduct.

• After cocoon formation by the clitellum, fertilized eggs pass from the female pore into the cocoon, the leech then slips over its head.

Excretory System

Seventeen pairs of nephridia opens on VII to XXIII.

A complete separation of the funnel from the rest of the nephridium (Outstanding).

The funnel is more of circulatory than excretory.

Nephridium is totally enclosed within a blood filled Sinus lying on top of a central capsule/reservoir

The parts of a typical Nephridium as seen in situ

Excretory System

The central capsule is studded with many small ciliated funnels.

The central capsule is the site of formation of corpuscles for the coelomic circulatory system.

The cilia in the funnel beats outwards wafting the corpuscles into the blood.

A winding intra-cellular canal from the testis sac follows to the main body of the nephridium.

The parts of a typical Nephridium as seen in situ

Excretory System

The nephridium’s glandular part originated from the nephridioblast which earlier cut off from ecto-mesoderm.

The vesicle & its duct to the exterior are ectodermal.

The nephridium is enriched with branches of blood sinus system, therefore, excretory products are derived within rather than from ciliated organ.

The parts of a typical Nephridium as seen in situ

Excretory System

The first six, and the last pair of nephridia are not associated with testes.

Absence of ciliated organ in nephridia 1st six & last pair, and the initial lobe ends blindly a little away from the ventral nerve cord

The parts of a typical Nephridium as seen in situ

Circulation (Blood System)

• Hirudo has no trace of blood vessels unlike Glossiphonia

• 4 main longitudinal sinuses, viz:– 1 dorsal above

the gut,– 1 ventral

containing the nerve cord &

– 1 posterior ganglionic masses

– 2 lateral sinuses.

Lateral sinuses have muscular walls and are responsible for circulating blood.

Three main branches originated and are: the latero-dorsals, latero-ventrals and latero-laterals.

Circulation (Blood System)

• The blood consists of:-– a plasma, coloured red by haemoglobin in solution

and – various amoeboid corpuscles – together with some chloragogenous cells.

Reconstruction of the coelomic sinus system of Hirudo

Nervous System

• The 1st six ganglion pair consists of 6 capsules dorsal to the gut.

• The 2nd consists of 3 capsules on each side of the gut.

• The remaining 4 pairs of ganglia form the ventral mass.

• One pair of ganglion encircled round the pharynx & closely associated with prostomial ganglia.

A contrast from earthworm, the cerebral ganglion buried under the oesophagus is a single ganglion pair associated with the prostomium

Nervous System

• There are seven peripheral nerves in the head region,

• The sympathetic (stomatogastric) nervous system(SNS) consists of nerve a ring lying on the wall of the pharynx.

• The SNS links with the central nervous system at two points on the circum-pharyngeal nerve ring.

Arthropods have several segmental ganglia which contribute to the brain

Histology

Transverse Section of H. medicinalis in the intestinal region

Histology

• The cuticle on the outer surface is secreted by epidermal cells and renewed at daily intervals

• Epidermal cells are columnar with pentagonal heads touching neighbouring cells.

• The inner ends are cylindrical with spaces penetrated by blood capillaries, nerve endings, pigment cells & dermal fibres.

• Various unicellular glands also originate from the epidermis (mucus, pear-shaped, elongated tubular).

• The anterior (head) region has mainly pear-shaped glandular cells which secretes to the surface of the anterior sucker (Similar to posterior sucker).

Histology

• The salivary glands lying between the pharynx and the body wall are modified epidermal cells.

• They are unicellular, pyriform glands with ductules leading to the jaws.

• The jaw epidermis of the jaws secretes cuticle which bears the apertures of these modified epidermal glands

• The clitellar region has mucus glands, chitogenous glands found within the circular muscles (secrete outer casing), albumen glands (longitudinal muscles)

Histology

HistologyBotryoidal and Vaso-fibrous tissue• Botryoidal tissue: found between the gut

and the muscles of the body wall.• It is characteristic of jawed leeches

(resembles bunches of grapes). • It consists of a network of very fine capillary

channels of the coelomic blood sinus system, lined by swollen globular cells which are heavily laden with brown pigment.

• Function• They correspond to chloragogen tissues and

serves as a center for synthesis of glycogen and fat, a function roughly equivalent to that of liver cells.

HistologyBotryoidal and Vaso-fibrous tissue• Vaso-fibrous tissue: consists of strands

running in the connective tissue which contain deposits of brown pigment. They have a small lumen which is continuous with that of the botryoidal tissue.

• Function• It is thought that the vaso-fibrous tissue

accumulates excretory products and is in some way complementary to the botryoidal tissue.

Leeches & MedicineToday, doctors use leeches for:- treating abscesses, painful joints, glaucoma, myasthenia, to heal venous diseases and thrombosis,used in plastic surgery, for improving brain

circulation and For curing infertility, in the treatment of children with cerebral palsy. May treat the following in the future:-multiple

sclerosis, effects of stroke, Parkinson's disease and Alzheimer's.

Hirudin Hirudin is an anticoagulant that is a polypeptide of 65-66

amino acids. Hirudin is the strongest direct specific inhibitor of

thrombin. Quickly reacting with thrombin, forming an inactive complex.

Thus, hirudin called "linking all the active sites of thrombin, can completely inhibit its proteolytic activity, inhibits the conversion of fibrinogen to fibrin and activates factors hemocoagulation.

Hirudin Clinical studies have established the effectiveness of

hirudin as an antithrombotic agent. It has two advantages over drugs of heparin:More stable anticoagulant effect;

1. Inhibition of thrombin contained in blood.

2. In addition, the effect of hirudin does not require the presence of anti-thrombin III or other endogenous

cofactors. Leeches have action against coagulation, resolving blood

clots, prevent their formation, stop hemoptysis. They have antiatherosclerotic and anesthetic action, help cleanse the body of toxins and poisonous substances.

End of Hirudinea Slides