ZOOLOGY OF INVERTEBRATES IN THREE VOLUMES - VOLUME 2 - H.Y. Shcherbak - 1996

PHYLUM ANNELIDA

CLASS LEECHES (HIRUDINEA)

Leeches are freshwater, less commonly marine or terrestrial, predatory or Blood-sucking animals, ranging in size from a few millimeters to 20 centimeters or more. About 400 species are known, of which about 30 occur in Ukraine.

The body of leeches is flattened, elongated, and consists of a fixed number of segments (30 or 33). The prostomium and pygidium are mostly poorly developed, and the body exhibits secondary annulation. There are anterior (oral) and posterior suckers. The anterior sucker is formed by the fusion of four segments, and the posterior by seven (Fig. 30).

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Fig. 30. Leeches:

a - Acanthobdella peledina (order Acanthobdellea); b - its anterior end with setae; c - Ozobranchus with gills; d - Piscicola geometra (order Rhynchobdellea); e - Hirudo medicinalis (order Arhynchobdellea); f - gills

The Skin of leeches forms many transverse rings that do not coincide with the true segmentation: each segment accounts for three to five such rings. Most leeches lack not only parapodia but also setae, with the exception of species belonging to the subclass Archihirudinea, which possess setae on the first five segments similar to those of oligochaetes (Fig. 30, b). In sexually mature individuals, as in oligochaetes, a thickening called the clitellum appears on certain segments.

The skin-muscular sac consists of a single-layered epidermis secreting a thin cuticle on its surface, an underlying Connective Tissue layer, and circular, diagonal, and longitudinal Muscle layers (Fig. 31). The thin, transparent cuticle is often shed in the form of delicate films, which can be observed when maintaining leeches in a laboratory. The epidermis consists of a single layer of cylindrical Cells, although in certain areas of the body (e.g., the clitellar region) the epidermis may be multilayered. Cilia are absent in epidermal cells. The integument is very rich in glands that secrete mucus, which facilitates the locomotion of leeches and the functioning of their suckers. The glandular Cells of the clitellum secrete material for forming the cocoon into which eggs are deposited. In most leeches, the basal ends of the epidermal cells are embedded in the connective tissue layer, the outer parenchyma, which contains various pigment cells, muscle fibers, and capillaries of the lacunar system (see below). Pigment cells determine the coloration. Almost every species of leech has a characteristic coloration and distinct pattern (stripes, spots).

The musculature in leeches is well developed. Muscle volume can account for up to 65% of their total body volume. In addition to the Muscles of the skin-muscular sac, they also possess bundles of dorsoventral muscles. The modes of locomotion in leeches are diverse. They crawl along the substrate by alternately attaching the anterior and posterior suckers and pulling the body forward; they are capable of swimming rapidly by undulating the body up and down. Respiratory movements are characteristic of many leeches: they attach themselves to the substrate with the posterior sucker and make wave-like body movements to facilitate gas exchange through the skin.

Fig. 31. Cytology/practical/72.html">Cross section of the medicinal leech Hirudo medicinalis:

1 - skin epithelium; 2 - outer layer of parenchyma; 3 - circular muscles; 4 - diagonal muscles; 5 - dorsal lacuna; 6 - longitudinal musculature; 7 - botryoidal tissue; 8 - inner parenchyma; 9 - dorsoventral muscles; 10 - lateral lacuna; 11 - Urinary Bladder; 12 - fine lacunar channels; 13 - Stomach; 14 - ventral lacuna with ventral nerve cord; 15 - vas deferens; 16 - Seminal Vesicle;

17 - nephridium; 18 - lateral caecum of The Stomach

Unlike other Annelids, leeches have a fairly well-developed parenchyma. It consists of two layers: outer and inner. The outer parenchyma develops between the epidermis and the layer of longitudinal muscles. The inner parenchyma fills the space between the Organs. The outer and inner Divisions of the parenchyma are closely interconnected. The parenchyma consists of a ground substance containing various cells: fat cells, pigment cells, and specialized cells that envelop muscle bundles like sheaths. Fat cells accumulate numerous nutrient reserves utilized during periods of starvation. Green, brown, and red pigment cells, together with skin pigments, determine the coloration of leeches. Decay products accumulate in various parenchymal cells.

The strong Development of the parenchyma, combined with highly developed musculature, imparts a special density and firmness to the body of leeches. Unlike other annelids, it is very difficult to tear a leech apart.

Seta-bearing leeches still possess a fairly large coelom, which houses the dorsal and ventral Blood Vessels, the ventral nerve cord, and the intestine. The parenchyma develops only between the skin-muscular sac and the walls of the coelom. In true leeches, the parenchyma proliferates much more extensively, leaving only four longitudinal lacunar channels from the coelom: dorsal, ventral, and two lateral, interconnected by a complex system of branching transverse lacunae (Fig. 32). In jawed leeches of the family Herpobdellidae, the dorsal channel also disappears.

Parallel to The Development of the parenchyma in leeches, a reduction of the Circulatory system is observed, replaced by a new transport system—the lacunar system, which is a system of coelomic channels. A true closed circulatory system, similar in Structure to that of oligochaetes, exists only in seta-bearing leeches. In rhynchobdellid leeches, the circulatory system is also closed, featuring dorsal and ventral blood vessels lying respectively within the dorsal and ventral coelomic lacunae; however, they lack ring vessels and a cutaneous capillary network. In arhinobdellid leeches, the circulatory system is completely absent, and its Functions have been taken over by the coelom. The longitudinal lacunar channels in arhinobdellid leeches are connected by numerous highly branched intermediate lacunae, forming a dense capillary network in the parenchyma between the skin-muscular sac and the intestine (Fig. 33). Lacunae also branch off from the longitudinal channels to form a dense network in the integument, with their ramifications located between the bases of the epidermal cells. The presence of a capillary network in the skin is of great importance for ensuring gas exchange. The coelomic fluid, which replaces blood in these leeches, moves due to the pulsation of the lateral lacunae, which possess muscular walls (see Fig. 32, c).

Fig. 32. Diagram of coelomic structure in leeches:

a - cross sections of Acanthobdella (order Acanthobdellea); b - Piscicola (order Rhynchobdellea); c - Hirudo (order Arhynchobdellea); 1 - dorsal blood vessel; 2 - coelom; 3 - ventral nerve cord; 4 - ventral blood vessel; 5 - intestine; 6 - dorsal, lateral, and ventral lacunae respectively

Fig. 33. Part of the lacunar system of Hirudo medicinalis:

1 - ventral lacuna; 2 - urinary bladder; 3 - cutaneous capillary network; 4 - glandular region of the nephridium; 5 - lacunar channel leading from the dorsal lacuna to the skin; 6 - dorsal lacuna; 7 - branch of the lateral lacuna; 8 - lateral lacuna; 9 - vas deferens; 10 - free end of the nephridium; 11 - ampulla

The Digestive System of leeches (Fig. 34) begins with the Mouth located, as a rule, at the bottom of the anterior sucker. In proboscis leeches (Rhynchobdellea), part of the Pharynx and the anterior end of the Esophagus have evolved into a proboscis capable of protruding and piercing the skin of animals. In jawed leeches (Arhynchobdellea), the Oral Cavity contains three ridged pads, or jaws, which the worm uses to bite through animal skin. These Teeth are formed of a hard organic substance impregnated with calcium carbonate. In some predatory leeches (family Herpobdellidae), the jaws are reduced.

Fig. 34. Diagram of the digestive system structure in leeches:

a - general view of the intestine; b, c - anterior end and jaw of the blood-sucking jawed leech Hirudo medicinalis, respectively; d - intestine of the predatory jawed leech Haemopis sanguisuga; e - anterior end of the proboscis leech Glossiphonia complanata; 1 - anterior sucker; 2 - pharynx; 3 - esophagus; 4 - stomach; 5 - transition of the stomach into the intestine; 6 - intestine; 7 - posterior gastric caecum; 8 - Water/144.html">Origin of the hindgut; 9 - hindgut; 10 - anus; 11 - posterior sucker; 12 - jaw; 13 - radial muscles of the pharynx; 14 - longitudinal muscles of the skin-muscular sac; 15 - ventral nerve cord; 16 - teeth; 17 - muscles moving the jaw; 18 - mouth; 19 - proboscis sheath; 20 - proboscis; 21 - Salivary Glands

The ducts of the salivary glands open into the oral cavity. The saliva of blood-sucking leeches contains a protein substance called hirudin, which prevents Blood Coagulation, enabling leeches to ingest large portions of blood without obstruction. Hirudin also possesses bactericidal properties, preventing the blood contained in the intestine from spoiling. In predatory leeches, which feed on other animals by swallowing them whole or in parts, hirudin is absent. The Pharynx and Esophagus are followed by the stomach, which belongs to the middle section of the intestine. In blood-sucking species, it bears from 6 to 11 pairs of lateral caeca, the rearmost of which are particularly large and extend toward the posterior end of the body. Ingested blood accumulates and is stored in the stomach (for over two months).

The stomach transitions into the posterior part of the midgut, which resembles a thin tube, at the beginning of which lies a pair of small glandular appendages. Here, food is digested and absorbed. The midgut opens into a short ectodermal hindgut, which opens to the exterior via the anus located above the posterior sucker. In predatory leeches, the stomach has only a single pair of lateral caeca (Fig. 34, d), and in some species, even these are absent.

The main excretory organs of leeches are metanephridia (Fig. 35). However, due to the reduction of the coelom, the metanephridia of leeches exhibit certain peculiarities: they are significantly fewer in number than the body segments (no more than 17 pairs). At their inner ends, the metanephridia are blindly closed, but the blind end of the nephridial tubule closely adheres to a ciliated funnel that opens with its wide end into a coelomic lacuna and with its narrowed end into a small reservoir sac. Metabolic products diffuse through the walls of the latter into the nephridial tubule. The tubule is long, forms loops, and is surrounded by numerous nephridial cells penetrated by intracellular canaliculi that empty into the main channel. This portion is known as the glandular region. The nephridial tubule empties into a urinary bladder, which opens to the exterior through an excretory pore, the nephridiopore.

Fig. 35. STRUCTURE OF THE nervous, excretory, lacunar, and reproductive systems of Hirudo medicinalis:

1 - supraesophageal ganglion; 2 - subesophageal ganglionic mass; 3 - copulatory organ; 4 - egg sac with Ovary; 5 - Vagina; 6 - Seminal Vesicles; 7 - vas deferens; 8 - ventral lacunar canal with nerve cord; 9 - urinary bladder; 10 - glandular region of the nephridium; 11 - lateral lacunar canal; 12 - Epididymis; 13 - Prostate Gland

The excretory function is also performed by the so-called botryoidal tissue (chloragogen cells). These are clusters of large granular cells lining the walls of part of the coelomic lacunae. They are particularly abundant around the capillary network in the parenchyma. They extract Metabolic waste products from the blood and accumulate them within themselves.

Leeches breathe through their entire body surface, and only in marine leeches of the genus Ozobranchus are gills present on certain segments in the form of branched paired lateral skin outgrowths (see Fig. 30, c).

Due to The formation of the sucker, the supraesophageal ganglion is shifted backward and located in the VI segment above the pharynx; on the ventral nerve cord, the ganglia of the segments forming the anterior sucker are fused into a single ganglion; a similar ganglion is also located near the posterior sucker (Fig. 35).

Sense Organs are represented by sensory papillae arranged in regular metameric rows on each segment. A sensory papilla (Fig. 36, a) is a cluster of sensory cells surrounded by heavily vacuolated epithelial cells; it features specialized arcuate muscles whose contraction causes the papilla to protrude. Nerve fibers extend from the sensory cells to the ventral nerve cord. Sensory papillae function as organs of Touch and chemical sense; sometimes they also include photosensitive cells. In many leeches, they transform into eyes located near the anterior end of the body in quantities of one to five pairs (Fig. 36, b, c).

Fig. 36. Sense organs of leeches

Leeches are hermaphrodites. The Gonads are located in specialized Regions of the coelom, the genital sacs. The Male Reproductive System (see Fig. 35) consists of one or several pairs of seminal vesicles (in the medicinal leech, for example, there are nine), each containing a Testis. A vas efferens extends from each sac; all of them empty into a pair of vas deferens, which form convoluted coils, or epididymides, in the X segment; sperm accumulates here. The walls of these coils secrete mucus. Paired ducts extend from the coils, fusing into an unpaired muscular ejaculatory duct. It has two appendages and is surrounded by a glandular mass (the "prostate"). Here, spermatophores—sacs containing sperm—are formed. In most jawed leeches, the ejaculatory duct passes through the copulatory organ.

A pair of egg sacs containing cord-like Ovaries inside is located in the XIII segment. The elongated ends of the sacs ("uteri") fuse into an unpaired oviduct that forms several loops and transitions into the vagina, which opens to the exterior on the XIII segment.

Fertilization is cross-fertilization. In proboscis leeches, spermatophores are attached to specific areas of the partner's body, their walls dissolve, and spermatozoa penetrate the partner's body to reach the female Gametes; in jawed leeches, spermatophores are absent, copulation takes place, and the male organ introduces sperm into the vagina of another individual. Glands of several anterior segments (the "clitellum") secrete mucus into which fertilized eggs are laid, forming a cocoon. After a certain period, young leeches emerge from the cocoon.

Among leeches, there are true ectoparasites that live on vertebrates, predominantly fish, throughout their entire lives and leave the host's body only for reproduction (Acanthobdella, Piscicola, etc.). Another ecological group consists of blood-suckers. They attack vertebrates solely for feeding (Hirudo, terrestrial leeches). Finally, some leeches are predators that feed on small invertebrates (Haemopis, etc.).

The class Hirudinea is divided into two subclasses: Archihirudinea and Euhirudinea.