ZOOLOGY OF INVERTEBRATES - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

SECTION TRIPLOBLASTICA, OR BILATERIA

SUBSECTION SPIRALIA

PHYLUM ANNELIDA

SUBPHYLUM CLITELLATA

CLASS OLIGOCHAETA

Oligochaetes inhabit freshwater bodies, soil, and more rarely (about 200 species) marine environments. The vast majority are burrowing forms, some live on the bottom surface among decaying organic matter, and a small number of species parasitize the gills of crustaceans. About 5,000 species are known. In the freshwaters of Ukraine, about 200 oligochaete species have been recorded, while the fauna of the Black and Azov Seas contains 33 species. Soil-dwelling oligochaetes remain insufficiently studied. Most oligochaetes range from 0.5 mm to 40 cm in length, whereas certain tropical earthworm species (Megascolides australis) can reach up to 3 m.

Morphology. The body is elongated, nearly cylindrical (Fig. 179), exhibiting homonomous metamerism. The prostomium generally lacks appendages and eyes, the peristomium is absent, and the Mouth is located on the first trunk segment. The number of segments varies from 5-6 to 500-600. Parapodia are reduced and replaced by paired bundles of setae: two dorsal and two ventral. Setae play a crucial role in locomotion: burrowing forms use them to anchor against the walls of their burrows, while aquatic species use them for swimming. Sexually mature specimens feature a glandular thickening in the anterior third of the body known as the clitellum.

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Fig. 179. Oligochaetes

(after Perel and Malevich): a - sludge worm Tubifex tubifex; b - Stylaria lacustris; c - Nais pseudobtusa

The integumentary-muscular sac has a typical annelid Structure (Fig. 180). Externally, the body is covered by a thin cuticle, beneath which lies the Skin epithelium (epidermis) containing numerous glandular Cells. Connective Tissue cells are loosely distributed among the Muscle fibers. In regions where setae emerge, cuticular and epidermal invaginations form seta sacs (setigerous follicles). Mucus secreted by glandular cells lubricates the body surface; in soil-dwelling oligochaetes, this keeps the skin moist, which is essential for cutaneous Respiration. Mucous and protein glands are particularly abundant in the epithelium of the clitellum, where they secrete the substance that forms the cocoon.

Fig. 180. Diagram of a cross-section through the middle region of an earthworm's body (after Perel and Malevich, modified):

1 - dorsal Blood vessel; 2 - typhlosole vessel; 3 - typhlosole;

4 - intestinal vascular plexus; 5 - midgut; 6 - chloragogen cells on the intestinal surface; 7 - ventral nerve cord;

8 - subneural vessel; 9 - ventral blood vessel; 10 - mesentery; 11 - setigerous follicle;

12 - metanephridium; 13 - coelomic epithelium; 14 - longitudinal musculature;

15 - circular musculature; 16 - epidermis; 17 — cuticle

The coelom is typically divided by septa (dissepiments) into individual segments, although within each segment the right and left coelomic cavities are not separated; oligochaetes lack a dorsal mesentery, and in some species, the ventral mesentery is also partially or entirely missing. The coelomic fluid contains amoebocytes, whose primary function is phagocytosis, as well as so-called excretory corpuscles. These are relatively large (up to 1-2 mm) aggregations composed of numerous amoebocytes surrounding the breakdown products of various cells and occasionally even parasites. In the middle and posterior segments of many soil-dwelling species (such as Lumbricus), unpaired dorsal pores are present, through which coelomic fluid oozes out; this lubricates the body and facilitates the animal's movement through the soil. Excretory corpuscles are expelled from the body cavity through the Pores in the posterior region.

Oligochaetes move through the contraction of the Muscles of the integumentary-muscular sac, with the coelomic fluid functioning as a hydrostatic Skeleton. Oligochaetes are capable of both crawling and swimming. Burrowing forms move continuously, tunneling through the soil. Burrowing occurs differently than in animals with a continuous body cavity (such as priapulids). The septa between segments prevent fluid from surging freely throughout the entire body; instead, it flows only within the range of two to five segments via apertures in the dissepiments.

The Digestive System begins with the mouth on the first trunk segment and extends through the entire body as a straight tube comprising the Pharynx, Esophagus, thin-walled crop, muscular gizzard, midgut, and hindgut (Fig. 181). Numerous Salivary Glands open into the pharynx. In species that ingest soil, three pairs of calciferous glands open into the esophagus. Their primary Functions are to remove carbonates from the blood, neutralize humic acids present in the soil (which are harmful to the digestive Organs), and bind excess carbon dioxide in the blood by forming soluble bicarbonates. The walls of the midgut are rich in ciliated

and secretory cells, where food Digestion AND ABSORPTION take place. In soil-dwelling oligochaetes, the DORSAL SIDE OF the midgut forms a longitudinal, trough-like invagination into the intestinal lumen—the typhlosole—which serves to increase the internal surface area of the intestine. It terminates near the posterior end of the body, where the midgut transitions into a short hindgut that opens via the anus on the pygidium. Aquatic species feed on microscopic Algae, Protozoa, and Bacteria, whereas soil-dwelling forms feed on PLANT AND ANIMAL remains by passing large quantities of soil through their digestive tracts.

Fig. 181. Internal Structure of an earthworm (ventral dissection, modified from Dogel):

1 - pharynx; 2 - supraoesophageal ganglion; 3 - oesophagus; 4 - hearts; 5 - seminal receptacles; 6 - Seminal Vesicles; 7 - crop;

8 - gizzard; 9 - dorsal blood vessel; 10 - midgut;

11 - lateral vessels; 12 - dissepiment; 13 - ventral blood vessel; 14 - metanephridium; 15 - typhlosole;

16 - ventral nerve cord; 17 - pharyngeal radial muscles

The excretory system is represented by a single pair of metanephridia in each segment, with the exception of a few anterior (larval) segments. In addition to nephridia, so-called chloragogen cells take part in excretion. These are specialized Cells of the coelomic epithelium located On the surface of the gut and Blood Vessels. Recent evidence indicates that their function is far more complex: they store Metabolic waste products as well as reserve nutrients (Glycogen and Lipids).

The Circulatory system is closed. The blood vessels lie beneath the coelomic epithelium of the intestine, dissepiments, and body walls; capillaries proliferate deep into the tissue layers. The circulatory system is similar to that of polychaetes, but with certain specific features: the circular vessels surrounding the oesophagus possess thick muscular walls and function as hearts (Lumbricus terrestris has five pairs of them), pumping blood from the dorsal vessel to the ventral one. Furthermore, in connection with cutaneous respiration, a dense subcutaneous network of small vessels and capillaries develops, penetrating the body wall musculature. In many oligochaetes, the blood is colorless; in some, it is red due to the presence of haemocruorin—a pigment structurally similar to haemoglobin, dissolved in the plasma or, more rarely, contained within specialized cells known as haemocytes. In tropical giant earthworms (genus Megascolides), the blood appears greenish due to another respiratory pigment, chlorocruorin. The blood also contains numerous colourless cells of various structures. The major vessels (dorsal and circular) have muscular walls.

Respiratory organs are generally absent: gas exchange occurs through the thin, moist skin and blood capillaries. However, some aquatic forms possess external gills.

The Nervous system has a typical annelid structure, consisting of a paired supraoesophageal ganglion, circumoesophageal connectives, and a ventral nerve cord. Only in a few species are the ventral trunks widely spaced and the ganglia connected by long commissures (for instance, in Aelosoma).

Sense Organs are poorly developed. Eyes are typically absent, yet earthworms exhibit sensitivity to light. Scattered throughout the epidermal epithelium are solitary cells or groups of sensory cells—sensilla, which serve as Touch and chemical receptors, as well as light-sensitive sensilla. They are especially abundant on the prostomium.

The Reproductive System is hermaphroditic, with the reproductive organs located across several segments and, in representatives of different families, in various Regions of the body. The reproductive apparatus comprises Gonads (Ovaries and Testes), egg sacs and seminal vesicles, male and female genital ducts (coelomoducts), and seminal receptacles (Fig. 182). In addition, the reproductive system includes the skin Glands of the clitellum and genital setae.

Fig. 182. Diagram of a sagittal section through the reproductive segments (IX-XV) of an earthworm (modified from Dogel):

1 - epidermis; 2 - circular muscles; 3 - longitudinal muscles; 4 - dissepiment;

5 - ovisac; 6 - male genital pore; 7 - female genital pore;

8 - oviduct funnel; 9 - Ovary; 10 - vas deferens; 11 - sperm duct funnel;

12 - seminal capsule; 13 - Testis; 14 - seminal receptacle; 15 - Seminal Vesicle

Reproduction. During mating, two worms join along their ventral surfaces with their anterior ends pointing in opposite directions, such that the clitellum of one worm lies opposite the seminal receptacle openings of the other. The clitellums secrete abundant mucus that envelops the bodies of both individuals, forming two mucous tubes. The worms discharge sperm from their sperm ducts, which is transferred by muscular contractions to the clitellum, where it enters the mucous tube and subsequently passes into the partner's seminal receptacles, which draw in the sperm via swallowing-like movements. Afterwards, the worms separate; thus, mating involves only sperm exchange rather than Fertilization. When a worm's eggs mature, the clitellum secretes a new mucous tube, which also contains nutritious secretions for the embryo (albumin). Through peristaltic body movements, the worm begins to push the mucous tube forward. As it moves, the eggs are first deposited into it, and then the sperm—previously received from the partner—is squeezed out from the seminal receptacles, after which fertilization of the eggs takes place within the tube.

The worm then sheds the mucous tube over its anterior end; its edges seal shut (first the anterior, then the posterior), forming a cocoon in which the eggs mature. Self-fertilization is known in some oligochaete species. There are also species capable of parthenogenesis.

Development proceeds without metamorphosis. The embryo, developing inside the cocoon, possesses a mouth, pharynx, and midgut; it actively ingests albumin and is therefore referred to as a "cryptic larva" (Fig. 183), which transforms into a young worm just before emerging from the cocoon. The young worms hatch by rupturing the cocoon membrane.

Fig. 183. Lumbricus embryo from the ventral side (from Davydov):

1 - mouth; 2 - pharynx; 3 - ectodermal bands; 4 - entoderm; 5 - ectodermal teloblasts;

6 - mesodermal teloblasts; 7 - mesodermal bands

Apart from sexual reproduction, oligochaetes also reproduce asexually through fission. In some species (Lumbriculus, Enchytraeus), the body can break down into several fragments, each of which subsequently regenerates the missing parts. This phenomenon is known as architomy. In other cases, the body divides by a constriction, yet both parts remain connected; the anterior part develops a new posterior end, while the posterior part grows a new anterior end. Eventually, the young individuals separate, though sometimes even before this happens, each of them divides in turn, forming chains of several individuals (e.g., in Aelosoma). This mode of asexual reproduction is called paratomy. Earthworms possess a remarkable capacity for regeneration: a severed worm can restore its missing parts.

Soil oligochaetes include earthworms, which belong to several families and do not form a single systematic group. The best-known among them are true earthworms (family Lumbricidae), ranging in length from 20 mm to several centimeters. All of them are saprophages (feeding on organic debris), while some also consume green plant parts, pulling them down from the soil surface into their burrows. Earthworms are widespread in various soils, even in deserts; in chernozems, their population can reach up to 10 million individuals per hectare. Over 50 species of earthworms are known in Ukraine, with the common earthworm (Lumbricus terrestris) being the most widespread.

Of particular interest are the endemic species of the Carpathians (Helodrilus cemosvitovianus, Allolobophora carpathica) and the lowland regions of our country (Allolobophora leoni, etc.). These species are potentially threatened and in need of conservation; Eisenia gordejeffi is listed in the Red Data Book of Ukraine. In the soils of southern and eastern Asia, Australia, South America, and the islands of Indonesia, species of the family Megascolecidae can be found, sometimes reaching a length of 3 m.

Small whitish worms—enchytraeids (family Enchytraeidae)—are also common in the soil, typically measuring 2–3 mm in length, but occasionally reaching 45 mm. They feed on humus and reproduce in vast numbers. Aquarists often breed them in pots with garden soil as fish food, which is why they are commonly called "potworms." Some species of enchytraeids inhabit marine and freshwater environments.

Freshwater oligochaetes include numerous species of the family Tubificidae, with the most common genus being Tubifex (Fig. 179, a). These are small worms, with a body length of up to 5 cm. About 40 species of this genus are known in Ukraine. They are abundant in silted soils where oxygen is scarce. All of them possess hemerythrin in their blood, which stores oxygen. Tubifex worms anchor the anterior end of their body in the mud, while waving their posterior end—rich in cutaneous blood capillaries—in the Water to intensify gas exchange. They feed on mud, digesting the organic matter it contains. They can withstand significant water pollution caused by various substances (petroleum products, pesticides, detergents, etc.). In organically rich water bodies, their concentration can reach 100,000 individuals per 1 m² of the bottom.

Among tubificids, there are also marine forms. In the coastal zone of tropical seas, species of the genus Phallodrilus are found, characterized by a unique mutualistic relationship with bacteria that settle under the cuticle of the worms: these bacteria are capable of utilizing hydrogen sulfide produced during the decomposition of organic remains. The gut in Phallodrilus is absent, and they subsist entirely on these bacteria.

Members of the family Branchiobdellidae attach themselves to the gills and body surface of freshwater higher crustaceans. Their adaptation to ectoparasitism has drastically altered their anatomy, leading earlier scientists to classify these oligochaetes as leeches. Their body is small (no more than 10–12 mm), thickened, and features a sucker at the posterior end. The mouth is equipped with chitinoid upper and lower jaws. Juveniles feed on detritus, while adults feed on crustacean blood.

Tubificids and enchytraeids are high-calorie food for fish, which is why they are harvested and cultivated. Due to their high resistance to pollution, tubificids hold great potential for the biological Treatment of wastewater. Certain giant megascolecid worms are consumed as food by the indigenous peoples of Southeast Asia and South America.

Certain species of earthworms are farmed industrially to produce nutritional supplements for livestock and even humans. The common species Eisenia foetida (commercially known as the "California red worm") naturally inhabits manure heaps. Through selective breeding, strains have been developed that can efficiently break down various types of manure and organic waste. These worms are successfully cultivated commercially and used—including in Ukraine—for Processing diverse biological waste. Passed through the worms' gut, this waste is converted into a valuable complex fertilizer, while the worms themselves, processed into meal, serve as a high-value Supplement that significantly enhances the nutritional efficiency of livestock feed.

The most significant role of earthworms lies in soil formation, a fact first highlighted by Charles Darwin. In his work The formation of Vegetable Mould, Through the Action of Worms, he demonstrated their positive impact on soil fertility. Worms loosen and mix the soil, improving its aeration and the penetration of water into deeper layers: the total length of worm burrows per 1 m² can reach 1–8 km. Soil that has passed through their digestive tract shows an increased content of essential plant minerals and reduced acidity. Over the course of a year, earthworms pass a 1 to 7 meter thick layer of soil through their bodies; their biomass, depending on the soil type, ranges from 10 to 3,000 kg/ha.

The NEGATIVE IMPACT OF oligochaetes is negligible. It is known that most earthworm species become toxic during their reproductive period (June–July in our region) and can cause the death of domestic poultry. Some microdriles serve as intermediate hosts for helminths. For instance, procercoids of Caryophyllaeus (Class Cestoda), a parasite of cyprinid fish, inhabit the coelom of Tubifex, while earthworms host nematode larvae responsible for severe diseases in pigs (Metastrongylus) and chickens (Syngamus).