ZOOLOGY OF INVERTEBRATES IN THREE BOOKS - BOOK 3 - H. Y. Shcherbak - 1997

PHYLUM ONYCHOPHORA

Onychophorans are a small group of terrestrial predatory invertebrates inhabiting exclusively high-humidity environments in the tropical and temperate Zones of the Southern Hemisphere. In the Northern Hemisphere, they occur only in Mexico and Southeast Asia. A total of over 70 species are known.

The body of onychophorans is indistinctly divided into a HEAD with three pairs of appendages and a trunk bearing paired, unsegmented limbs of primitive Structure. The body is covered by a thin, elastic cuticle that does not function as an exoskeleton. A well-developed integumentary-muscular sac is present. The body cavity is a mixocoel. The Digestive System is weakly differentiated.

The Circulatory system is open and represented by a tubular Heart with metameric ostia. The excretory Organs are numerous metameric celomoducts. Respiration occurs via tracheae that form numerous unbranched bundles.

The Nervous system consists of a paired supraesophageal ganglion and two widely spaced, non-ganglionated ventral nerve cords interconnected by numerous commissures. Onychophorans are dioecious; Fertilization is internal; development proceeds without metamorphosis; the majority of species are viviparous.

The phylum Onychophora includes a single Class — Protracheata.

CLASS PROTRACHEATA

As already mentioned, protracheates are highly hygrophilous animals. They find optimal living conditions (along with humidity, also a constant Temperature) in the litter of tropical and subtropical forests, under stones and logs, and in various shelters near Water. They are very sensitive to desiccation and therefore burrow deeper into the soil in drier locations.

The size of onychophorans ranges from 2 to 15 cm. Most species are dark brown or brown, though green, blue-green, orange, and light blue ones also occur; they may be unicolor or feature stripes or spots.

The body of protracheates is worm-like, elongated, cylindrical, and slightly flattened ventrally (Fig. 5). Externally, the body exhibits annulation that does not correspond to true segmentation. It is divided into a head and trunk without a sharp boundary.

Fig. 5. Paraperipatus amboinensis

The head bears three pairs of unsegmented appendages (Fig. 6). Two long antennae project anteriorly to the Mouth, near the base of which, on the dorsal side, most onychophorans possess a pair of eyes. The antennae have external annulation but are not divided into true segments. On the ventral side, anterior to the Oral Cavity, lie two muscular outgrowths, each bearing a robust sclerotized plate with serrated edges. This is the only pair of jaws present in protracheates.

Fig. 6. Anterior body region of Peripatoides novaezealandiae:

1 — leg; 2 — perioral papillae; 3 — jaws; 4 — lateral papillae; 5 — antenna

Flanking the mouth are short papilla-like outgrowths bearing the openings of specialized slime glands at their tips (Fig. 6). When disturbed, they eject a sticky slime. These serve as defensive organs and simultaneously enable the extremely slow onychophorans to capture prey—such as small insects, spiders, etc.—by gluing and immobilizing them.

The trunk bears from 13 to 43 pairs of legs, with the posterior pair occasionally reduced. Each leg is a simple muscular body outgrowth terminating in two claws, hence the phylum name Onychophora, meaning "claw-bearing". It is believed that the number of leg pairs corresponds to the number of segments.

Externally, the body is covered by a thin cuticle 1 – 3 µm thick, beneath which lies an epithelium layer, followed by a thick layer of Fibrous Connective Tissue. The integuments do not protect onychophorans from desiccation, yet they remain unwettable due to the presence of micro papillae whose apices bear a well-developed, robust cuticle.

Muscles are situated beneath the integuments: a layer of circular muscles, two intersecting layers of oblique muscles, and two longitudinal layers, which together form a true continuous integumentary-muscular sac. In addition, well-developed dorsoventral muscles are present (Fig. 7).

Fig. 7. Cross section through the trunk of Peripatoides novaezealandiae:

1 — cuticle; 2 — Diaphragm separating the Pericardium; 3 — heart; 4, 5 — oblique and longitudinal muscles; 6 — slime gland duct; 7 — circular muscles; 8 — salivary gland; 9 — celomoduct; 10 — claws; 11 — excretory opening; 12 — midgut; 13 — nerve cord; 14 — FOOT; 15 — leg muscles; 16 — mixocoel; 17 — dorsoventral muscles

The cuticle of onychophorans does not function as an exoskeleton; it is soft and elastic. Instead, the integuments together with the fibrous connective tissue provide structural support. By altering the thickness and length of specific body regions through muscular activity, onychophorans crawl into narrow crevices or burrow through the soil, much like earthworms. Serpentine movements are not characteristic of them.

The longitudinal Muscle layer bounds the body cavity, which is of mixed origin, known as the mixocoel, and lacks its own distinct lining. It is filled with hemolymph, which simultaneously serves as Blood.

The digestive system features a relatively simple structure (Fig. 8). The mouth is located on the ventral side of the head at the bottom of a specialized preoral cavity formed by an invagination of the integument. The oral cavity can open and close thanks to a ring of cutaneous projections (papillae) that form its outer convex wall. As previously mentioned, at the bottom of the preoral cavity, on small muscular pads, lies a pair of sickle-shaped jaws that move back and forth with the aid of specialized muscles to tear apart prey. The unpaired duct of two large Salivary Glands also opens here; their secretion reaches the prey, initiating Digestion right on the spot.

Fig. 8. Internal Structure of a female Peripatoides novaezealandiae:

I — antenna; 2 — lateral papilla; 3 — supraesophageal ganglion; 4 — Pharynx; 5 — Esophagus; 6 — midgut; 7 — ventral nerve cord; 8 — salivary gland duct; 9 — slime gland; 10 — Uterus with embryos;

II — trunk leg; 12 — Ovary; 13 — anus; 14 — oviduct; 15 — nerve commissures; 16 — slime gland

duct; 17 — eyes

The mouth leads into the ectodermal foregut, which consists of a muscular pharynx and an esophagus, followed by the endodermal midgut. The latter appears as an undifferentiated tube extending through the entire body and transitioning into a short ectodermal hindgut that opens via the anus at the posterior end of the body. Onychophorans are voracious predators, and feeding on succulent insects and spiders compensates for their moisture loss.

The excretory system is represented by metameric coelomoducts. Each coelomoduct consists of a coelomic vesicle into which opens a ciliated funnel leading into a convoluted tubule. At the end of the tubule lies an expansion—the Urinary Bladder—which opens via an excretory pore near the Base of the appendage. Part of the canal is lined with ciliated epithelium. Excretory organs are present in all trunk segments except the last two, where the genital pores are located.

The circulatory system is open and highly simplified, reduced to a single long, tubular vessel—The Heart—lying above the gut. There are no peripheral vessels whatsoever. The posterior end of the heart is blindly closed, while the anterior end is open; along the sides are paired metameric openings, or ostia, through which hemolymph is drawn into the heart, propelled forward by its pulsation, and poured into the mixocoel through the anterior end. The ostia are equipped with Valves that ensure the hemolymph moves forward during heart pulsation and does not leak out through the lateral openings. The heart lies within the pericardium—a portion of the mixocoel separated from the general body cavity by a thin, incomplete partition, the diaphragm.

Onychophorans respire across their entire body surface and via tracheae, which open through numerous randomly distributed stigmas, or spiracles, on the body surface. Each stigma leads into a bundle of unbranched, blindly ending tracheal tubules that are bathed in hemolymph (Fig. 9).

Fig. 9. Section through a stigma and tracheal bundle of Peripatopsis capensis

The nervous system is represented by a relatively large paired supraesophageal ganglion connected by two circumesophageal connectives to a pair of widely spaced ventral cords that extend along the body and join together above the anus. Numerous commissures run between the cords. The cords are evenly covered with Nerve Cells that do not form any distinct ganglionic clusters. The supraesophageal ganglion consists of three divisions—the proto-, deuto-, and tritocerebrum—which innervate the eyes, antennae, and the anterior section of the gut, respectively.

Sense Organs are represented by antennae, which function as organs of Touch and chemical sense, numerous cutaneous tactile papillae, and eyes.

The cup-shaped simple eyes are located directly above the Brain and resemble the eyes of polychaetes in structure. Vision helps these animals avoid light; in the dark, they orient themselves by tactile exploration of objects with their antennae.

Onychophorans are dioecious animals, with males being overwhelmingly noticeably smaller than females. The Gonads are paired and located in the posterior half of the body. In the male, two hose-like Testes continue into vas deferens, each featuring an expansion at the beginning—the Seminal Vesicle—and then transitioning into a long, convoluted canal where spermatophores are formed. At the end, the vas deferens merge into an unpaired muscular ejaculatory duct that opens outwardly between the last pair of legs. An accessory gland is associated with the ejaculatory duct, the secretion of which is used to construct spermatophores.

The FEMALE Reproductive System (see Fig. 8) consists of a pair of tubular Ovaries from which oviducts extend. The expanded PARTS OF THE latter form uteri that merge into an unpaired Vagina opening outwardly between the bases of the penultimate pair of legs.

Fertilization in onychophorans is spermatophoric, but the process itself varies. In African species of the genus Peripatopsis, the male attaches several spermatophores to The surface of the female's body; spermatozoa emerge from these and penetrate the body through cracks in the integument. Fertilization likely occurs within the ovary, as already fertilized eggs enter the oviducts. In other species, spermatophores or even droplets of seminal fluid are deposited onto the substrate, and the females pick them up using the edges of their genital pore.

Only onychophorans of the genus Symperipatus and species of the genus Ooperipatus, distributed in Australia and New Guinea, lay relatively large eggs (up to 2 mm in diameter) in damp places. In most other onychophorans, ovoviviparity or true viviparity occurs, with egg development taking place entirely within the uterus in both cases. In ovoviviparity, embryonic development relies on the yolk's nutrient reserves (found in species of the genera Eoperipatus from Southeast Asia and Opisthopatus from South Africa). In viviparity (found in the majority of onychophoran species), development is sustained by the maternal Organism: the eggs firmly attach to the uterine walls, and the developing embryo is nourished by maternal secretions, much like through a Placenta.

Development in various species lasts from six to 13 months, after which young individuals are born, externally resembling adults but with an underdeveloped reproductive system. During embryonic development, paired coelomic sacs form in the embryo. Each of them is divided into three sections: dorsal, ventral, and lateral. Subsequently, the dorsal sections separate and arrange themselves into two metameric rows, with all sacs of each row fusing together to form two long tubular gonads. The ventral and partially lateral sections of the coelomic sacs transform into metameric coelomoducts, while the walls of most of the lateral sections break down, forming the mixed body cavity (mixocoel) in their place. Consequently, in adult onychophorans, the remnants of the coelom are represented solely by the small vesicles of the excretory organs and the gonads.

The formation of the reproductive system in onychophorans concludes only in their fourth year of life. Primitive tracheates live for an average of six to seven years, periodically molting throughout their lives while consuming their old cuticle.

The class Protracheata includes only a single order of the same name, which is divided into two families. The family Peripatopsidae includes green, blue-green, and white onychophorans (in soil-dwelling species). It is among these that egg-laying species or those exhibiting ovoviviparity are found. Peripatopsidae are distributed exclusively in the Southern Hemisphere.

In Representatives of the second family, Peripatidae, the general coloration tone is reddish-brown. All species of this family are viviparous, and most of them inhabit the equatorial belt.