INVERTEBRATE ZOOLOGY - G. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

SECTION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBSECTION MOLTING ANIMALS (ECDYSOZOA)

PHYLUM ONYCHOPHORA

CLASS PROTRACTEATA

Onychophorans find optimal living conditions (constant Temperature In addition to high humidity) in the litter of tropical and subtropical forests, under stones and logs, and in various shelters near Water. They are extremely sensitive to desiccation, which is why in drier habitats they burrow deeper into the soil.

Their body length ranges from 2 to 15 cm. Most species are dark brown or brown, though some are green, bluish-green, orange, or light blue, either uniformly colored or adorned with stripes or spots.

Morphology. The body of onychophorans is worm-like, elongated, cylindrical, slightly flattened ventrally, and lacks a distinct boundary between the HEAD and trunk. Externally, the body exhibits annulation that does not correspond to true segmentation (Fig. 398).

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Fig. 398. Onychophorans (from Krombein):

a - general view of Paraperipatus amboinensis; b - anterior part of the body of Peripatoides novae-zealandiae:

1 - leg; 2 - oral papillae; 3 - jaws; 4 - lateral papillae; 5 - antenna

The head bears three pairs of unsegmented appendages. Two long antennae project anteriorly. Near their base on the dorsal side, most onychophorans possess a pair of eyes. On the ventral side, in front of the Mouth opening, lie two muscular outgrowths, each bearing a robust sclerotized plate with serrated edges—the jaw. Flanking the mouth are short papillar outgrowths with the openings of slime glands at their tips. When disturbed, they eject a stream of sticky slime. This mechanism allows the extremely slow-moving onychophorans to capture prey—such as small insects, spiders, etc.—by gluing and immobilizing them. At the same time, this serves as a defense mechanism.

The trunk bears 13 to 43 pairs of legs, with the posterior pair sometimes reduced. Each leg is a simple muscular body outgrowth equipped with two claws (hence the phylum name, from Greek *onyx* - claw, and *phora* - bearing).

Skin-muscular sac. 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 integument does not protect onychophorans from desiccation, yet it prevents wetting due to the presence of tiny outgrowths—micropapillae, whose tips bear a well-developed, durable cuticle. Muscles are situated beneath the integument: a layer of circular muscles, two layers of intersecting oblique muscles, and two longitudinal Muscle layers, which collectively form a true continuous skin-muscular sac. In addition, well-developed dorsoventral muscles are present (Fig. 399).

The cuticle of onychophorans does not function as an exoskeleton; it is soft and elastic. Instead, the supportive function is performed by the integument combined with the fibrous connective tissue. By altering the thickness and length of specific body regions through muscular activity, onychophorans can squeeze into narrow crevices or burrow through soil in a manner similar to earthworms.

Body cavity. The longitudinal muscle layer bounds the body cavity, which is of mixed origin (mixocoel) and lacks a proper epithelial lining. It is filled with hemolymph.

The Digestive System has a relatively simple Structure. The mouth opening is located on the ventral side of the head at the bottom of the preoral cavity, which can open and close thanks to a ring of cutaneous outgrowths (papillae) that form its outer convex wall. At the bottom of the preoral cavity, as mentioned earlier, a pair of sickle-shaped jaws is situated on small muscular cushions; these jaws are moved forward and backward by specialized muscles to tear apart prey. The single duct of two large Salivary Glands also opens here, and its secretion contacts the prey, initiating Digestion right away. The ectodermal foregut consists of a muscular Pharynx and Esophagus, followed by the endodermal midgut. The latter extends as

an undifferentiated tube through the entire body, transitioning into a short ectodermal hindgut that opens via the anus at the posterior end of the body. Onychophorans are voracious predators, and consuming succulent insects and spiders compensates for their water loss.

The excretory system is represented by metameric coelomoducts.

Each coelomoduct consists of a coelomic vesicle into which a ciliated funnel opens, continuing into a convoluted tubule. The terminal part of the tubule features an expansion—the Urinary Bladder—which opens via an excretory pore near the Base of the appendage. Part of the tubule is lined with ciliated epithelium. Excretory Organs are present in all trunk segments except the last two, where the genital openings are located.

The Circulatory system is open and highly simplified, reduced to a single long tubular vessel—The Heart—situated above the gut. The posterior end of the heart is blindly closed while the anterior end is open; paired metameric openings, or ostia, are located laterally, through which hemolymph is drawn into the heart.

The ostia are equipped with Valves that ensure the hemolymph moves forward during heart pulsations without leaking through the lateral openings, instead pouring out into the mixocoel through the anterior end. The heart lies within the Pericardium—a portion of the mixocoel separated from the general body cavity by a thin, incomplete partition, the Diaphragm.

Fig. 399. Internal anatomy of a female Peripatoides novae-zealandiae (adapted from Dogel, with modifications):

a - dissection of the body; b - cross-section through a stigma and tracheal bundle: 1 - antenna; 2 - lateral papillae; 3 - supraesophageal ganglion; 4 - pharynx;

5 - esophagus; 6 - midgut; 7 - ventral nerve cord; 8 - salivary gland duct; 9 - slime gland;

10 - Uterus with embryos; 11 - body leg; 12 - Ovary; 13 - anus; 14 - oviduct;

15 - nerve commissures; 16 - slime gland duct; 17 — eyes

Onychophorans respire across their entire body surface and via tracheae, which open through numerous randomly distributed spiracles (stigmas) on the body surface. Each spiracle leads to a bundle of unbranched, blind-ending tracheal tubules bathed in hemolymph (Fig. 398, c).

The Nervous system includes a relatively large paired supraesophageal ganglion connected by two circumesophageal connectives to a pair of widely spaced ventral nerve cords that run along the length of the body and join above the anus. Numerous commissures connect the two cords. The cords are uniformly covered with Nerve Cells that do not form discrete ganglionic clusters. The supraesophageal ganglion consists of three regions — the proto-, deuto-, and tritocerebrum, which innervate the eyes, antennae, and foregut, respectively.

Sense Organs are represented by antennae, which function as organs of Touch and chemoreception, numerous cutaneous tactile papillae, and eyes. The cup-shaped simple eyes lie directly above the Brain and structurally resemble the eyes of polychaetes. Vision helps the animals avoid light, whereas in the dark they orient themselves by palpating objects with their antennae.

Reproductive System. Onychophorans are dioecious, with males being overwhelmingly noticeably smaller than females. The Gonads are paired and located in the posterior half of the body.

Reproduction. Fertilization in onychophorans is spermatophore-based, though the process varies. In African species of the genus Peripatopsis, the male attaches several spermatophores to the female's body surface; spermatozoa emerge from these and penetrate the body through cracks in the integument. Fertilization presumably occurs in the ovary, as already fertilized eggs enter the oviducts. In other species, spermatophores or even droplets of seminal fluid are deposited on the substrate, and the females pick them up with the margins of their genital opening.

Only onychophorans of the genera Symperipatus and Ooperipatus, distributed in Australia and New Guinea, lay relatively large eggs (up to 2 mm in diameter) in damp places, whereas most species exhibit ovoviviparity or true viviparity, with eggs developing within the uterus in both cases. In ovoviviparity, embryonic development relies on the yolk reserves of the egg (found in species of the genera Eoperipatus from Southeast Asia and Opisthopatus from South Africa). In viviparity (in the majority of onychophoran species), development is fueled by the maternal organis

m: the eggs firmly attach to the uterine walls, and the developing embryo is nourished by uterine secretions, functioning much like a Placenta.

Embryonic development in various species lasts from 6 to 13 months, after which young individuals are born that externally resemble adults but have an undeveloped reproductive system. Its maturation is completed only in the fourth year of life. Onychophorans live for an average of six to seven years, undergoing periodic molting throughout their lives, during which the old cuticle is consumed.