Phylum Onychophora
Phylum
Onychophora
Members of phylum Onychophora (on-y-kof´o-ra) (Gr. onyx, claw, + pherein, to bear) are commonly called “velvet worms,” or “walking worms.” They compose approximately 70 species of caterpillar-like animals, ranging from 1.4 to 15 cm in length. They live in rain forests and other moist, leafy habitats in tropical and subtropical regions and in some temperate regions of the Southern Hemisphere.
The fossil record of onychophorans shows that they have changed little in their 500-million-year history. A fossil form, Aysheaia, discovered in the Burgess shale deposit of British Columbia and dating back to mid-Cambrian times, is very much like modern onychophorans. Onychophorans have been of unusual interest to zoologists because they share so many characteristics with both annelids and arthropods. They have been called, a bit too hopefully perhaps, a “missing link” between the two phyla. Onychophorans were probably far more common at one time than they are now. Today they are terrestrial and extremely retiring, becoming active only at night or when the air is nearly saturated with moisture.
Form and Function
External Features
Onychophorans are more or less cylindrical and show no external segmentation except for the paired appendages (Figure 21-11). The skin is soft, velvety, and covered with a thin, flexible cuticle that contains protein and chitin. In structure and chemical composition it resembles arthropod cuticle; however, it never hardens like arthropod cuticle, and it is molted in patches rather than all at one time. The body is studded with tiny tubercles, some of which bear sensory bristles. The color may be green, blue, orange, dark gray, or black, and minute scales on the tubercles give the body an iridescent and velvety appearance. The head bears a pair of large antennae, each with an annelid-like eye at the base (Figure 21-11). The ventral mouth has a pair of clawlike mandibles and is flanked by a pair of oral papillae which can expel a defensive secretion.
The unjointed legs are short, stubby, and clawed. Onychophorans crawl by passing waves of contraction from anterior to posterior. When a segment extends, the legs lift up and move forward. The legs are more ventrally located than are parapodia of annelids.
Internal Features
The body wall is muscular like that of annelids. The body cavity is a hemocoel, imperfectly divided into compartments, or sinuses, much like those of arthropods. Slime glands on each side of the body cavity open on the oral papillae. When disturbed by a predator, the animal can eject from the slime glands two streams of a sticky substance that rapidly hardens.
The mouth, surrounded by lobes of skin, contains a dorsal tooth and a pair of lateral mandibles for grasping and cutting prey. There is a muscular pharynx and a straight digestive tract (Figure 21-12). Most velvet worms are predaceous, feeding on caterpillars, insects, snails, and worms. Some onychophorans live in termite nests and feed on termites.
Each segment contains a pair of nephridia, each nephridium with a vesicle, ciliated funnel and duct, and nephridiopore opening at the base of a leg. Absorptive cells in the midgut excrete crystalline uric acid, and certain pericardial cells function as nephrocytes, storing excretory products taken from the blood.
For respiration there is a tracheal system that ramifies to all parts of the body and communicates with the outside by many openings, or spiracles, scattered all over the body. Onychophorans cannot close their spiracles to prevent water loss, so although the tracheae are efficient, the animals are restricted to moist habitats. The tracheal system is somewhat different from that of arthropods and probably has originated independently.
The open circulatory system has, in the pericardial sinus, a dorsal, tubular heart with a pair of ostia in each segment.
There are a pair of cerebral ganglia with connectives and a pair of widely separated nerve cords with connecting commissures. The brain sends nerves to the antennae and head region, and the nerve cords send nerves to the legs and body wall. Sense organs include pigment cup ocelli, taste spines around the mouth, tactile papillae on the integument, and hygroscopic receptors that orient the animal toward water vapor.
Onychophorans are dioecious, with paired reproductive organs. Males usually deposit their sperm in spermatophores in the female seminal receptacle. A male deposits the spermatophores on a female’s back, which may accumulate a number of them. White blood cells dissolve the skin beneath the spermatophores. Sperm can then enter the body cavity and migrate in the blood to the ovaries to fertilize the eggs. Onychophorans may be oviparous, ovoviviparous, or viviparous. Only two Australian genera are oviparous, laying shell-covered eggs in moist places. In all other onychophorans eggs develop in the uterus, and living young are produced. In some species there is a placental attachment between mother and young (viviparous); in others young develop in the uterus without attachment (ovoviviparous).
Members of phylum Onychophora (on-y-kof´o-ra) (Gr. onyx, claw, + pherein, to bear) are commonly called “velvet worms,” or “walking worms.” They compose approximately 70 species of caterpillar-like animals, ranging from 1.4 to 15 cm in length. They live in rain forests and other moist, leafy habitats in tropical and subtropical regions and in some temperate regions of the Southern Hemisphere.
The fossil record of onychophorans shows that they have changed little in their 500-million-year history. A fossil form, Aysheaia, discovered in the Burgess shale deposit of British Columbia and dating back to mid-Cambrian times, is very much like modern onychophorans. Onychophorans have been of unusual interest to zoologists because they share so many characteristics with both annelids and arthropods. They have been called, a bit too hopefully perhaps, a “missing link” between the two phyla. Onychophorans were probably far more common at one time than they are now. Today they are terrestrial and extremely retiring, becoming active only at night or when the air is nearly saturated with moisture.
Form and Function
External Features
Onychophorans are more or less cylindrical and show no external segmentation except for the paired appendages (Figure 21-11). The skin is soft, velvety, and covered with a thin, flexible cuticle that contains protein and chitin. In structure and chemical composition it resembles arthropod cuticle; however, it never hardens like arthropod cuticle, and it is molted in patches rather than all at one time. The body is studded with tiny tubercles, some of which bear sensory bristles. The color may be green, blue, orange, dark gray, or black, and minute scales on the tubercles give the body an iridescent and velvety appearance. The head bears a pair of large antennae, each with an annelid-like eye at the base (Figure 21-11). The ventral mouth has a pair of clawlike mandibles and is flanked by a pair of oral papillae which can expel a defensive secretion.
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| Figure 21-11 Peripatus, a caterpillar-like onychophoran that has characteristics in common with both annelids and arthropods. A, Ventral view of head. B, In natural habitat. |
The unjointed legs are short, stubby, and clawed. Onychophorans crawl by passing waves of contraction from anterior to posterior. When a segment extends, the legs lift up and move forward. The legs are more ventrally located than are parapodia of annelids.
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| Figure 21-12 Internal anatomy of an onychophoran. |
Internal Features
The body wall is muscular like that of annelids. The body cavity is a hemocoel, imperfectly divided into compartments, or sinuses, much like those of arthropods. Slime glands on each side of the body cavity open on the oral papillae. When disturbed by a predator, the animal can eject from the slime glands two streams of a sticky substance that rapidly hardens.
The mouth, surrounded by lobes of skin, contains a dorsal tooth and a pair of lateral mandibles for grasping and cutting prey. There is a muscular pharynx and a straight digestive tract (Figure 21-12). Most velvet worms are predaceous, feeding on caterpillars, insects, snails, and worms. Some onychophorans live in termite nests and feed on termites.
Each segment contains a pair of nephridia, each nephridium with a vesicle, ciliated funnel and duct, and nephridiopore opening at the base of a leg. Absorptive cells in the midgut excrete crystalline uric acid, and certain pericardial cells function as nephrocytes, storing excretory products taken from the blood.
For respiration there is a tracheal system that ramifies to all parts of the body and communicates with the outside by many openings, or spiracles, scattered all over the body. Onychophorans cannot close their spiracles to prevent water loss, so although the tracheae are efficient, the animals are restricted to moist habitats. The tracheal system is somewhat different from that of arthropods and probably has originated independently.
The open circulatory system has, in the pericardial sinus, a dorsal, tubular heart with a pair of ostia in each segment.
There are a pair of cerebral ganglia with connectives and a pair of widely separated nerve cords with connecting commissures. The brain sends nerves to the antennae and head region, and the nerve cords send nerves to the legs and body wall. Sense organs include pigment cup ocelli, taste spines around the mouth, tactile papillae on the integument, and hygroscopic receptors that orient the animal toward water vapor.
Onychophorans are dioecious, with paired reproductive organs. Males usually deposit their sperm in spermatophores in the female seminal receptacle. A male deposits the spermatophores on a female’s back, which may accumulate a number of them. White blood cells dissolve the skin beneath the spermatophores. Sperm can then enter the body cavity and migrate in the blood to the ovaries to fertilize the eggs. Onychophorans may be oviparous, ovoviviparous, or viviparous. Only two Australian genera are oviparous, laying shell-covered eggs in moist places. In all other onychophorans eggs develop in the uterus, and living young are produced. In some species there is a placental attachment between mother and young (viviparous); in others young develop in the uterus without attachment (ovoviviparous).
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