The word Onychophora is derived from ancient Greek words “onyches” meaning ‘claws’ and “pherein” meaning ‘to carry’. Onychophora is, therefore, a phylum of elongate, soft-bodied, claw-bearing, vermiform animals. The external morphology of these animals resembles the worms with legs, caterpillars, and slugs. Due to their velvety dermal texture and vermiform appearance, these animals are commonly called as “Velvet Worms” . These are also often referred to as a ‘connecting link’ between annelids and arthropods due to close resemblance in certain characteristic features of both these phyla.
These animals prey upon other invertebrates by catching them with the ejection of an adhesive slime. Approximately 200 species of velvet worms have been described till date, but the actual number of species is thought to be much greater. The two extant families of velvet worms are Peripatidae and Peripatopsidae. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are found towards the south of the equator. It is the only phylum in Kingdom Animalia that is wholly endemic to terrestrial environments, at least among extant members. Velvet worms are generally considered to be the close relatives of arthropods and tardigrades, with which they form the proposed taxon Panarthropoda. This makes them of great palaeontological interest, as they can help reconstruct the ancestral arthropod and its lineage. In modern zoology, they are particularly renowned for their curious mating behaviours and the bearing of live young in some species.
Representative organism:
This phylum is represented by Peripatus.
Affinities with Annelida
- Vermiform body with truncated extremities.
- Absence of true head.
- Dermo-muscular body wall, with a thin flexible cuticle and underlying circular and longitudinal muscles.
- Locomotion is slow and by peristalsis like in an earthworm.
- Structure of simple eyes like polychaetes.
- Presence of cilia in reproductive and excretory ducts.
- Slime and coxal glands are present in both.
- Simple, straight alimentary canal with terminal mouth and anus.
- Segmentally arranged paired nephridia.
Affinities with Arthropoda
- Presence of antennae.
- Jaws are modified appendages.
- Locomotion by definite legs, having definite musculature and claws.
- Cuticle has thin deposits of chitin.
- Body cavity is a Haemocoel.
- Coelom reduced to small cavities that surround the excretory and reproductive ducts.
- Similar salivary glands.
- Tracheal respiratory system.
- Brain is large like arthropods.
- Structure of reproductive organs and developmental stages like insects.
Status of Onychophora:
Onychophora was conventionally classified as a class in phylum Arthopoda. However, the modern system of classification gave it the status of an independent phylum since it has characteristic differentiating features of its own. As there are only about 200 species in this phylum, therefore, it is considered as a minor phylum. Therefore, the current status of Onychophora is that of an independent minor phylum.
For more details, refer to Peripatus.