Phylum Onychophora

(Gr., onyx = claw + pherein = to bear)

Peripatus

• Peripatus is a connecting link between Annelida and Arthropoda.
• Peripatus and related animals belong to the phylum Onychophora, which was conventionally classified with arthropoda as one of its classes. But the modern classification has given to it the status of an independent phylum.

Geographical distribution

Peripatus exhibits discontinuous distribution with its species scattered in most of the warmer parts of the world: Africa, Australia, New Zealand, Central America, Mexico, West Indies, India and other localities.

Habit and Habitat

• Peripatus is a terrestrial animal, living in moist places, in crevices of rocks, under stones, logs and barks, and other dark and damp places where it is protected both from loss of water and from the predatory arthropods.
• It is nocturnal in nature and predaceous and carnivorous in feeding habit.

External morphology

• Body of Peripatus is elongated, cylindrical caterpillar-like and bilaterally symmetrical measuring between 5-10 cm in length.
• The external segmentation is unclear and the legs, which number from 14 to 43 pairs, are unjointed.
• Each leg is a stumpy protuberance ending in two claws and provided with 3 to 6 pads on ventral side which serve as walking soles which contact the substratum. That is why they are also called walking worms.
• Outer covering or skin velvety-like which is thrown into transverse ridges bearing numerous conical papillae or tubercles armed with chitinous spines.
• Head is not distinct. The anterior end bears a pair of simple dorsal eyes, a pair of pre antennae or preoral antennae, a pair of oral papillae, and a mid-ventral mouth with a pair of chitinous jaws.
• Both the jaws and the oral papillae are modified legs.
• The unjointed legs are short stumpy and each ends in two claws.
• The anus lies at the posterior end, slightly towards the ventral side, and the genital aperture is located in front of the anus.
• Peripatus shows sexual dimorphism; males are smaller than females.

Affinities of Peripatus

• Peripatus has no economic importance; but it is zoologically very important because it exhibits both arthropod and annelid characteristics as well as peculiarities of its own.

Annelidan characteristics

• Absence of a true head.
• Locomotion slow and by peristalsis as in an earthworm.
• Unjointed, hollow, stimpy appendages of the nature of extensions of the body-wall, like parapodia of Polychaeta.
• Simple, straight alimentary canal with terminal mouth and anus.
• Segmentally arranged paired nephridia.
• Slime and coxal glands correspond with similar glands of Chaetopoda.
• Presence of cilia in the excretory and reproductive ducts.

Arthropodan characteristics

• Presence of antennae.
• Jaws are modified appendages provided with striped muscles.
• Locomotion by definite legs, having definite musculature and provided with claws.
• Cuticle has a thin deposition of chitin, like that of Arthropods.
• Body-cavity is haemocoel.
• Coelom reduced to small cavities that surround the excretory and reproductive ducts.
• Brain is large and typically arthropodian.
• General structure of reproductive organs and development mainly arthropodan.