Why is Peripatus said to be a connecting link between Annelida and Arthropoda?

Peripatus (a velvet worm) shows features of both segmented worms (Annelida) and joint-legged animals (Arthropoda), so biologists call it a “connecting link” between these two groups.

Meet Peripatus

Peripatus belongs to the phylum Onychophora and looks like a soft, velvety caterpillar. It lives in moist places, hides under logs or leaves, and moves with many short, fleshy legs called lobopods. Because it shares traits with both annelids and arthropods, it helps scientists understand how complex arthropods might have evolved from simpler worm-like ancestors.

Features similar to Annelida (segmented worms)

• Body form and movement: Soft, elongated body with a worm-like appearance; locomotion is by peristaltic-like waves along the body, resembling annelid movement.
• Excretory organs: Segmentally arranged nephridia (excretory units) are present, a hallmark of annelids.
• Body segmentation (internal hints): Though not sharply marked externally, internal organization shows repeated units reminiscent of segmentation.

Features similar to Arthropoda (insects, spiders, crustaceans)

• Cuticle and molting: A thin chitinous cuticle is present, and the animal molts (ecdysis) as it grows—typical arthropod behavior.
• Respiration: A system of tracheae opens to the outside through spiracles, much like insects.
• Circulation: An open circulatory system with hemocoel (blood spaces), again like arthropods.
• Nervous system: A pair of ventral nerve cords with segmental ganglia shows arthropod-style organization.
• Head appendages: Antennae and jaws (modified appendages) resemble primitive arthropod features; slime glands with oral papillae are unique but echo the idea of specialized mouthparts.

Special adaptations

• Lobopod legs: The legs are soft and unjointed (unlike true arthropod joints), ending in claws. This mixes annelid-like softness with arthropod-like appendages.
• Slime defense: Peripatus ejects sticky slime from oral papillae to trap prey—an unusual but effective hunting and defense tool.

Why we call it a “connecting link”

Scientists use “connecting link” for living organisms that bridge two big groups by sharing traits with both. Peripatus does that clearly: annelid-style excretion and body plan hints on one side; arthropod-style cuticle, tracheae, open blood system, and nervous organization on the other. This mix supports the idea that arthropods may have evolved from worm-like ancestors with soft legs (lobopods), and that Onychophorans sit near that evolutionary pathway.