Affinities of Paripatus
"Paripatus" likely refers to the velvet worms, which are members of the phylum Onychophora. The common name "velvet worms" comes from the fine and velvety appearance of their skin. Onychophorans are fascinating organisms that share characteristics with both arthropods (such as insects and arachnids) and annelids (such as earthworms and leeches). Let's explore the affinities of Paripatus (velvet worms) in more detail:
1. Morphological Characteristics:
- Segmentation: Velvet worms exhibit a segmented body, a feature they share with annelids. However, unlike annelids, the segments do not have internal septa.
- Soft Exoskeleton: The exoskeleton of velvet worms is soft and flexible, more like the skin of annelids than the rigid exoskeleton of arthropods.
2. Appendages:
- Unjointed Legs: Velvet worms have multiple pairs of unjointed, fleshy legs (onychophoran legs) that are more similar to the lobopods of some extinct arthropods than the jointed legs of typical arthropods.
- Onychophoran Limbs: The limbs bear tiny claws, known as onyches, which are characteristic of onychophorans.
3. Sensory Organs:
- Antennae: Velvet worms possess antennae, a feature they share with some arthropods.
4. Reproductive Features:
- Live Birth: Velvet worms give birth to live young, a characteristic they share with certain arthropods like some insects.
5. Circulatory System:
- Tubular Heart: The circulatory system of velvet worms is more similar to that of annelids, featuring a tubular heart, unlike the open circulatory system found in many arthropods.
6. Nervous System:
- Ventral Nerve Cord: Like arthropods, velvet worms have a ventral nerve cord. However, the arrangement and structure may vary.
7. Ecological Affinities:
- Terrestrial Lifestyle: Velvet worms are primarily terrestrial, a trait they share with many arthropods. However, their lifestyle and behavior are distinct from most arthropods.
8. Evolutionary Affinities:
- Basal Arthropods: Velvet worms are often considered to be a sister group to arthropods, suggesting that they share a common ancestor with the arthropod lineage.
While velvet worms share certain characteristics with both annelids and arthropods, they also exhibit unique features that set them apart as a distinct group. Their position in the evolutionary tree has been a subject of scientific debate, and they are often considered a key group for understanding the early evolution of panarthropods, which include onychophorans, arthropods, and tardigrades.