Onychophora (Peripatus): Structure and Affinities

Onychophora, commonly known as Peripatus or velvet worms, possess segmented bodies, numerous legs, and retractable appendages. Their unique characteristics have sparked scientific interest, as they exhibit features that connect them to both arthropods (such as jointed legs and exoskeleton) and annelids (like the presence of circular muscles and nephridia). This intriguing mix of traits has led to ongoing debates among taxonomists regarding their precise affinities and evolutionary relationships.

Peripatus

Peripatus is a soft-bodied worm-like, bilaterally symmetrical animal. It shows the tracheal mode of respiration. It is grouped in Phylum Arthropoda and Class Onychophora. This class is the smallest and it includes only one genus and seventy species. Class Onychopora includes primitive worm-like, Arthropods. The study of this Class infers the Annelid ancestry of Arthropod, It is a connecting link between annelids and arthropods. Peripatus is seen in neotropical regions like West Indies, America, Congo, Australia, Tasmania, New Zealand, Malaya, etc. It exhibits "discontinuous distribution".

The distribution of these groups together with their general names is in the given table.

Peripatus is a terrestrial animal. It lives under stones; bark of trees and in shady places. It is nocturnal in habit.

During the day time it remains in the dark place and at night time it comes out in search of food. It feeds on small insects, worms, and termites.

External Characters

It is a soft-bodied, worm-like animal. It grows up to 1 to 2" in length. The soft body shows deep black color on the dorsal side and light red color on the ventral side.

The integument exhibits a large number of wrinkles or ring-like constructions that are superficial. The skin is soft and bears many minute papillae and bristles along the mid-dorsal line of the body. It produces slimy secretion which is protective and helps in capturing the food organisms.

The body is divisible into two parts, (a) Head, and (b) Long trunk.

1. Head: The head is composed of three segments. They are fused. It has a pair of antennae. Each antenna shows a large number of segments. On the ventral side of the head, the mouth is seen. The mouth is surrounded by a lip that has ridges. A pair of jaws with teeth surrounds the mouth. A tongue is also seen in the mouth. A pair of oral papillae will be present. Each oral papilla contains the openings of slime glands. On the dorsal side of the head, a pair of simple eyes will be seen.
2. Trunk: It contains 14-42 segments arranged serially. All the segments are alike. Each segment shows a pair of appendages. The anus is terminal and it is present at the posterior end of the body. On the ventral side below the anus, the genital pore is present.

Appendages: Each segment contains a pair of appendages. Each appendage shows 2 parts. (1) A conical proximal leg, and (2) A short distal foot with a pair of horny claws. The leg shows 2 spiniferous pads at its distal end. The leg bears rings of papillae with bristles. The foot is attached to the distal end of the leg. All the appendages are hollow.

Peripatus-Phylogenetic Relations

Peripatus belongs to Phylum Arthropoda. Class Onychophora. Some scientists do not agree to give the status of a class to Onychophora but prefer to consider it as an appendix to Phylum Arthropoda. Because these animals exhibit certain peculiar and unique features which are not shown by a majority of Arthropods. Peripatus exhibits close affinities with the Annelids on one hand and with Arthropods on the other.

Peripatus was first discovered by Guilding in 1826. By considering the external characteristics of Peripatus, he considered it ap a slug and kept it under Phylum Mollusca. Since the animal bears several "annelidan characters" many scientists grouped it under Phylum "Annelida". Only in the year 1874 did "Mosely" observe the tracheal mode of respiration and included it under the class Onychophora. The entire structure, physiology, and development of peripatus were worked out by "Adam Sedgwick".

Peripatus is a unique organism. It forms a connecting link between Annelids are Arthropods. It exhibits several important Annelida characters and some Arthropodan characters. In addition to these, peripatus exhibits some peculiar characteristics not shown by either Annelids or Arthropods.

Annelidan Characters

1. Presence of worm-like body with bilateral symmetry.
2. Presence of thin flexible cuticle.
3. Absence of distinct head region.
4. Presence of dermomuscular body wall with circular and longitudinal muscles.
5. Presence of a pair of simple eyes on the dorsal side of the head.
6. Presence of hollow stumpy appendages in the trunk segments.
7. Presence of muscular pharynx.
8. Presence of short stomodaeum and proctodaeum regions of the alimentary canal.
9. Presence of salivary glands which are modified Nephridia.
10. Presence of paired Nephridia in every segment.
11. Presence of segmentally-arranged coxal glands which are homologous with settiparous glands of chaetopods.
12. Presence of cilia in the reproductive ducts.

Arthropodan Characters

1. Presence of a chitinous cuticle.
2. Presence of jointed appendages arranged in one pair in each segment.
3. Presence of jaws in the mouth which are modified appendages.
4. Presence of hemocoel.
5. Presence of colorless blood.
6. Presence of long, tubular, and dorsal heart enclosed by the pericardium.
7. Presence of tracheal system with stigmata.
8. Presence of tubular gonads around the gut.

Primitive characters of Peripatus

1. Presence of superficial segmentation of the body.
2. Presence of one pair of jaws in the mouth cavity.
3. Presence of trachea which opens out with diffused stigmata.
4. Presence of a ladder-like nervous system.
5. Presence of velvety skin.
6. Genital organs are arranged in a manner that is not seen in any other animals.

In 1930 Onychophora fossils "Aysheaia" were discovered in Cambrian period stones. Hence we can conclude that Onychophora included many genera along with Peripatus. We can consider this as a very primitive group. Peripatus shows annelidan, arthropods, and special characters. Because it shows more Arthropoda characters it was kept in Arthropoda by some. Because it shows both Annelida and Arthropoda characters it was considered a connecting link between Annelida and Arthropoda. Onychophorans are not worms and also they are not arthropods. Hence they are treated as a separate phylum.