Study Notes on Peripatus:- 1. Geographical Distribution of Peripatus 2. Habit and Habitat of Peripatus 3. External Structures 4. Integumentary System 5. Muscular System 6. Locomotion 7. Body Cavity 8. Digestive System 9. Respiratory System 10. Vascular System 11. Excretory System 12. Nervous System 13. Reproductive System.
Contents:
- Geographical Distribution of Peripatus
- Habit and Habitat of Peripatus
- External Structures of Peripatus
- Integumentary System of Peripatus
- Muscular System of Peripatus
- Locomotion of Peripatus
- Body Cavity of Peripatus
- Digestive System of Peripatus
- Respiratory System of Peripatus
- Vascular System of Peripatus
- Excretory System of Peripatus
- Nervous System of Peripatus
- Reproductive System of Peripatus
1. Geographical Distribution of Peripatus:
About 200 species are included among 49 genera and 2 families—Family Peripatopsidae and Family Peripatidae. The members of the Peripatopsidae are distributed among the following countries—Chilie, South Africa, Australia, New Zealand, Tasmania, New Britain and New Guinea.
The species of Peripatidae is distributed to the equatorial West Africa, West Indies, Mexico to the Rio de Janeiro, N.E. India and southern part of South East Asia.
The fossils have been found in Baltic amber, indicating that they were widely distributed in the Northern Hemisphere in suitable condition. The present distribution of onychophorans indicates that the countries were the part of supercontinent Gondwana and the countries such as central and South America, Africa, S.E. Asia and Australasia have derived with the breakup of the Gondwana supercontinent.
The following genera are found in different parts of the world (Table 19.1):
2. Habit and Habitat of Peripatus:
These are terrestrial animals and are denizens of humid localities and crepuscular in habit. Its favourite residing places are within litter, underneath of logs or stones in forests or in areas near water. They are mainly predaceous and feeding on insects, other small arthropods and worms.
The prey is captured by spurting mucus from the slime glands which open to the apical end of the oral papillae. The slime glands are a pair of glands situated in the perivisceral haemocoel, one on either side of the body cavity. Each gland of peripatus consists of a tube from which side branches develop at the rear end and in the first half of the tube it is dilated and the part is called slime gland reservoir.
The tube and branches are lined with columnar epithelium which secretes adhesive substance and this substance is stored within slime gland reservoir. The spurting of mucus may go up to 15 cm. On the body of the victim the secretion forms a sticky web and kills the prey by entangling. There is at least one white cavernicolous species, e.g., Speleoperipatus.
3. External Structures of Peripatus:
The body of Peripatus is caterpillar-like (Fig. 19.1). It is soft, elongated and roughly cylindrical. The segmentation is indistinct and marked only by the presence of paired appendages. Numerous superficial lines or annuli mark the body but such annuli do not correspond to segmentation. The colouration usually varies from dark gray to brown. But red, blue and green colourations are also marked.
The entire body may be divided into:
(a) An indistinctly set of head and
(b) An elongated trunk.
(a) Head:
The head of Peripatus (Fig. 19.2A) consists of following structures:
1. Prostomium or acron:
It is the anterior- most part of the head. It bears a pair of eyes but paired appendages are absent.
2. Antennae:
One pair of antennae represents the first pair of appendages. The entire surface of each antenna is surrounded by numerous spiny rings. The slightly narrow anterior end of each antenna is enclosed by a sheath of spiny tissue.
3. Mandibles:
A pair of mandibles (jaws) constitutes the second paired appendages. These are present deep inside the mouth cavity. Each mandible is small, muscular, stumpy and provided at its free end with a pair of sharp cutting blades.
The base of the mandible is extended posteriorly up to the second leg-bearing segment. A chitinous rod, apodeme, is secreted from the base of the mandible and acts as site for the attachment of muscles. The salivary duct from salivary gland opens behind the mandible.
4. Oral papillae:
Third paired appendage is a pair of oral papillae. Each papilla is situated at the sides of the head. The terminal end of the papilla serves as the exit of a special kind of gland, called slime gland. Mouth is present immediately behind the oral papillae and on the ventral side.
(b) Trunk:
The trunk is devoid of exoskeletal coverings and its skin is drawn into a number of ridges along which wart-like papillae are placed. The trunk possesses 14-43 pairs of legs (lobopods). Legs are all alike and are placed at regular intervals. Each leg is hollow and cone-shaped but appears as if squared off at the tip (Fig. 19.2C).
The tip bears a white flat pad and two slender claws within a pocket. Entire surface of the leg consists of numerous papillae. The anus which serves as the outlet of alimentary canal is present at the posterior end and behind the last pair of legs. The reproductive opening or genital pore is placed just in front of the anus (Fig. 19.2B).
4. Integumentary System of Peripatus:
The epidermis is made up of a single layer of cells and situated immediately beneath the cuticle. The body of Peripatus is covered externally by a flexible chitinous cuticle which is nearly 1 mm in thickness. The hypodermis bears numerous short papillae, each containing a small fine seta. Various pigments which are responsible for its colouration are present in the hypodermis.
5. Muscular System of Peripatus:
Beneath the hypodermis, following annelid-like smooth muscles are arranged from outside to inside:
(i) Circular,
(ii) Diagonal and
(iii) Longitudinal.
The dorsoventrally arranged muscles divide the body cavity into two lateral and one median cavity.
All the muscles are operated by the turger pressure of the body fluid. In addition to the muscles of the body wall, there are muscles which remain connected with the mouth, pharynx, jaws and alimentary canal. It may be mentioned that the jaw muscles are striated.
6. Locomotion of Peripatus:
Peripatus moves by walking on all its legs excepting the first three anterior pairs. While walking, the legs raise the body above the ground. On slippery surface, the claws are extended. Each leg, while on ground, pushes backward thus its proximal attachment with the body moves forward. The leg is then raised and carried forward.
The leg touches the ground only after the completion of forward movement. It has been calculated that a group of peripatus, Peripatopsis, measuring 5-6 cm in length, can attain a speed of 50-60 cm per minute. Peripatus can easily crawl through crevices or holes which arc much smaller than the diameter of its body.
7. Body Cavity of Peripatus:
The major part of the body cavity is haemocoelic and is known as mixocoel. The mixocoel extends as four compartments— one central, two laterals and one pericardial. The lateral compartments extend within the legs. True coelom is restricted to the gonadal cavities and excretory organs.
8. Digestive System of Peripatus:
The digestive system of peripatus consists of alimentary canal and digestive glands (Fig. 19.2A).
(i) Alimentary canal:
It begins with mouth which is present immediately behind the oral papillae. The mouth is enclosed by the oral or peribuccal lobes which form a pre-oral cavity. The mandibles or jaws are present within the cavity and remain posteriorly directed. Each mandible is provided with a small base and a pair of sickle-shaped, tooth-like claws.
The jaws work independent of each other and operate from anterior to posterior end. The mouth leads into a pharynx. The cavity of pharynx is lined by chitin and in cross section it appears X-shaped. The wall of the pharynx is raised to increase the cavity. Such movement of pharynx produces a sucking action.
The pharynx opens to the chitinous oesophagus having weak muscular wall. The oesophagus leads to a straight midgut which is lined by endoderm. Like insects and other arthropods, a thin chitinous membrane is called peritrophic membrane which is formed by the secretion of midgut epithelium and helps to confine the food mass in the midgut.
The caeca are absent. The midgut enters into the hindgut which in turn communicates to the exterior by an aperture, anus. The hindgut is lined by chitin.
(ii) Digestive glands:
Most important digestive gland is a pair of salivary glands. Each salivary gland is slender, tubular and elongated. The salivary glands extend from third to tenth and sometimes up to thirty first segments. The two salivary ducts fuse and open within the pre-oral lobe. In some forms, a reservoir is present. The lining of midgut is also responsible for producing digestive juices.
Mechanism of nutrition:
Food:
Peripatus is insectivorous and its food includes insects like grasshopper.
Feeding:
A prey, when detected is captured by the spurting of slime from slime gland. It is then held firmly by the lips and sucking action of the pharynx.
Digestion:
The jaws work to slit open the prey and saliva, containing various enzymes, is poured over the victim through the slit. The saliva digests the soft part of the prey, and these nutritive fluids are drawn in. The peristaltic action of the midgut wall forces the material to pass posteriorly through its lumen. The cells of the midgut secrete a thin membrane (Peritrophic membrane) around the food particles.
Various juices from the lining of the midgut penetrate this membranous envelope and digests the food further. The digested materials come out of the membrane and are absorbed by the absorptive cells in the midgut lining. The residual matters which remain within membrane gradually pass to the hindgut and are finally ejected through the anus.
9. Respiratory System of Peripatus:
Respiratory organs are network of tracheae. The tracheal tubes are usually delicate, distally un-branched or rarely branched and possess faint, transverse striations. The tracheal openings or stigmata are scattered over the surface of the body and are placed in the depressions between the papillae or ridges of the skin.
Each metamere contains nearly 75 stigmata or spiracles. Each stigma opens into a short, narrow atrium which contains a bundle of minute tracheae (Fig. 19.4). The tracheal system of peripatus is markedly different from that of cockroach, in that the spiracles in this case are devoid of any closing mechanism. The condition, however, adversely affects the animal by loss of water in great quantity.
10. Vascular System of Peripatus:
Heart is a median tube placed on the dorsal side of the gut. It extends from the metamere bearing first pair of legs to the metamere immediately before last. It is enclosed by a pericardial sinus with which it communicates through paired ostia.
The ostia are segmentally arranged and their number depends upon the number of segments. No other definite blood vessel of found. The pericardial sinus is separated from the general body cavity by a longitudinal partition.
11. Excretory System of Peripatus:
Excretion is carried by:
(1) Nephridia,
(2) Midgut lining and
(3) Pericardial cells.
The nephridia (Fig. 19.5B) are paired structures, segmentally arranged in the lateral chamber of the body cavity and correspond to the pairs of legs. A typical nephridium consists of a long ciliated funnel, a coiled duct and a contractile bladder, called vesicle. The first three pairs of nephridia are poorly developed and each bears a vesicle and a duct.
The coiled part consists of the following portions—a tightly packed part that ends through a spacious opening and the elongated part that leads to the vesicle. The vesicle communicates to the exterior on the ventral surface of the legs by a narrow passage. It has been observed in Peripatopsis that nephridia release a drop of urine after 3-4 days.
The urine contains various nitrogenous compounds but not urea. Nephridia of Peripatus are, however, analogous to the nephridia of annelids but homologous to such excretory organs of arthropods. In both, these are in the form of modified coelomoducts. For this reason, recent workers prefer to call them coelomoducts and not nephridia.
Certain cells in the midgut lining collect uric acid crystals as excretory products. Pericardial cells act as nephrocytes to remove excretory products from blood. Following glands of peculiar nature are present in the body of peripatus. These act as excretory organs.
Coxal or Crural glands:
Many paired and serially arranged glands are present on the compartments belonging to the sides of the body cavity. Their ducts open to the under-surface of the legs just outside the excretory pores. The glands are usually present only in males.
Anal glands:
A pair of glands having uncertain functions opens close to the anus. These are probably modified excretory organs.
12. Nervous System of Peripatus:
Nervous system (Fig. 19.5B) of Peripatus consists of brain and a pair of longitudinal nerve cords, peripheral nerves and sense organs. Brain is present in the head region and is constituted by a pair of compactly united supraoesophageal ganglia. A pair of circumoesophageal connectives unites the brain with the ventral nerve cord.
The entire area of brain may be divided into three parts:
(a) Protocerebrum:
It is the portion which lies within the prostomium and it contains a central body and a pair of globuli. It sends optic nerves.
(b) Deutocerebrum:
It consists of a prominent antennal glommerulus and sends antennary nerves.
(c) Tritocerebrum:
It is the smallest part of the brain which sends nerves to the mouth and the anterior region of the gut. The two longitudinal ventral nerve cords extend up to the posterior end along the two sides of the alimentary canal and unite near the anus.
Along its entire length the two cords are connected by several transverse connectives and are separated from each other by dorsoventrally muscle strands. One pair of simple and small eyes is present at the base of the antenna. Each eye is 0.2-0.3 mm in diameter, cup-like in appearance and has an outer cuticular cornea and thick lens (Fig. 19.6).
In the inner layer the rod cells of retina are projected towards the lens at one end. The other end is connected with the branches of the optic nerve. Eyes can only feel the difference between light and shade and the animal moves away from light. The sensory cells which are present on the antenna are responsible to feel the way during locomotion.
Numerous sensory papillae are distributed all over the body. Each papilla consists of a seta and group of sensory cells. These are responsible for determining air currents and also act as tactile receptors Specialised sensory cells are also present in the lining of buccal cavity to determine the taste of the food.
13. Reproductive System of Peripatus:
Sexes are separate. Males are usually smaller than females. In males, the reproductive organs are one pair of tubular structures, called testes. From each testis a slender vas deferens opens through a funnel-like aperture into the vesicula seminalis. An elongated slender and coiled vas deferens arises from the vesicula seminalis and unites with its fellow from the other side to form a central tube, called ejaculatory duct.
The proximal part of the ejaculatory duct secretes a substance which packs the sperm cells to form spermatophores. The ejaculatory duct opens ventrally and between the last pairs of legs. In females, the reproductive organs, called ovaries, are also a pair of tubular organs. Two ovaries are fused at the anterior and posterior ends.
From each ovary arises an oviduct which runs anteriorly and then bends to form a curved uterus. The two uteri unite to form a median vagina, which opens to the exterior through an opening which occupies the same position as in males. In some forms, a process, called ovipositor, is present near the genital aperture.
