In this article we will discuss about Peripatus:- 1. Habit and Habitat of Peripatus 2. External Morphology of Peripatus 3. Body Wall 4. Locomotion 5. Body Cavity 6. Slime Glands 7. Digestive System 8. Circulatory System 9. Respiratory System 10. Excretory System 11. Nervous System 12. Sense Organs 13. Reproductive System 14. Development 15. Geographical Distribution 16. Affinities 17. Taxonomic Position.
Contents:
- Habit and Habitat of Peripatus
- External Morphology of Peripatus
- Body Wall of Peripatus
- Locomotion of Peripatus
- Body Cavity of Peripatus
- Slime Glands of Peripatus
- Digestive System of Peripatus
- Circulatory System of Peripatus
- Respiratory System of Peripatus
- Excretory System of Peripatus
- Nervous System of Peripatus
- Sense Organs of Peripatus
- Reproductive System of Peripatus
- Development of Peripatus
- Geographical Distribution of Peripatus
- Affinities of Peripatus
- Taxonomic Position of Peripatus
1. Habit and Habitat of Peripatus:
Peripatus is a terrestrial animal found living in moist places, in crevices of rocks, under bark, stones, logs and beneath fallen leaves and other dark and damp places or along stream banks. It is generally confined to humid habitats.
It is nocturnal in nature and predaceous and carnivorous in feeding habit. Most species of Peripatus are predaceous and feed on small invertebrates, such as snails, insects and worms. A number of species display a particular preference for termites. Most species of Peripatus are viviparous and a single large female may produce 30 or 40 young ones in a year.
2. External Morphology of Peripatus:
The body of Peripatus is caterpillar-like. It is soft, elongated, bilaterally symmetrical and more or less cylindrical and ranges from 1.4 cm to 15.00 cm in length. The external segmentation is indistinct and marked only by the presence of paired appendages. Numerous annuli or superficial lines mark the body, but such annuli do not correspond to the segmentation.
The skin covered by the thin cuticle is soft and has a velvety texture and thrown into a number of fine transverse ridges bearing numerous conical papillae or tubercles armed with little chitinous spines. The colouration varies considerably in different species from dark grey to brown.
The ventral surface is nearly always flesh-coloured, while the dorsal surface has a darker colour. But blue, green, red-orange or black colourations are also marked. The entire body of Peripatus may be divided into an indistinctly marked off head and an elongated trunk.
1. Head:
The head of Peripatus bears a pair of eyes, a pair of antennae, a pair of jaws or mandibles, and a pair of oral papillae.
(i) Eyes:
The eyes are simple and dorsal in position. They resemble the eyes of chaetopods.
(ii) Antennae:
One pair of antennae represents the first pair of appendages. The antennae are ringed and taper slightly till near their termination, where they are slightly enlarged. The rings bear a number of spines. The free end of each antenna is covered by a sheath of spiny tissue.
(iii) Jaws or Mandibles:
One pair of jaws or mandibles constitute the second pair of appendages. These are present deep inside the mouth cavity. Each jaw is small, muscular, stumpy and provided at its free end with a pair of sharp cutting blades. Each jaw is composed of two curved, falciform pointed chitinous plates, the inner toothed and the outer concave edge.
The jaws have their convex edge directed forwards and their concave or cutting edge turned backwards. The inner cutting plate usually bears a number of cutting teeth in addition to the main tooth. The jaws appear to be used for tearing the food.
(iv) Oral Papillae:
Third paired appendage is a pair of oral papillae. The oral papillae are placed at the sides of the head, one on each side. On the terminal end of each oral papilla is situated an aperture of a special kind of gland called slime gland. The oral papillae possess two main rings of the projecting tissue and their extremities bear irregularly arranged papillae. Mouth is situated on the ventral side immediately behind the oral papillae.
2. Trunk:
The trunk is devoid of exoskeletal coverings and its skin is thrown out into a number of ridges, along which wart-like papillae are placed. The trunk possesses appendages or legs which vary in number from 14 to 43 pairs, depending on the species and the sex. The legs are all alike and are placed at regular intervals.
The appendages or legs consist of two main divisions—the leg and the foot. Each leg is a large, hollow, conical, un-jointed protuberance bearing a pair of terminal claws (Fig. 83.3). At the distal end of each leg on the ventral side are three to six transverse spiniferous pads, on which the leg rests when walking.
The entire surface of the leg consists of numerous papillae. The foot is attached to the distal end of the leg. It is slightly narrower at its attached extremity than at its free end. It bears two sickle- shaped claws and two, three or four papillae.
The part of the foot which carries the claws is specially retractile and is generally found more or less telescoped into the proximal part.
The anus which serves as the outlet of the alimentary canal lies at the posterior end of the body and behind the last pair of legs. The genital pore or reproductive opening is situated on the ventral surface between the last pair of legs just in front of the anus. A nephridiopore lies at the base of each leg.
3. Body Wall of Peripatus:
The body wall is dermomuscular, consisting of cuticle, epidermis, dermis and muscles.
1. Cuticle:
It is a thin, chitinous outer layer which covers the body surface. The cuticle of onychophorans is only one micron thick, flexible and very permeable. It is not divided into articulating plates.
The absence of a rigid exoskeleton enables onychophorans to squeeze their bodies into very confining places. The cuticle is ridged and covered with microscopic small and large tubercles which give it a velvety texture which is unknown in other animals.
2. Epidermis:
Beneath the cuticle is a single layer of epidermis, which overlies the dermis and muscle fibres. The epidermis is composed of a single layer of cells. It also secretes the cuticle on its outer surface.
3. Dermis:
It is a thin layer comprising of connective tissue. It lies just below the epidermis.
4. Muscles:
There are three layers of muscle fibres which lie beneath the dermis. The outer layer is composed of circular muscles. The inner layer consists of longitudinal muscles. Between the outer and inner layers, there is a layer of diagonal muscles.
The main muscles of the body wall are un-striated and divided into fibres, each invested by a delicate membrane. The muscles of the jaws are transversely striated. The body wall of Peripatus is, thus, constructed on the typical annelidan plan.
4. Locomotion of Peripatus:
Peripatus crawls by means of the legs and by extension and contraction of the body, which is held off the ground. Waves of contraction progress from the anterior to the posterior. When a segment is extended, the legs are lifted from the ground and moved forward.
A pushing force is exerted in the effective stroke, as in arthropods, the legs are located more ventrally than are the Para podia of annelids. Peripatus exhibits a slow movement. Peripatus can also easily crawl through crevices or holes which are much smaller than the diameter of its body.
5. Body Cavity of Peripatus:
In Peripatus, the body cavity is a haemocoel. It is lined with epithelium and consists of four compartments—one central, two laterals and one pericardial. The central compartment is the largest and contains the alimentary canal, the reproductive organs and the slime glands.
The lateral compartments are much smaller than the central and they extend within the legs. The pericardium contains a peculiar cellular tissue. However, the true coelom is restricted to the gonadial cavities and to small sacs associated with the nephridia.
6. Slime Glands of Peripatus:
A pair of slime glands is situated, one on either side of the body cavity. The slime glands are tubular and their ducts are considerably dilated to form the large slime reservoirs.
These glands open on the terminal ends of the oral papillae and secrete an adhesive slime for entangling the prey. The secretion of the slime glands is discharged as two streams for a considerable distance as great as 50 cm. This secretion hardens very rapidly into threads in the air.
7. Digestive System of Peripatus:
The digestive system consists of alimentary canal and digestive glands.
(i) Alimentary Canal:
The alimentary canal is very simple and tube-like. It begins with the mouth and consists of foregut of a pharynx and oesophagus, the midgut composed of stomach-intestine, and the hindgut or rectum.
Mouth:
The mouth is located at the base of pre-buccal depression. It is enclosed by the pre-buccal or oral papillae which form a preoral cavity. Within the preoral cavity lie the lateral claw-like mandibles which are used for grasping and cutting the prey. The mouth leads into a pharynx.
Pharynx:
The pharynx is thick-walled and muscular. The cavity of the 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 into the oesophagus.
Oesophagus:
The oesophagus is short having a weak muscular wall. It is lined internally by chitin. The oesophagus leads to a straight midgut.
Stomach-intestine:
The stomach-intestine is a straight tube which forms the largest part of the alimentary canal. It is dilated soft-walled tube lined by ectoderm. The stomach-intestine is the chief site of digestion and absorption. It leads behind into the hindgut or rectum.
Rectum:
The hindgut or rectum is a short tubular and narrow terminal part of the alimentary canal. It loops forward over the intestine before passing posterioly to the anus. The rectum is internally lined by chitin. The anus opens on the ventral side at the posterior end of the body.
(ii) Digestive Glands:
The most important digestive gland is a pair of salivary glands.
(iii) Salivary Glands:
A pair of salivary glands opens into a median dorsal groove in the preoral cavity. Each salivary gland is slender, tubular and elongated. The salivary glands extend from third to tenth and sometimes up to thirty-first segment. The two salivary ducts fuse to open within the preoral cavity. In some forms, a reservoir is also present.
The salivary secretions are passed into the body of the prey, and the digested tissues are then sucked into the mouth. The salivary glands are supposed to be modified nephridia.
Ingestion and Digestion of Food in Peripatus:
Peripatus is predaceous and carnivorous and its food comprises small invertebrates, such as snails, insects and worms. A prey, when detected, is captured by the spurting of slime from the slime gland. It is then held firmly by the lips and sucking of the pharynx. The jaws split open the prey and the saliva, containing various enzymes, is poured over the prey through the slit. The soft parts of the prey are digested by the saliva.
The nutritive fluids of the digested soft parts are drawn in by the sucking action of the pharynx and oesophagus. The peristaltic action of the midgut wall carries the material to pass posteriorly through its lumen. The midgut cells secrete a thin membrane around the food particles.
Various juices secreted by the lining of the midgut penetrate this membrane envelope and digest the food further. The digested materials come out of the membrane and are absorbed by the absorptive cells in the midgut lining. The residual matter which remains within the membrane gradually passes to the hindgut and is finally ejected from the anus.
8. Circulatory System of Peripatus:
The circulatory system consists of a single mid-dorsal tube, the heart placed on the dorsal surface of the gut. The heart is tubular and contractile, lies within the pericardial sinus. It is provided with a pair of lateral ostia in each segment. The heart extends from the segment bearing first pair of legs to the segment immediately before last.
The heart propels the blood forward into the general haemocoel. The partitions between sinuses are perforated by openings that facilitate blood circulation. The pericardial sinus is separated from the general body cavity by a longitudinal partition. No other definite blood vessel is found. The blood is colourless and contains phagocytic amoebocytes.
9. Respiratory System of Peripatus:
The respiratory system consists of air tubes called tracheae or tracheal tubes. The tracheae are delicate, un-branched or rarely branched tubes. They are lined by a thin chitinous layer exhibiting faint transverse striations. The tracheae extend throughout the body and communicate to outside through minute spiracles.
The spiracles are minute openings. These are present in large numbers all over the surface of the body between bands of tubercles. The spiracle opens into a very short atrium, at the end of which arises a tuft of minute tracheae. By means of tracheal tubes the air is conveyed to all parts of the body for respiration.
The tracheal system of Peripatus is markedly different from that of the cockroach in that the spiracles in this case are devoid of any closing mechanism. This condition, however, adversely affects the animal by loss of water in great quantity.
10. Excretory System of Peripatus:
The excretory organs are nephridia. The nephridia are segmentally arranged pairs of coiled tubes. Each segment contains a single pair of nephridia located in the ventrolateral sinuses. The nephridia 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 ciliated funnel and nephrostome lie within an end sac, which represent a vestige of the coelom. Before opening to the outside, the tubule becomes enlarged to form a contractile bladder. The nephridiopore is located on the inner base of each leg except for the fourth and fifth segments in which it is mounted on a more distal tubercle.
The first three pairs of nephridia are poorly developed consisting of a vesicle and duct. The anterior nephridia are modified as salivary glands and the posterior nephridia are modified as gonoducts in the female. The nature of the excretory waste is not known. 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. Due to this reason, recent workers prefer to call them coelomoducts and not nephridia. Following peculiar glands are also found in the body of Peripatus to function as excretory organs.
Coxal or Crural Glands:
A series of paired glands are present in the lateral compartments of the body cavity. Their ducts open on the lower surface of the legs just outside the apertures of the excretory ducts. As a rule they are only present in male, except in Peripatus capensis in which they are present in both sexes.
Their number and arrangement differ in males in different species. Also opening on the ventral surfaces of the legs is a series of thin-walled vesicles, the coxal organs. These occur in both the sexes and are capable of eversion and retraction. A pair of large slime glands opens on the extremities of the oral papillae.
These are supposed to be modified coxal glands. When the animal is irritated their secretion is discharged in the form of a number of fine viscid threads probably as a measure of defence. A pair of anal glands having uncertain function opens close to the anus. These are probably modified excretory organs.
11. Nervous System of Peripatus:
The nervous system is primitive. It consists of a pair of supra-pharyngeal ganglia or brain, two circumpharyngeal connectives, a pair of lateral longitudinal nerve cords connected together by transverse commissures. The brain is situated in the head over the pharynx and it supplies nerves to the antennae, the eyes and the mouth region.
A pair of circumpharyngeal connectives unite the brain with the ventral nerve cord. Two widely separated lateral longitudinal nerve cords run parallel to one another to the posterior end of the body, where they join together behind the anal aperture.
Several transverse commissures connect the two cords imparting them a ladder-like appearance. In each segment, the ventral nerve cords contain a ganglionic swelling and give rise to a number of paired nerves supplying the legs and the body wall.
12. Sense Organs of Peripatus:
The sensory organs are a pair of eyes, sensory papillae and specialised sensory cells in the buccal cavity. There is a simple and small eye at the base of each antenna. Each eye is 0.2 to 0.3 mm in diameter and cup-like in appearance. It has an outer cuticular cornea and thick lens (Fig. 83.7).
In the inner layer the rod cells of the retina are projected towards the lens at one end and the other end is connected with the branches of the optic nerves. Eyes can feel the difference between light and dark and the animal moves away from light.
The sensory cells which are present on the antennae are responsible to feel the way during locomotion. Numerous sensory papillae are distributed all over the body. Each papilla consists of a seta and a group of sensory cells. These papillae are responsible for determining air currents and also act as tactile receptors.
Several specialised sensory cells are also present in the lining of buccal cavity. These sensory cells determine the taste of food.
13. Reproductive System of Peripatus:
The sexes are always separate and the cavities of the reproductive organs are coelomic. The males are usually smaller than females.
Male Reproductive Organs:
The male reproductive organs consist of a pair of testes, a pair of seminal vesicles, a pair of vasa deferentia, an ejaculatory duct and a male gonopore. The testes are elongated and tubular structures. From each testis a narrow and slender vas efferens opens through a funnel-like aperture into the seminal vesicle.
On each side an elongated, narrow, slender and coiled vas deferens arises from the seminal vesicle and unites with its fellow from the other side to form a central tube called ejaculatory duct. The proximal part of the ejaculatory duct is glandular and secretes a substance which packs the sperm cells into spermatophores.
The spermatophores are as long as 1.0 mm and are enclosed in a chitinous envelope. The ejaculatory duct opens to exterior on the ventral surface between the legs of the last pair or behind them. The male gonopore is ventral and posterior like that of the female.
Female Reproductive Organs:
The female reproductive organs include a pair of ovaries, a pair of oviducts, a pair of uteri, a vagina, and a female genital pore. The ovaries are a pair of elongated tubular structures located in the posterior part of the body. 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 uteri are beaded in appearance. The two uteri unite to form a median vagina (or common atrium) which opens to the exterior through a common genital pore, situated ventrally near the posterior end of the body. In some forms, a process called ovipositor is present near the genital aperture.
14. Development of Peripatus:
Onychophorans are oviparous, ovoviviparous, or viviparous. In most species the reproduction appears to be continuous. Oviparous forms are limited to the Australian genera, Ooperipatus and Symperipatus. The females of these genera are provided with an ovipositor. The large yolky eggs are enclosed in chitinous shells and are laid in moist places. The cleavage is superficial.
All other onychophorans are ovoviviparous or viviparous. The fertilisation is internal and the egg develops within the uterus. In the mating of South African Peripatopsis, which lacks seminal receptacles, the male crawls over the body of the female and deposits a spermatophore at random on her sides or back. Over a period of time a female may accumulate many spermatophores.
The spermatophore stimulates blood amoebocytes and these amoebocytes dissolve the cuticle and skin (integument) of the female and that of the spermatophores. The sperms then pass from the spermatophore into the female haemolymph. They eventually reach the ovaries , where fertilisation of eggs takes place.
The eggs of viviparous onychophorans are small and have little yolk. The cleavage is either superficial or holoblastic. The uterine secretion provides for the nutrition of the embryo. The nutritive material is obtained by the embryo through a special embryonic membrane or through a placental connection to the uterine wall. The pattern of development varies in different forms.
15. Geographical Distribution of Peripatus:
The geographical distribution of onychophorans is peculiar in a number of respects. The phylum consists of two families. Each has a wide, discontinuous distribution around the world, but neither is found in the same area with species of the other family. The family Peripatidae is more or less equatorial in distribution, while the family Peripatopsidae is limited to the Southern Hemisphere.
The genus Peripatus is found restricted in the various places of the world such as South Africa, Australia, New Zealand, Tasmania, New Britain, South America and West Indies, Malaya, Equatorial Africa and Chile. The species of Peripatus, of which between 50 and 60 are known, fall into seven discontinuous geographical groups, the members of which are more closely related to each other than those of any other group.
The names and distribution of these groups, together with the generic names which have been applied to them by various authors are as follows:
1. Neo-Peripatus (Peripatus). Neotropical region (West Indies and America from Mexico in the North to Rio de Janeiro in the South).29 Species.
2. Congo-Peripatus (Mesoperipatus Evans). Congo district in West Africa.1 Species.
3. Eo-Peripatus (Eoperipatus Evans ). Malaya (Malacca and Sumatra).4 Species.
4. Capo-Peripatus (including Peripatopsis Pocock and Opisthopatus Purcell). South Africa (Natal to Cape Town) 7 Species.
5. Melano-peripatus (Paraperipatus Willey). New Britain. 1 Species.
6. Austro-Peripatus (Peripatoides Pocock and Ooperipatus Dendy). Australia, Tasmania, New Zealand. 8 Species.
7. Chilio-Peripatus (Opisthopatus Bouvier). Chile. 1 Species,
16. Affinities of Peripatus:
The genus Peripatus was established in 1826 by Guilding, who first obtained specimens of it from St. Vincent in the Antilles. He regarded it as a mollusc, misled by its slug-like antennae and by its moist skin. Specimens were subsequently obtained from other parts of the neo-tropical region and from South Africa and Australia.
The animal was variously assigned by the zoologists of the day to the Annelida and Myriapoda. In 1874 Moseley first of all established the position of Peripatus as a primitive member of phylum Arthropoda by discovering the tracheae. Peripatus exhibits affinities with Annelida, Arthropoda and Mollusca as well as peculiarities of its own.
A. Annelidan Affinities:
Peripatus resembles the annelids in the following features:
1. Vermiform body with truncated extemities.
2. True head is absent.
3. Segmentation in both is homonymus.
4. Outer body covering is cuticle, skin is thin and flexible.
5. Musculature is smooth and muscles operate in identical way.
6. Appendages are hollow and unjointed.
7. Locomotion slow and by peristalsis as in an earthworm.
8. Simple and straight alimentary canal.
9. Paired segmentally arranged nephridia.
10. Slime glands and coxal glands correspond with similar glands of Chaetopoda.
11. Structure of the eye is same as in polychaetes.
12. Presence of cilia in the reproductive tracts.
B. Arthropodan Affinities:
Peripatus resembles the arthropods in the following characteristic features:
1. Presence of antennae.
2. Body is covered with chitinous cuticle like that of arthropods.
3. The appendages are provided with claws.
4. Jaws are modified appendages.
5. Locomotion is not annelid-like but by legs having definite musculature.
6. Presence of haemocoel.
7. Absence of perivisceral part of coelom.
8. Coelom reduced to the cavities of excretory and reproductive organs.
9. Peculiar salivary glands, supposed to be modified nephridia.
10. Presence of lateral ostia as doors between heart and pericardial sinus.
11. Presence of tracheae as respiratory organs.
12. Brain is large and resembles the brain of typical arthropod.
13. General structure of reproductive organs and the pattern of development is same as in other arthropods.
C. Molluscan Affinities:
Peripatus exhibits the following molluscan characteristics:
1. Slug-like appearance of the body.
2. Ladder-like nervous system as found in Polypiacophora and Prosobranchiata.
The above characteristics led to the inclusion of Peripatus among Mollusca. But according to other evidences, these are only superficial resemblances.
D. Onychophoran Characteristics:
The following characteristic features are peculiar to Onychophora in which they differ from other phyla:
1. Body shows no or indistinct segmentation.
2. Appearance of skin is not like that of arthropods.
3. Antennae not homologous to the antennae of other arthropods.
4. Three segmental head of Peripatus exhibits a condition mid-way between that of Annelida and Arthropoda.
5. Segments behind head are simple and identical.
6. Restriction of jaws to a single pair. Movement of jaws is from anterior to the posterior end.
7. Presence of slime and coxal glands.
8. Arrangement of tracheae is not arthropod-like. Here in each segment there are numerous permanently opened spiracles.
9. Two ventral nerve cords are widely separted and without true ganglia.
10. Structure of eye is less complicated.
11. The disposition of reproductive organs.
17. Taxonomic Position of Peripatus:
Peripatus is of special interest because its body exhibits certain structures characteristic of annelids and other structures found only in arthropods. Therefore, they are regarded to be an intermediate stage or connecting link between Annelida and Arthropoda.
However, onychophorans appear to be more closely allied to arthropods than to the annelids. They may represent an offshoot from near the base of the arthropod line. Based on such phylogenetic considerations, Manton (1970) and other contemporary zoologists have included onychophorans within the Arthropoda as a subphylum or class.
But, absence of an exoskeleton and jointed limbs and the primary annelidan characters in Onychophora present serious problems. As a matter of fact, onychophorans are neither annelids nor arthropods but possess distinctive characteristics of their own, therefore. Onychophora are nowadays considered as a separate phylum.
But the modern view is somewhat different. It is presumed that the group is very ancient and once had been more widespread and diversified containing a great variety of forms.
This is supported by the discovery of eleven fossil specimens of the mid-Cambrian, Aysheaia which closely resembles the modern Onychophora. The latest view holds that Onychophora is not an evolutionary link between Annelida and Arthropoda but a distinct surviving branch of segmental animals.