Introduction:

Arthropods having inhabited all kinds of aquatic habitats are first major group of animals to contain invaded successfully terrestrial environment occupying every possible niche there. Like annelids arthropods are coelomates and segmented, and perhaps both phyla arose from the common ancestor.

Characteristics:

1) Body bilaterally symmetrical and metamerically segmented demonstrates the tendency to combine or fuse together to create functional units known as tagmata, such as cephalothorax and abdomen: head and truck, or head, thorax and abdomen.

2) Segments carry jointed appendages.

3) Exoskeleton comprises of tough cuticle composed of chitin, protein and lipid, at times strengthened with calcium carbonate. Cuticle, secreted by underlying epidermis, is pushed periodically to allowed growth of body.

4) Absence of cilia.

5) Coelom present but extremely decreased and obliterated in adult. Main body cavity is haemocoel, a characteristics space between organs and tissues, filled with blood.

6) Circulatory systems, open type.

7) Mouth parts altered from appendages well developed alimentary canal.

8) Respiratory organs are generally tracheae, booklungs or gills.

One of the distinguishing characteristics of arthropods is presence of the tough, chitinous exoskeleton known as cutile that covers complete body surface. It is product of give flexibility. Cuticle between two segments and at joints remains very thin and flexible and is known as articular membrane. Cuticle in each segment form the dorsal plate tergum, ventral plate sternum and lateral structure pleura.

Excretion is performed by structures known as Malpighian tubules usually found in terrestrial arthropods as blind tubular elongations of gut, and lie freely in haemocoel. Aquatic forms have usually paired coxal glands, antennary glands or maxillary glands that are homologous to metameric nephridia of annelids Arthropod digestive tract is usually separated in three parts foregut or stomadaeum, midgut or mesenteron and hindgut or proctodaeum.

Subphylum Trilobitomorpha:

Subphylum Trilobitomorpha comprises trilobites. All species are extinct and fossils point to that they were all marine forms belonging to palaeozoic era. They are the most primitive of arthropods. Body was separated in three lobes through two furrows longitudinally; thorax, distinct head, and abdomen were present. Appendages biramous. It seems that trilobites had mixture of habits; they included burrowing, epibenthic, crawling, planktonic and swimming forms.

Subphylum Chelicerata:

Chelicerata body is separated in two parts an anterior cephalothorax or prosoma and posterior abdomen or opisthosoma. Antennae are absent. First pair of appendages is called as Chelicerae that are food capturing structures. Second pair of appendages are pedipalpi performing a variety of functions in different groups. Following pedipalpi, cephalothoracic region contains four pairs of walking legs. Subphylum chelicerata comprises three classes: Merostomata, Arachnida and Pycnogonida.

Class 1 Merostomata: Aquatic chelicertates in which 5 or 6 pairs of abdominal appendages are altered in gills for respiration. At the end of body there is spike like telson. Group comprises 2 subclasses: (i) Xiphosura, horseshore crabs and (ii) Eurypterida that is extinct now.

i) Subclass 1 Xiphosura: Xiphosurans are identified to exist from Cambrian period. Many species have become extinct. Only 4 species are known to exist from Cambrian period. Most common genus of horseshoe crab is Limulus. This dwells in shallow water with soft bottom. They reach length of 60cm and are dark brown in colour. Cephalothorax is enclosed through a shield - like or horseshoe shaped convex exosketeal plate called carapace which facilitates organism to push through sand.

ii) 2 Eurypterida: Eurypterida are the group of gland, extinct merostomes. They were aquatic forms and existed from Ordovician to permain period. The species of genus pterygotus was about 3 meters long. Body plan of eurypterids was like xiphosurans. They also resembled scorpions. Though, their cephalothorax was smaller.

Class 2 Arachnida: Body separated in cephalothorax and abdomen. Cephalothorax with 4 pairs of legs; abdomen segmented, with or without appendages. Respiratory organs are either, tracheae, or book lungs. Excretory organs are Malpighian tubules or coxal glands. Brain bilobed joined to ventral ganglionic mass forming ring. Arachnids, the largest of all chelicerate classes, comprise some of the common and familiar but varied form, such as ticks, spiders, mites, scorpions, pseudoscorpions, Whipscorpions, harvestmen (daddy longlegs) etc. The body of arachnids, shows 3 district regions; prosoma that is unsegmented and covered by the carapace; mesosoma or preabdomen and metasoma or postabdomen except in scorpions.

Class 3 pycnogonida: measures generally 3 - 4 mm; body mainly composed of cephalothorax, abdomen being very small; usually four pairs of walking legs; long proboscis with the mouth; simple eyes, four in number; no excretory or respiratory organs. These are usually called as sea spiders happen in all seas. Narrow body is formed of the number of distinct segments.

Subphylum Crustacea:

Crustacea are generally aquatic arthropods with gills for respiration. Cephalothorax has generally a carapace; appendages biramous but altered for different functions. Head has pair of antennules, a pair of antennae, a pair of mandibles and 2 pairs of maxillae. Development with the nauplius stage, but this way be absent in higher forms. Crustacea comprises arthropods most of which have the aquatic existence. Group abounds in species diversity and biomass. It comprises shrimps, crabs, lobsters, cxrayfish, woodlice etc. Though mainly aquatic there are some semiterrestrial and terrestrial species.

The crustacea are accommodated into six major classes. These are:

i)  1 Branchiopoda: These are small fresh water crustaceans. The trunk appendages are flattened and leaf like and are useful for locomotion as well as respiration hence the name Branchiopoda. The first antennae and second maxillae are much reduced. The anal segments bears a pair of large terminal processes called cercopods eg. Triops, the tadpole shrimp Branchinecta,

ii) Class 2 Ostracoda: Commonly called mussel or seed - shrimps, ostracods include both fresh water and marine forms. The small crustaceans, measuring a few mm have their body covered in a carapace formed by two elliptical values. The head is the dominant part of the body, with well developed appendages. The trunk is much reduced and segmented.

iii) Class 3 Copepoda: Copepoda is a large class of small (1 -5mm) crustaceans occupying both marine and freshwater environments. Copepoda form the most abundant and conspicuous members of planktonic collection. Many are parastic. Most copepods are briglty coloured and some of the species are also bioluminescent. The body is cylindrical and tapering antero - posteriorly. The trunk is divided into thorax and abdomen.

iv) Class 4 Branchiura: Branchiura includes only around 130 species of ectoparasitic crustaceans living mostly on the integument and gill cavities of fresh water and marine fish feeding on the muscus and blood of their hosts. A pair of sessile compound eyes and a large shield like carapace that covers both the head and thorax are the distinguishing features of brachiran morphology. Abdomen is small, unsegmented and formed of two lobes. Both the pairs of antennae are much reduced.

v) Class 5 Cirripedia: These crustaceans are exclusively marine and include the barnacles. Most species are free living attached to rock, shell, coral, timber and other objects. Some are parasites. Free living forms are either stalked or sessile. The stalked froms (goose barnacles) measure a few mm to 75cm in length. Sessile forms are a few centimenters long. Stalked forms have a muscular. Flexible stalk or peduncle which is attached to the substratum as its lower end and ahs the major parts of thebody, the capitulum, at the other end.

vi) Class 6 Malacostraca: Malacostraca includes most of the larger forms such as crabs, lobsters, shrimps etc. and constitute the majority of crustacean species. The cephalic region is formed by the dusion of five segments; the trunk region consists of five thoracic and six abdominal segments. Additionally, a postabdominal telson forms part of the tail fin. A carapace covering the thorax may or may not be present.

Subphylum Uniramia:

Uniramia, the largest subphylum of phylum Arthropoda includes myriapods and insects. Because of the unbranched nature of the appendages of its members, the subphylum is designed uniramia, as against those of crustaceans and primitive chelicerates that are branched and hence biramous. The uniramians have mandibles which are non - jointed, unbranched appendages without any palps; there have only a single pair of antennae which correspond to the second cephalic segments (second anbtennae). Uniramians having taken to terrestrial mode of life, have developed trachea as gas exchange organs, hence the subphylum is also known as Tracheata. calss characters of each class of uniramia.

i) Class 1 Chilopoda: Chilopoda comprise the centipedes. The class contains some 2500 specoes that have been described so far. They inhabit both tropical and temperate regions of the world, living in soil and humus, beneath stones, barks and logs.

ii) Class 2 Diplopoda: Diplopoda comprise the millipedes which are anoctural and live beneath leaves, stones, bark and logs as well as in soil an in caves. As the name indicates, these have large number of legs. They are mostly cydrindical and are abundant in the tropics.

iii) Class 3 Pauropoda: A small group of uniramains, there are 500 speices of pauropods described so far. The minute organisms measuring about 1.5mm in length live in leaf mold or in soil. The body is eleventh segements are legless. The tergal plates are very large.

iv) Class 4 Symphyla: Symphyla is yet another small myriapodous group that includes around 160 described species. These are also soil living forms and live in leaf molds as well. They measure around 1 to 8mm in length and have a trunk made of 13 segments but 15 - 22 tergal plates.

v) Class 5 Insecta: Class insecta contins nearly a million described species. There are more species of insects that all the other species of animals combined. The most important characteristics features of insects are the division of body into three tagmata namely, head, thorax and abdomen; presence of three pairs of legs and two apirs of wings on the thoracic region of the body, though some insects may have no wings.

Phylumm Onychophora:

Features of Phylumm Onychophora:

1. Free living terrestrial

2. Body is bilaterally symmetrical, lengthened, cydrindrical, vermiform, having tissues and organs.

3. Body wall contains thin flexible, layers of circular, chitinous cuticle over epidermis, oblique and longitudinal smooth muscles beneath.

4. 14 - 43 pairs of short, unjointed, fleshy legs that are hollow invaginations of body wall having terminal pad and terminal claws.

5. Gut straight, complete, with the pair of claw - such as mandible; foregut and hindgut lined with cuticle; does not digestive diverticula.

6. Body cavity is a well developed haemocoel

7. Open circualroty sytem having tubulart heart but no other blood vessels; paired ostia present.

Onychophora are small group of invertebrates closely connected to arthropods. They haven't changed much in structure since cambrain period. Most common genus is peripatus. Group illustrates discontinuous distribution and are generally confirmed to tropical; regions of world, found between stones, logs and leaves or along banks of streams. Body is covered by the exosketelon, cuticle. Compostion of cuticle is like arthropods. But there here it is thin, flexible, permeable and untanned (not sclerotised). Beaneth cuticle there is layer of epidermis and circular, diagonal and longitudinal layers of smooth muscle fibres.

Affinities:

Onychophorans have both annelidan and arthropodan characters. Arthropodan characters comprise reduced coelom, chitionuous cuticle. Moulting Possession of appendages altered for feeding, tubular heart and haemocoel for circulation. Annelidan characters are recommended by structure of body wall, nephridia, and thin and flexible cuticle and non jointed appendages. Also onychophorans look like annelids in embryonic development. Onychophorans were once thought to be missing link between Annelida and Arthropoda. It is now thought that onychophorans possibly shared common ancestor with arthropods.

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