Phylum Arthropoda, characters of Arthropoda, function, classification, example, top question,

 Phylum Arthropoda

Phylum Arthropoda (Gr., arthros = joint + podos = foot) is the largest phylum and most varied in the animal kingdom. Von Siebold gave the name Arthropoda. It includes well over one million described species and many million remain unstudied. Some of the more well-known arthropods include insects, crustaceans, spiders, scorpion and centipede as well as the fossil trilobites. Arthropods are mainly terrestrial but marine and freshwater species are also well known. Arthropoda varies tremendously in their habitats, life histories, and dietary preferences.

Arthropod bodies are divided into segments. However, a number of segments are sometimes fused to form integrated body parts known as tagmata. This process of fusion is called tagmosis. The head, thorax, and abdomen are examples of tagmata. Exoskeleton serves as protection and provides places for muscle attachment. Arthropods must molt because their exoskeletons don’t grow with them. However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs. Arthropod appendages may be either biramous (branched) or uniramous (unbranched). In insects, the anterior portion of the heart is extended into atube that is called an aorta which directs the blood forward as it goes out into the body cavity. Arthropods have a well-developed, mesodermal, solid nerve cord, ventral and well-developed sense organs. They range in size from microscopic plankton to life-forms that are a few meters long. Arthropods primary internal cavity is known as hemocoel, which accommodates their internal organs, and through which their hemolymph - analogue of blood - circulates; they have open circulatory systems.

Respiration occurs in various ways for e.g. some species have gills, while others employ tracheae, or book lungs. The tracheal respiratory system consists of external openings called spiracles that are linked to a system of branched tubules which allow respiratory gases to reach internal tissues. Arthropods are characterized by a brain as well as a nerve ring around the area of the pharynx, in theoral cavity. A double nerve cord extends backwards along the ventral surface of the body, and each body segment is associated with its own ganglion, or mass of nerve cells. Generally, the sexes areseparate in phylum Arthropoda. Fertilization usually occurs internally, and most species are egg laying. While some species exhibit direct development, in which eggs hatch as miniature versions of adults, other species pass through an immature larval stage and undergo a dramatic metamorphosis before reaching adult form. Arthropods contribute directly (as food) as well as indirectly (crop pollination) to human food supply chain. Some specific species are known to spread severe disease to humans, livestock, and crops.

GENERAL CHARACTERS: -

• Arthropoda is bilaterally symmetrical, triploblastic and metamerically segmented animals. 
• Body can be divided into head, thorax and abdomen; head and thorax are often fused to form Cephalothorax. 
• Body is covered with a thick chitinous cuticle forming an exoskeleton. 
• Body segments usually bear paired and jointed appendages. 
• Coelom largely a blood-filled haemocoel. 
• Muscles are mostly striated, usually capable of rapid contraction. 
• Digestive tract is complete. The mouth and anus lie at opposite ends of the body. 
• Circulatory system is open with dorsal heart and arteries and blood sinuses. 
• Respiration through by general body surface, by gills in aquatic forms, by tracheae and by book lungs. 
• Nervous system has dorsal nerve ring. 
• Excretory organs are Malpighian tubules (in insects) and green glands (in Crabs and prawn). 
• Cilia are absent from all parts of the body. 
• Sexes are generally separate. 
• Fertilization is internal, oviparous and ovoviviparous. 
• Parental care is well marked in many arthropods. 

Other types of Arthropoda: - 

1. Limulus: -

Limulus or king crab belongs to the subclass xiphosure and class Merostomata of subphylum Chelicerata. It is a large-sized marine animal, up to 60 cm long. It lives in shallow waters partly buried in the bottom mud. It feeds on worms, algae and mollusks. It is bilaterally symmetrical, triploblastic metamerically segmented animal with jointed appendages. The body is differentiated into prosoma and hinged opisthosoma. Prosoma bears a pair of 3-jointed chelicerae and 5 pairs of 6-jointed legs. First four legs are chelated; fifth is non-chelate. The 7th pair of appendages, chilaria are degenerate and of doubtful functions. (Yadav and Varshney, 2015). The broad hexagonal opisthosoma is divisible into an anterior 6-segmented mesosoma and a posterior 3 segmented metasoma. Excretion takes place through 4 pair of coxal glands. Sexes are separate. In breeding season males and females copulate in shallow water. Eggs, laid by females in holes in sand, are fertilized by sperms. Limulus is found along the eastern coastline of Asia and North America (Kotpal, 2005). 

2. Aranea: -

Aranea belong to the order Araneae of class Arachnida. Body can be divided into prosoma and opisthosoma. Prosoma is covered by carapace that bears 8 simple eyes anteriorly. Ventrally prosoma bears 6 pair of appendages, a pair of chelicerae, a pair of pedipalps and 4 pair of walking legs. Opisthosoma bears spinnerets or spinning organs just anterior to the terminal anus. Respiration happens via book lungs or tracheae or both. Excretory organs are Malpighian tubules. The silk secreted by most spiders is used for construction of egg sacs and for constructing webs to trap insects, as well as a guideline for males in finding sexual partners.

3. Daphnia: -

 Daphnia, also called "water fleas", are found in order Cladocera of subclass branchiopod and are sometimes called cladocerans. Body is about 2 mm long, oval, laterally flattened with a ventral beak on head and a sharp posterior caudal spine. Daphnia has a single compound eye and there are normally five or six appendages attached to the thorax. The body is compressed and enclosed in a flattened, transparent carapace. Antennules are much reduced and uniramous. Daphnia moves by beating its antennae which are moved by large muscles. They feed on algae, protozoa, bacteria, and decaying organic material. Most are filter feeders that consume phytoplankton which they remove from water using their setose thoracic appendages. A few are carnivores preying on other cladocerans. Daphnia swims by rapid jerks of its two large biramous antennae.

4. Cyclops: -

 The Cyclops is named after one-eyed monster of Greek legend. Cyclops is crustacean invertebrate with a hard outer shell. It is the most familiar copepod crustacean found in freshwater ponds. Pear-shaped or elongated body measures 1.5 to 5 mm in length. Head and first thoracic segment become fused to form cephalothorax, which is covered dorsally by a carapace. The cyclops has 5 pair of legs and a divided tail-like appendage called a Furca. It is greenish, straw yellow, or grayish in color. It goes jerking through the water usually in very large numbers. The females carry the eggs in little side sacs and they multiply rapidly. The Cyclops is often seen near water fleas or Daphnia. Many water animals feed upon the Cyclops. It has a very important role in the food chain. Cyclops serves as intermediate hosts for the guineaworm of man.

5. Lepas: -

 Lepas is commonly known as “ship or goose barnacle”. It is a marine animal that inhabits warmer seas. It is found attached to ships and vegetation with the help of peduncle. Lepas is bilaterally symmetrical, triploblastic metamerically
segmented animal with jointed appendages. The body is covered by a thick chitinous exoskeleton and by 5 calcareous plates-a pair of scuta, a pair of terga and a median dorsal carina. They feed upon minute organisms gathered from water and kicked into mouth by the thread like feet. It is bisexual. Development includes a free-swimming cypris larva (Yadav and Varshney, 2015). 

6. Sacculina: -

Sacculina is commonly known as root-headed barnacle. The adult looks like a large tumor or ovoid sax fastened to the abdomen of crab by a short stalk. Appendages, segmentation, sense organs and alimentary canal etc. are absent. The body is made up of two parts- a thin sac present on the ventral abdomen of the host crab and a peduncle. The female sacculina infects a crab. The male sacculina lives within the body of the female and fertilize its eggs. Larva is cirripede-nauplius.

7. Squilla: -

 Squilla is a large marine crustacean and belongs to order stomatopod of subclass malacostraca. Squilla is active, predatory and nocturnal; it catches hold of the prey with powerful maxillipedes. It is found in burrows in the sand or mud at the bottom of the sea. Body is bilaterally symmetrical, metamerically segmented, triploblastic, Dorso-ventrally flattened and 25 cm long. Body is divisible into cephalic, thoracic and abdominal parts. Head bears a pair of stalked movable compound eyes, antennules and antennae. The thoracic region bears 5 pairs of maxillipedes and 3 pairs of walking legs. The second pair of maxillipedes is exceptionally large. The large abdomen is made up of 6 segments and each segment bears a pair of biramous pleopods. The last pair of pleopods are called uropod. It feeds on small fishes, crustaceans, molluscs etc.

8. Eupagurus: -

Eupagurus is commonly known as ‘hermit crab’. It is found inhabiting the empty shells of gastropods. It leads a commensal life. Cephalothorax is broad and flattened. Head bears a pair of stalked elongated eyes, a pair of large antennae and a pair of short antennules. Thorax bears 5 pair of legs. First, fourth and fifth pair of legs are chelated whereas rest of the legs are non-chelate (Verma, 2015). Abdomen appendages of the left side are reduced while those of the right side are absent. Eupagarus is a peculiar crustacean having extreme modifications in order to adjust in the coils of molluscan shells.

9. Cancer: -

Cancer is commonly called rock crab or true crab. Body is Dorso-ventrally compressed and consists of a cephalothorax and an abdomen. Cephalothorax is broader than long and is covered by a carapace. It bears small antennules, antennae, maxillipedes, compound eyes and thoracic legs. Mouth parts present on the ventral surface are covered by the flat, plate-like third maxillipedes. Fifth pairs of thoracic legs are present. The first pair is chelate whereas the remaining are non-chelate. The eggs are carried about by the female attached to abdominal legs (Verma, 2015). Development is indirect through zoea and megalopa larval stages.

10. Gryllus: -

Gryllus belong to the order Orthoptera. Gryllus is the common house cricket which lives in damp warm places like under logs, boxes, and stones and in holes behind boods and crevices and in the kitchens. The body can be divided into head, thorax and abdomen. Head bears a pair of compound eyes and a pair of antennae which are filiform and longer than the body. Mouth parts are mandibulate and well developed. Female possess a well-developed oviposterior that serves for depositing eggs in holes or crevices, devours all sorts of animal and vegetable matter such as clothing paper, skin, fruits and vegetables etc.

11. Mantis: -

 Mantis is commonly called praying mantis. The colour of Mantis is usually green. It is a large insect (5-10cm), with a small triangular head, a long prothorax and abdomen consisting of10-segments. Wings are folded flat and overlap the sides of the body. It is found in areas of green vegetation. It feed voraciously on other insects. Female eats up the male after copulation. The female lays about 200 eggs in frothy mass which soon hardens into a waterproof egg case. 

12. Carausius: -

Carausius is commonly known as stick insect because it mimics a twig to escape enemies. Carausius is herbivorous and is found in tropical forests in thick vegetation. Body is elongated and slender and can be divided into head, thorax and abdomen. Head is small, bears a pair of antennae and a pair of small compound eyes. Thorax is elongated bearing pairs of long slender legs. Abdomen has 10 segments. Mouthparts are of chewing type. Sexes are separate. While male is small, winged and active, the female is large, sluggish and apterous.

Important Point

• Phylum arthropoda (Gr., arthros = joint + podos = foot) is the largest phylum and most varied in the animal kingdom. Arthropods are mainly terrestrial but marine and freshwater species are also well known. Arthropod bodies are divided into segments. 
• A number of segments are sometimes fused to form integrated body parts known as tagmata. This process of fusion is called tagmosis. The head, thorax, and abdomen are examples of tagmata. However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs. Arthropod appendages may be either biramous (branched) or uniramous (unbranched). 
• Arthropods have a well-developed, mesodermal, solid nerve cord, ventral and well-developed sense organs. They range in size from microscopic plankton to life-forms that are a few meter long. Arthropods primary internal cavity is known as hemocoel. 
• Respiration occurs in various ways for e.g. some species have gills, while others employ tracheae, or book lungs. Generally, the sexes are separate in phylum arthropoda. 
• Fertilization usually occurs internally, and most species are egg laying. While some species exhibit direct development, in which eggs hatch as miniature versions of adults, other species pass through an immature larval stage and undergo a dramatic metamorphosis before reaching adult form. Some specific species are known to spread severe disease to humans, livestock, and crops.
• Palaemon is commonly known as prawn. It is found in freshwater streams, ditches, lakes, ponds, rivers and reservoirs. It is a nocturnal animal hiding at the bottom during the day and coming to the surface at night in search of food. 

Objective Question of Phylum Arthropoda:-

(1) Metamerically segmented, bilaterally symmetrical animals bearing jointed appendages. These are characteristics of: 

(a) Annilida        (b) Helminths   (c) Arthropoda   (d) Mollusca

(2) An animal without economic importance is: 

(a) Honeybee    (b) Pheritma     (c) Peripatus    (d) Parasite 

(3) Peripatus is connecting link between: 

(a) Annelida and Platyhelminthes      (b) Annelida and Mollusca     (c) Annelida and Arthropoda (d) Mollusca and Arthropoda 

(4) Peripatus is: 

(a) Herbivorous     (b) Carnivorous     (c) Omnivorous     (d) Parasite

 (5) The excretory structure in Peripatus are: 

(a) Nephridia (b) Malpighian tubules (c) Coxal glands (d) Solenocytes

(6) Biting and chewing type of mouth parts occurs in:

(a) Grasshopper  (b) Cockroarch  (c) Cricket  (d) All 

(7) Respiratory organs of crustaceans are: 

(a) Gills (b) Lungs (c) General surface (d) both gills and general surface 

(8) Means of respiration in insects: 

(a) Integuments (b) Blood gills (c) Tracheal gills (d) Spiracular gills (e) All 

(9) The mouth parts of housefly are: 

(a)Biting and chewing type (b) Sucking and sponging type (c) Biting, sucking and lapping type (d) Piercing and sucking type 

(10) Rearing of silkworm is known as: 

(a) Aquaculture (b) Floriculture (c) Sericulture (d) Apiculture 

💥Answer key :-

 (1) c (2) c (3) c (4) a (5) c (6) d (7) a (8) e (9) b (10) c 

💥Terminal Question For external Examination:-

1. To describe the classification of Arthropoda? 

2. What is the economic importance of social insects? 

3. What is the zoological importance of Peripatus? 

4. Describe the mouth parts of insects? 

5. Discuss the reproduction of Palaemon? 

6. Write about the nervous system of Palaemon?