In this article we will discuss about the General Characters and Classification of Phylum Arthropods.
General Characteristics of Phylum Arthropods:
Some of the general characters of Phylum Arthropoda are listed below:
1. Habitat:
They occur on land, in the soil, in sea water, in fresh water and in the bodies of animals and plants as parasites.
2. Body Form:
It varies considerably. They have jointed legs. The body is segmented externally. The body consists of head, thorax and abdomen.
3. Body wall:
The body is covered with a thick, tough and non-living chitinous cuticle, which forms the exoskeleton.
4. Body cavity:
The true coelom is greatly reduced in adults, and is only represented by the cavities of the reproductive and excretory organs. The body cavity is a haemocoel viz., cavity filled with blood.
5. Digestive tract:
It is complete. The alimentary canal consists of stomodaeum (fore gut), mesenteron (mid gut) and proctodaeum (hind gut).
6. Blood vascular system:
It is of open type viz., blood does not flow in definite vessels. There are present irregular spaces, known as lacunae or sinuses, filled with blood.
7. Respiratory organs:
These are gills or book gills in aquatic forms and tracheae or book-lungs in terrestrial forms.
8. Excretory organs:
These are either green glands or Malpighian tubules. In some forms coxal glands are excretory organs.
9. Nervous system:
The annelidian type of nervous system is present, viz., it consists of a nerve ring and a solid double ventral nerve cord with ganglia.
10. Sense organs:
Sensory organs like antennae and eyes are present. In many arthropods, compound eyes are present, in which mosaic vision is developed. Some forms also have statocysts (balancing organs).
11. No cilia:
An important feature of the arthropods is the complete absence of the cilia.
12. Muscles:
The muscles are generally striped, which are capable of rapid action. Un-striped muscles are also found.
13. Endocrine glands:
Endocrine glands are present which secrete hormones. Some arthropods particularly insects excrete pheromones. The latter are chemicals which communicate message. Some pheromones act as sex attractants.
14. Sexes:
Sexes are separate (dioecious) and the sexual dimorphism is observed in many forms.
15. Development:
Fertilization is usually internal. They are mostly oviparous. Development may be direct or indirect. In indirect development there is metamorphosis.
16. Parental care:
It is often seen in many arthropods.
17. Parthenogenesis:
Development of an egg (ovum) into a complete individual without fertilization by a sperm is called parthenogensis. Male honey bees (drones) are formed by parthenogenesis.
Examples:
Economically important insects—Apis (Honey bee), Bombyx (Silk worm), Laccifer (Lac insect).
Vectors — Anopheles, Culex and Aedes (all the three are mosquitoes). Gregarious pest — Locusta (Locust). Living fossil — Limulus (King crab).
Trilobites (Fig. 4.24) are fossil arthropods which are over 600 million years old.
Unique Features:
(i) Jointed appendages,
(ii) Thick exoskeleton.
(iii) Compound eyes,
(iv) Haemocoel.
(v) Antennary glands, Malpighian tubules and coxal glands for excretion and osmoregulation.
(vi) Тracheae, gills, book gills, and book lungs as respiratory organs,
(vii) Pheromones.
Advancement over Annelids:
(i) Distinct head,
(ii) Well developed exoskeleton,
(iii) Jointed appendages for different functions,
(iv) Striped muscles,
(v) Endocrine glands and pheromones.
(vi) Special respiratory organs such as tracheae, book lungs,
(vii) Compound eyes.
Classification of Phylum Arthropods:
On the basis of body divisions and presence or absence of certain appendages, the phylum arthopoda is divided into seven classes.
Class 1. Crustacea:
The body is divisible into cephalothorax (head + thorax) and abdomen. Dorsally, the cephalothorax is covered by a thick exoskeleton carapace. There are present two pairs of antennae and a pair of stalked compound eyes.
Examples: Palaemon, (Prawn), Astacus (Cray fish), Palinurus (Lobster), Cancer (Crab), Lucifer (shrimp) Eupagarus (Hermit Crab), Lepus (Goose barnacle), Balanus (Acorn barnacle), Sacculina (a parasite on crab), Oniscus (Wood louse— terrestrial), Squilla, Daphnia (Water flea), Cyclops.
Class 2. Chilopoda:
Body is divisible into head and trunk. Each trunk segment bears a pair of legs. The first pair of legs are modified into poison claws. There is a single pair of antennae.
Example: Scolopendra (centipede).
Class 3. Diplopoda:
Body is divisible into head, thorax and abdomen. There is a single pair of antennae. Except first thoracic segment, (it does not have legs) each thoracic segment bears a pair of legs, however, each abdominal segment has two pairs of legs.
Example: Julus (millipede).
Class 4. Insecta (Hexapoda):
Body is divisible into head, thorax and abdomen. There is a pair of antennae, and a pair of compound eyes. The thorax consists of three segments with three pairs of legs and usually two pairs of wings. Uric acid is chief excretory waste.
Examples: silver fish, cockroach, bedbug, locust, termites, butterflies, rat flea, beetle, wasp, aphid, silk moth, etc.
Class 5. Arachnida:
The body is usually divisible into cephalothorax and abdomen. The cephalothorax bears simple eyes and six pairs of appendages (one pair of chelicerae, one pair of pedipalpi and four pairs of legs). Antennae are absent. Respiratory organs are book lungs or tracheae or both.
Examples: scorpion, spider, tick, mite.
Class 6. Onychophora:
Example Peripatus.
Class 7. Merostomata:
Example Limulus.
Palaemon (Prawn):
Gills are the chief respiratory organs. The body is divisible into cephalothorax and abdomen. The cephalothorax bears 13 pairs of appendages. The abdomen has 6 pairs of appendages. The appendages are useful in feeding, swimming, balancing etc. The posterior end of abdomen forms a conical telson. Prawn is edible.
Cancer (Crab):
Five pairs of thoracic legs are present in which first pair of legs forms enormous chelipedes.
Peripatus (The walking worm):
It has characters of phylum Annelida and phylum Arthropoda. Hence it is a “connecting link” between Annelida and Arthropoda.
Annelidian characters:
(i) Long, Worm-like body,
(ii) Segmentally arranged nephridia.
Arthropod characters:
(i) Antennae on the head,
(ii) Tracheae for gas exchange.
Limulus (The king Crab or Horse-shoe crab):
The king crab continues to remain unchanged for the past about 190 million years. It is, therefore, also called “living fossil”. Excretion takes place through four pairs of coxal glands. Respiratory organs are book gills.