The study of cellular Biology Topics is essential to understanding the workings of all living organisms.
Introduction to Phylum Arthropoda
Phylum Arthropoda (Gr., arthros – jointed; podos – foot; jointed – legged animals)
1. Triploblastic, bilaterally symmetrical, and metamerically segmented animals.
2. Body segments are grouped into two regions- cephalothorax (head and thorax together) and abdomen, or three regions – head, thorax, and abdomen. The anterior part of the body forms a distinct head, bearing sense organs and a brain.
3. Exoskeleton of the cuticle, containing protein, lipid, chitin, and often calcium carbonate is secreted by the underlying epidermis and shed (moulted) at intervals.
4. They have a complex muscular system, with an exoskeleton for attachment, striated muscles for rapid actions, and smooth muscles for visceral organs. Cilia are absent.
5. Each body segment (somite) usually bears paired lateral and jointed legs or appendages.
6. Body cavity is hemocoel (i.e., filled with blood). The coeloms are reduced to the spaces of the genital and excretory organs.
7. Alimentary canal is complete, the mouth and anus lie at opposite ends of the body.
8. Respiration through the general body surface, by gills, air tubes (tracheae), or book lungs.
9. Circulatory system is open with the dorsal heart, arteries, and hemocoel (blood sinuses) but without
capillaries and veins. It means that blood does not flow in well-defined blood vessels.
10. True nephridia is absent. Excretion by coelomoducts, Malpighian tubules, or green or coxal glands.
11. Sexes are usually separate; sexual dimorphism is well-marked in several forms.
12. Fertilization is usually internal, oviparous, or ovoviviparous and often with metamorphosis.
13. Terrestrial or aquatic (fresh water and marine), free-living, commensal, or parasitic (on other animals and plants) forms.
14. Arthropoda forms the largest phylum of the Animal kingdom. About 900,000 species are known. Phylum
Arthropoda is divided into the following five classes:
Class 1. Onychophora – The body is elongated, segmented, and covered with a thin cuticle.
Example: Peripatus
Class 2. Crustacea – Head and thorax are fused together to form a cephalothorax, Respiration by gills.
Examples: Palaemon (prawn), Daphnia (water flea), Cyclops (one-eyed gian), Cancer (True crab).
Class 3. Myriapoda – The body is elongated, and metamerically segmented with each segment having one or two pairs of legs, Respiration by tracheae.
Examples: Scolopendra (centipede), and Julus (millepede).
Class 4. Insecta – The body is divisible into distinct head, thorax, and abdomen, Abdomen lacks appendages, the head has a pair of antennae and mouth parts and the thorax has three pairs of legs and two pairs of wings, Respiration by tracheae.
Examples: Lepisma (silverfish), Melanopus (grasshopper), Gryllus (house cricket), Periplaneta (cockroach), Musca (house fly), Apis (honey bee), Anopheles (mosquito), Culex (mosquito), Pieries (butterfly), Cimex (bed bug).
Class 5. Arachnida – Body divisible into cephalothorax and abdomen, Antennae and true jaws are absent, Respiration by tracheae, book-lungs or book-gills.
Examples: Limulus (king-crab), Palamnaeus (scorpion); Aranea (spider), Sarcoptes (itch-mites), Argus (tick).
Differences between Annelids and Arthropods
| Annelids | Arthropods |
| 1. Their appendages are unjointed. | 1. Their appendages are jointed. |
| 2. They have a closed circulatory system, i.e., their blood flows inside blood vessels. | 2. They have an open circulatory system, i.e., their blood flows through large sinuses or spaces. |
| 3. They have a well-developed true coelom. | 3. They have a small coelom. Instead, they have blood. Filled body cavities called hemocoel. |
| 4. A chitinous exoskeleton is absent in them. | 4. A chitinous exoskeleton is present in them. |
| 5. Their excretory organs are nephridia. | 5. Their excretory organs are green glands and Malpighian tubules. |
| 6. Their sensory system is less developed. | 6. Their sensory system is well developed. |
| 7. Their locomotory organs include parapodia and chaetae. | 7. Their locomotory organs are legs and wings. |
| 8. Examples: Earthworm, Nereis. | 8. Examples: Centipede, crab, cockroach. |