Animal Kingdom: A Comprehensive Overview
Sponges: Porifera
Sponges are living organisms that are fixed to the substrate. Their body is perforated by many pores that communicate with each other. They maintain their structure thanks to a skeleton made of a soft material called spongin. Sponges feed by filtering water.
Polyps and Medusas: Cnidaria
Cnidarians exhibit radial symmetry and can take two forms:
- Polyps: Have a sac-like body with a suction cup for attaching to the substrate.
- Medusas: Have an umbrella-shaped body. Polyps appear inverted and flattened.
Cnidarians are carnivorous and capture their prey with tentacles. Their skin contains tiny capsules filled with venom to paralyze their prey.
Worms: Annelida
Most annelids are aquatic. They have a soft, elongated body divided into rings. Each ring bears fine needles called setae, which aid in movement. The clitellum is involved in reproduction. Annelids have bilateral symmetry. Their skin is moist and serves as a respiratory organ. Earthworms excavate galleries in wet soils rich in organic matter, which serves as their food.
Mollusks
Most mollusks are marine. Their body is not divided into rings and is soft. They have a muscular foot used for movement. They have a fold called the mantle, which covers the dorsal body surface and contains respiratory organs.
Mussels: Attach to rocks with a black shell consisting of two valves. They have a hatchet-shaped foot that they use to move around. Mussels are filter feeders, and water enters and leaves their body through two holes or siphons, leaving behind particles that serve as food.
Squid: Are very active animals. They have a shell reduced to a thin sheet, called a pen. They have a number of tentacles with suckers that extend from their head. Squid are carnivores and use their tentacles to catch their prey. They swim quickly by forcefully expelling water from their mantle cavity through a siphon. The siphon is mobile, allowing squid to change direction. When attacked, squid eject ink to confuse predators and escape.
Arthropods: Arachnids, Crustaceans, and Myriapods
The main groups of arthropods are arachnids, crustaceans, and myriapods. Arthropods have a segmented body, consisting of a head, thorax, and abdomen. They have jointed appendages and a hard exoskeleton.
Arachnids
Arachnids have a fused head and thorax, forming a cephalothorax. Their abdomen is not segmented, and the spinnerets, which produce silk, are located at its end. Arachnids are carnivorous.
Crustaceans
Crustaceans are aquatic arthropods with a hard exoskeleton made of calcium carbonate. Their cephalothorax bears two pairs of sensory antennae and five pairs of legs, the first pair modified into claws. Their abdomen is segmented, and their appendages are not used for locomotion. Some crustaceans are scavengers, filter feeders, or parasites.
Myriapods
Myriapods are terrestrial arthropods with a head bearing a pair of antennae. They have numerous body segments, each with one or two pairs of legs.
Arthropods: Insects
Insects have a head with a pair of antennae and eyes. The thorax has three segments and bears three pairs of legs, one pair on each segment. The abdomen is segmented. Insects, like most invertebrates, lay eggs.
Echinoderms
Echinoderms are marine invertebrates with a skeleton made of calcareous plates. Sea stars have a flattened body consisting of a central disc from which five arms extend. They exhibit radial symmetry. Many sea stars are carnivorous and feed on other invertebrates, including other sea stars.