Invertebrate Animals: Porifera, Cnidarians, Mollusks, Annelids, Arthropods
Invertebrate Animals: Porifera, Cnidarians, Mollusks, Annelids, and Arthropods
1. Porifera (Sponges)
Porifera, also known as sponges, are aquatic animals, generally marine, living fixed to rocks in shallow water. The body of a sponge has the form of a bag and is full of holes. Its walls are perforated by small pores, and part of it has a larger hole called an osculum.
2. Cnidarians
This group includes jellyfish, sea anemones, hydras, and corals. All these animals are aquatic and live in both fresh and saltwater, although most are marine. The body of cnidarians is reduced to a gastrointestinal cavity, which communicates with the outside through a single orifice, surrounded by tentacles, which serves as both the mouth and anus.
Cnidarians can exist in two different forms:
- Polyps: Sack-shaped organisms that live fixed to the ground.
- Medusa: Umbrella-shaped with a gelatinous consistency. They move through contraction of the body or are carried away by marine currents.
3. Mollusks
This group includes cuttlefish, squid, calamari, mussels, clams, oysters, slugs, and snails. Most of these invertebrates are aquatic, both freshwater and saltwater, though some, like land snails and slugs, live on land.
Despite their variety, all mollusks share common characteristics:
- A soft body, consisting of an outer layer called the mantle, which surrounds a set of internal organs (the visceral mass).
- An internal or external shell (with rare exceptions, such as the slug or octopus) which provides support and body protection.
- A muscular foot used for movement.
Most mollusks have a mouth and anus, as well as sensory organs for balance, smell, taste, touch, and sight.
Major Classes of Mollusks:
- Bivalves: Shells contain two parts joined by a hinge and have no distinct head. Examples include mussels, oysters, and razor clams.
- Gastropods: Shell usually consists of a coiled piece. They have a separate head with a mouth and four tentacles. This class includes slugs and snails, both aquatic and terrestrial.
- Cephalopods: The head has well-developed eyes and a mouth. Tentacles surround the mouth and are used to catch prey. Animals belonging to this class include cuttlefish, squid, and octopus.
4. Annelids
This group of invertebrates includes earthworms and leeches. Earthworms live in terrestrial environments with some moisture, while leeches live in aquatic environments.
The body of these animals is cylindrical and divided into rings or segments, both externally and internally, known as metamerism. In each segment, certain organs are repeated, such as digestive and excretory tubes, nerves, and some blood vessels. The mouth is located at the front of the body, and the anus is at the posterior.
5. Arthropods
Arthropods constitute the largest and most varied group in the animal kingdom, including creatures such as centipedes and prawns. They live in all known environments, both aquatic (fresh and saltwater) and terrestrial.
Arthropods are characterized by the following features:
- Segmented body
- Exoskeleton
- Jointed appendages