Sponges and Coelenterates: Characteristics and Classification
Characteristics of Sponges
There are some 5,000 species of sponges. These multicellular animals are aquatic, primarily marine, and come in different colors, ranging from 1 to 200 cm in size. They often exhibit cup or glass-like shapes. Bath sponges are derived from the spongin skeletons of certain species.
In a vertical section of a sponge, different organized cells can be observed:
- Flagellated cells are present inside.
- Epithelial cells line the outer surface.
- Central amoeboid cells exist, which perform various roles.
Other cells, such as those forming the inorganic spicules of the skeleton, are also present. These spicules are the only remaining cells when the sponge dies. In a simple sponge, water enters through the pores, passes through the central cavity, and exits through the top opening.
Sponges can reproduce asexually by budding, where buds grow at the base of the sponge and eventually become independent. Most sponges are hermaphroditic, meaning the same individual produces both eggs and sperm.
Classification of Sponges
- Calcarea: Skeleton consists of calcium carbonate spicules.
- Hexactinellida: Skeleton formed by spongin fibers.
- Demospongiae: Skeleton formed by spongin fibers.
Coelenterates
Hydras, jellyfish, sea anemones, and corals are examples of coelenterates. This phylum encompasses about 10,000 species of extraordinary beauty. They constitute the only important group that exhibits radial symmetry throughout its life cycle.
The body of a coelenterate is organized like a bag, with its interior being a digestive cavity that communicates with the outside through a mouth. It consists of two tissue layers: the epidermis and the gastrodermis. Between these two layers is a jelly-filled layer called the mesoglea.
Coelenterates have small bodies capable of producing skin irritation. In bathing waters where hydras and jellyfish abound, they often cause a rash on the skin. They have gelatinous bodies without skeletal structures, although some, like corals, are sessile. They live in both marine and freshwater environments. Inside the digestive cavity, enzymes are released to break down food, which is then partially digested.
Coelenterates present in two forms: polyp and medusa.
Classification of Coelenterates
- Hydrozoa: Includes hydra and some types of jellyfish found in marine and freshwater environments.
- Scyphozoa: Free-living animals, including umbrella-shaped jellyfish.
- Anthozoa: All are sedentary polyps with tentacles.
External Morphology of Coelenterates (e.g., Hydra)
At its rear end, a hydra has a basal disc, which it uses to attach to aquatic plants. At the anterior end, the mouth opens into a region called the hypostome. Around the mouth are tentacles, which are long, extensible, mobile, and vary in number from 6 to 10.
Internal Morphology of Coelenterates (e.g., Hydra)
The body wall encloses an internal cavity called the gastrovascular cavity, which communicates with the outside through the mouth. The tentacles are also hollow, closed at their ends, and open on the side of the gastrovascular cavity. The ectoderm cells contain nematocysts, which are specialized stinging cells with a coiled filament.
Classes of Nematocysts
- Penetrating: The discharge is stuck into the victim’s skin.
- Enclosing: The discharge is fixed on the hairs or bristles of the victim.
- Thorny binders: Its filament is arranged in a helix with small spines.
- Smooth binders: The filament has no spines.
Locomotion, Feeding, Respiration, Excretion, and Reproduction of Coelenterates (e.g., Hydra)
- Locomotion: They move by rotation or dragging. To rotate from one place to another, they bend and fix their tentacles.
- Feeding: They feed mainly on small aquatic animals, such as fish fry, crustaceans, and insect larvae.
- Respiration: They have no special respiratory organs. Oxygen is taken directly from the dissolved oxygen in the aquatic environment.
- Excretion: Waste products, such as nitrogenous compounds and carbon dioxide, are expelled to the outside by diffusion.
- Reproduction: They reproduce both asexually and sexually.