Sponge vs Hydra: Structure, Function, and Key Differences
Every organism is unique in its shape and characteristic in kingdom plantae.
Hydra has a cylindrical and radially symmetric body from 2 to 20 mm in length. It is visible to the naked eye when fully extended. Hydras are attached to some underwater objects, waving their tentacles slowly with the current. Sponges are multicellular organisms belonging to the phylum Porifera. They are characterised by the presence of pores all over the body.
The body of the hydra retracts from sensing any danger and appears to be shorter and rounder. A small hydra appears to be transparent in its tentacles and yellowish in its tube-like body. Hydra oligarchies are brown, also known as the brown hydra.
What is a Hydra?
Hydra is a simple invertebrate animal, having two layers of body cells. They are found in freshwater and other water bodies including ditches, ponds, slow-flowing rivers, and streams. Their body is tubular and radially symmetric. They have a central opening through which they take in food and expel waste.
Hydra is classified under the phylum cnidaria and belongs to the class hydrozoa. They consist of stinging cells on their tentacles and are microscopic in size. They belong to mostly unpolluted bodies of water and have a low tolerance for pollution. They are reproduced by budding.
Budding refers to the forming of a lump by the offspring of the parent and they break off as soon as they become big enough to live by themselves. They move in a kind of slow somersaulting motion if they feel the need to.
Hydra Structure
Hydra viridissima is commonly called green hydra because of its distinctive colour due to the symbiotic green algae Chlorella that lives within its body.
The outer layer of the hydra is the epidermis which can secret a protective layer of hydramacin against bacterial infection. Respiration and excretion in hydra occur by diffusion through the surface of the epidermis.
The inner layer is called the gastrodermis lining the stomach. Mesoglea is a gel-like structure present between the two layers. The bottom end of a hydra is adhered to the surface by a simple foot called the basal disc. Gland cells in the basal disc secrete a sticky fluid that is responsible for its adhesive properties. Hypostome is a mouth opening at the free end of the body that is surrounded by tentacles. The mouth is also connected to the body cavity.
The waste after digestion will be discharged from the same opening where the food is engulfed since hydra has only one opening. Cnidocytes are the cells that cover the tentacles.
Hydra Structure
What is a Sponge?
Sponge refers to the multicellular aquatic animal that belongs to the phylum Porifera.
Mostly 98 percent of sponge species are marine; however, the members of one family, the spongillidae, are found in freshwater.
Structure of Sponge
Sponges are hollow cylinders having a large opening at the top through which wastes and water are excreted. Their body is a fingerlike or treelike shapeless mass ranging in diameter from approximately 1 inch to several yards. Sponges can reproduce asexually or sexually. Sponges have been used in holding water, bathing, and scrubbing; because of overharvesting and newer technologies, most sponges sold today are synthetic.
Types of Digestion Occur in Sponges
Sponges live in close contact with water, which plays an important role in their feeding, gas exchange, and excretion. Most of the part of the sponge is attached to moving water through the body, so it can filter out food, absorb oxygen, and excrete wastes. Water is entered through the spongocoel from numerous pores in the body wall and flows out through a large opening known as the osculum. Sponges consist of an outer layer and an inner layer of flattened cells called choanocytes which are separated by a jelly-like substance called mesohyl.
The mesohyl consists of embedded amoeboid cells that secrete tiny needles called spicules or protein fibres which help the sponge to attain its structural strength. The process through which digestion occurs in sponges is intracellular digestion.
The beating of flagella from all choanocytes helps water move through the sponge. Food particles are confined in mucus produced by the sieve-like collar of the choanocytes and are ingested by phagocytosis. Amoebocytes are responsible for nutrients repackaged in food vacuoles of the choanocytes and help them deliver to other cells within the sponge.
Difference Between Hydra and Sponge
Interesting Facts
A single hydra consists of 50,000 to 1,00,000 cells. Some species of hydra can extend longer up to 5 cm to 20cm.
The unique feature of Sponges is that they lack organs and individual cells digest food, excrete waste, and absorb oxygen.
Important Questions
Which kingdom does sponge and hydra belong to?
Ans: Sponge and hydra belong to the kingdom plantae. Sponge belongs to phylum Cnidaria and Hydra belongs to phylum Porifera.
What type of reproduction does sponge and hydra undergo?
Ans: Sponges undergoes both asexual and sexual reproduction and hydra undergoes budding. In budding, a small bud grows near the basal part of the hydra.
Key Features
Sponges reproduce both sexually and asexually. Asexual reproduction takes place either by fragmentation or budding.
Asexual reproduction found in freshwater sponges occurs through the formation of gemmules, clusters of cells surrounded by a tough outer layer.
Ninety percent of the world's sponges are found within the class Demosponge, which contains 4,750 species. These sponges vary from 3 millimetres to over 2 metres in length.
FAQs on Difference Between Sponge and Hydra
1. What are sponges and Hydra in biology?
Sponges and Hydra are simple, multicellular aquatic invertebrates. Sponges belong to the Phylum Porifera and are characterised by a porous body and a cellular level of organisation. They are among the simplest of all animals. Hydra belongs to the Phylum Cnidaria (or Coelenterata) and represents a more complex tissue level of organisation, known for its stinging cells called cnidoblasts.
2. What are the primary differences between a sponge and a Hydra?
The primary differences between a sponge and a Hydra can be understood based on their biological classification and body plan:
Phylum: Sponges are in Phylum Porifera, while Hydra is in Phylum Cnidaria.
Symmetry: Sponges are mostly asymmetrical, meaning they have no defined body shape. Hydra has a distinct radial symmetry.
Level of Organisation: Sponges have a cellular level of organisation where cells are relatively independent. Hydra has a tissue level of organisation with distinct cell layers (epidermis and gastrodermis).
Body Cavity: Sponges have a central cavity called the spongocoel, which is part of their water canal system. Hydra has a single body cavity called the gastrovascular cavity or coelenteron, which functions as both a mouth and an anus.
Unique Features: Sponges are defined by their unique water canal system for feeding and respiration. Hydra is defined by its tentacles and specialised stinging cells called cnidoblasts (or nematocysts) for capturing prey and defence.
3. Why is a sponge's body organisation called 'cellular level' while Hydra's is 'tissue level'?
This distinction highlights a major evolutionary step. A sponge's body has different types of cells (like choanocytes and amoebocytes) that perform specific functions, but these cells do not form coordinated tissues or organs. They exhibit a division of labour at a cellular level. In contrast, Hydra's cells are organised into two distinct layers: an outer epidermis for protection and an inner gastrodermis for digestion. The cells within each layer work together as a cohesive unit, which is the definition of a tissue. This tissue-level organisation allows Hydra to perform more complex, coordinated actions like movement and systematic digestion.
4. How do the feeding and digestion processes differ between sponges and Hydra?
The feeding and digestion methods reflect their different body structures. Sponges are filter-feeders. They draw water into their body through numerous pores (ostia) and capture microscopic food particles using specialised collared cells (choanocytes). Digestion is entirely intracellular, meaning it occurs inside individual cells. In contrast, Hydra is a predator. It uses its tentacles armed with stinging cnidoblasts to paralyse and capture larger prey like small crustaceans. The prey is then pushed into the gastrovascular cavity where digestion begins extracellularly (outside the cells) before being completed intracellularly.
5. Why do sponges and Hydra not require specialised respiratory or circulatory systems like blood?
Sponges and Hydra do not need complex systems like blood because of their simple body plans. Both organisms have bodies that are only two cell layers thick, and every cell is in direct or very close contact with the surrounding water. This proximity allows for the exchange of gases (oxygen and carbon dioxide) and the removal of metabolic waste directly with the environment through a process called simple diffusion. The water that flows through a sponge's canal system or into a Hydra's gastrovascular cavity effectively acts as a transport medium, making a formal circulatory system unnecessary.
6. What are the common methods of reproduction in sponges and Hydra?
Both sponges and Hydra can reproduce asexually and sexually, which is common in simple animals.
Asexual Reproduction: The most common method for both is budding, where a new individual grows as an offshoot from the parent's body. Sponges can also reproduce through fragmentation and by forming internal buds called gemmules to survive harsh conditions.
Sexual Reproduction: Both produce gametes (sperm and eggs). Sponges are typically hermaphrodites, releasing sperm into the water to fertilise eggs in another sponge. Hydra can have separate sexes or be hermaphroditic, developing temporary gonads (testes and ovaries) on the body wall to produce gametes for fertilisation.
