What are Lower Invertebrates?
The phyla of Invertebrates were commonly called lower and higher invertebrates. In body organization the lower invertebrates are basic and usually smaller in size. The lower invertebrates include various phyla such as Porifera, Coelenterata(Cnidaria), Nematoda and Platyhelminthes.
Classification of Lower Invertebrates
Phylum Porifera
Species of this phylum are widely referred to as sponges and about 5000 species are known.
Porifera Characteristics
These are typically marine animals, and mostly asymmetric.
These are multicellular animals and possess cellular level of organisation.
Sponges have a system for transporting water or the canal. Water enters a central cavity (termed as spongocoel) through minute pores (ostia) in the body wall, from which it flows through the osculum. This water transport pathway is useful in gathering food, exchanging breaths and removing waste.
Spongocoel is lined with Choanocytes or collar cells and the canals. Digestion occurs intracellularly. Skeleton supports body and it is made up of spicules or spongin fibres.
Sponges are hermaphrodites (eggs and sperms are produced by the same individual).
Sponges reproduce asexually through division, and sexually through gametes development. Fertilization occurs internally, and indirect development is observed which has a larval stage.
Phylum Porifera Examples: Euplectella, Sycon, Spongilla.
[Image will be uploaded soon]
Phylum Platyhelminthes
These species are commonly called as flatworms because of their dorso-ventrally flattened body. These are mostly endoparasites found in animals, including humans. About 6500 species are known.
Platyhelminthes Characteristics
They have bilateral symmetrical body and they are triploblastic and acoelomate animals possessing organ level of organisation.
Parasitic forms include hooks and suckers. Some of them absorb nutrients directly from the host through the surface of their body.
Osmoregulation and Excretion occur through specialised cells called flame cells.
Fertilisation occurs internally and development undergoes many larval stages. Some species like Planaria have high regeneration capacity.
Phylum Platyhelminthes Examples: Taenia (Tapeworm), Fasciola (Liver fluke).
[Image will be uploaded soon]
Phylum Cnidaria / Coelenterata
These species are aquatic, mostly marine, sessile/free-swimming and they are radially symmetrical animals.
The name “cnidaria” is taken from the term cnidoblasts or cnidocytes (stinging capsules or nematocytes) seen on the tentacles and the body.
Cnidoblasts are used for defense, anchorage and for the capture of prey. Cnidarians show tissue level of organisation and they are diploblastic.
Cnidarians possess a central gastro-vascular cavity with one opening and a mouth on hypostome. Both extracellular and intracellular digestion is seen
Cnidarians exhibit two Basic Body types. They are: Polyp and Medusa.
Polyp: It is a sessile and cylindrical form example: Hydra, Adamsia, etc.
Medusa: It is an umbrella-shaped and free-swimming example: Aurelia or jelly fish.
Both forms show Metagenesis (alternation of generation).
Polyps generate medusae asexually and medusae generate the polyps sexually (example: Obelia)
For Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).
[Image will be uploaded soon]
Phylum Ctenophora
Ctenophores, usually called as sea walnuts or comb jellies. They are exclusively marine possessing radial symmetry. They are diploblastic organisms with tissue level of organisation.
Phylum Ctenophora Characteristics
The body possesses eight external rows of ciliated comb plates, which help in locomotion of an animal.
Digestion occurs both extracellularly and intracellularly. It has a well-marked property called Bioluminescence (living organism to produce light).
Fertilisation occurs externally with indirect development.
Phylum Ctenophora Examples: Pleurobrachia and Ctenoplana.
Classification of Phylum Ctenophora
Ctenophora classification is based on presence of tentacles on the body and they are classified into two classes, they are: Tentaculata and Nuda.
Class 1 Tentaculata
They exhibit tentacles and small stomodaeum.
Ctenophora examples with names: Hormiphora (The Sea Walnut), Ctenoplana, Cestum (The Venus’ Girdle), Pleurobrachia (The Sea gooseberry).
Class 2 Nuda
Species under this class don’t have tentacles. They exhibit spacious mouth and stomodaeum.
Example: Beroe.
[Image will be uploaded soon]
Phylum Aschelminthes/ Nematoda
The body of the aschelminthes is circular in cross-section, therefore, they are named as roundworms. They may be free living, terrestrial, aquatic and about 10000 species are well known.
Phylum Aschelminthes Characteristics
Roundworms possess organ-system level of body organisation.
These species are bilaterally symmetrical and triploblastic.
Well-developed muscular pharynx is observed.
They consist of excretory tube which removes body wastes from the body cavity via excretory pore.
They are dioecious (Sexes are separate), it means males and females are distinct.
Fertilisation occurs internally and direct development is observed
Phylum Aschelminthes Examples: Ascaris (Roundworm), Ancylostoma (Hookworm), Wuchereria (Filaria worm).
[Image will be uploaded soon]
FAQs on Animal Animalia Lower Invertebrates
1. What are lower invertebrates?
Lower invertebrates are animals that lack a backbone and are characterised by a relatively simple body structure. As per the CBSE syllabus for 2025-26, this group primarily includes organisms from the phyla Porifera, Cnidaria, Ctenophora, Platyhelminthes, and Aschelminthes. They represent the earliest and most primitive forms of animal life.
2. What are the main characteristics of lower invertebrates?
Lower invertebrates share several fundamental characteristics that distinguish them from more complex animals. These features include:
Level of Organisation: They exhibit either a cellular (e.g., Porifera) or tissue-grade (e.g., Cnidaria) level of body organisation, lacking complex organs and organ systems.
Germ Layers: Most are diploblastic, possessing only two primary germ layers (ectoderm and endoderm).
Body Cavity (Coelom): They are typically acoelomates (lacking a body cavity) or pseudocoelomates (possessing a false coelom).
Symmetry: Body symmetry is often radial, biradial, or bilateral.
3. What are some examples of lower invertebrates from different phyla?
Here are some common examples of lower invertebrates, classified by their phylum:
Phylum Porifera: Sponges like Sycon and Spongilla.
Phylum Cnidaria: Hydra, jellyfish (Aurelia), and sea anemones (Adamsia).
Phylum Ctenophora: Comb jellies like Pleurobrachia.
Phylum Platyhelminthes: Flatworms such as tapeworms (Taenia) and liver flukes (Fasciola).
Phylum Aschelminthes: Roundworms like Ascaris (the common roundworm).
4. How do lower invertebrates differ from higher invertebrates?
The primary difference between lower and higher invertebrates lies in their structural complexity. Lower invertebrates possess a simple body plan with cellular or tissue-level organisation and are mostly acoelomates. In contrast, higher invertebrates (like Annelids, Arthropods, and Molluscs) exhibit an organ-system level of organisation, are triploblastic (having three germ layers), and possess a true body cavity (coelom), which allows for the development of more complex organ systems.
5. Why are sponges (Phylum Porifera) considered the most primitive of the lower invertebrates?
Sponges are considered the most primitive multicellular animals because they have a cellular level of organisation. This means their body is just an aggregation of different types of cells that are not organised into true tissues or organs. The presence of a unique water canal system for feeding, respiration, and excretion, rather than specialised organ systems, further highlights their primitive nature in the animal kingdom.
6. What is the key difference between Cnidaria and Ctenophora?
The most fundamental difference lies in their method of capturing prey. Cnidarians possess stinging cells called cnidoblasts or nematocysts. Ctenophores, on the other hand, lack these stinging cells and instead have specialised sticky cells called colloblasts (or lasso cells) on their tentacles to trap prey. Additionally, cnidarians typically show radial symmetry, while ctenophores exhibit biradial symmetry.
7. What is the evolutionary significance of the 'tube-within-a-tube' body plan seen in Aschelminthes?
The 'tube-within-a-tube' body plan, seen for the first time in phylum Aschelminthes, is a major evolutionary advancement. It signifies the development of a complete digestive tract with a separate mouth for ingestion and an anus for egestion. This allows for more efficient, one-way digestion and absorption of food, a significant improvement over the incomplete 'blind-sac' plan of phyla like Platyhelminthes where a single opening serves as both mouth and anus.
8. What does it mean for an animal to be diploblastic, and which lower invertebrate phyla show this?
An animal is described as diploblastic if its cells are organised into two primary embryonic layers: an outer ectoderm and an inner endoderm. A non-cellular layer called mesoglea may be present between them. This level of organisation is characteristic of lower invertebrates from the phyla Cnidaria (like hydra and jellyfish) and Ctenophora (comb jellies).
