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Lower Invertebrates belong to the kingdom Animalia. The kingdom Animalia is divided into two groups i.e., Vertebrates and Invertebrates. The vertebrates can be distinguished from the invertebrate as the former have vertebral columns which the latter do not possess. Again, the phyla Invertebrates are divided into lower and higher invertebrates. They differ in the essential body organization and sizes. From the point of view of body structure, lower invertebrates are basic and they are usually smaller in size when compared to vertebrates.
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Key terms: Animal Kingdom Vertebrates, Invertebrates, Porifera, Platyhelminthes, Cnidaria/ Coelenterata, Ctenophore, Aschelminthes
About Lower invertebrates
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Lower invertebrates are those organisms that neither possess nor develop a vertebral column (it is commonly known as backbone or spine), which is derived from the notochord. Lower invertebrate includes all animals apart from the chordate subphylum Vertebrata. Lower invertebrates have a simple body structure. The phyla included in Lower invertebrates are: Porifera, Platyhelminthes, Cnidaria, Ctenophora, and Aschelminthes.
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Classification of lower invertebrates
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Phylum – Porifera
The species of this phylum is commonly called as sponges due to their appearance and there are about 5000 known species of this phylum.
Characteristics of Porifera
- The species belonging to this phylum are typically marine and are mostly asymmetric.
- These are multicellular animals i.e., have many cells and possess a cellular level of organization.
- Porifera possesses a system of transporting water or the canal. Water generally enters through a central cavity which is called spongocoel through a very minute pore called an ostiole in the body wall, from which it flows through the osculum. This water transport mechanism in Porifera is of great importance as it is useful in gathering food, gaseous exchange and removing waste.
- The spongocoel is lined with collar cells called Choanocytes and the canals.
- Digestion occurs intracellularly.
- The skeleton which is made up of spicules or spongin fibre supports the body.
- They are hermaphrodites i.e., the same individual produces eggs and sperms.
- Reproduction occurs asexually via division and sexually through gamete formation. Fertilization is internal and indirect development is observed which is marked by the larval stage.
Some examples of Porifera are Euplectella, Sycon, Spongilla.
Phylum- Platyhelminthes
The species belonging to this group are commonly called Flatworms due to their dorso-ventrally flattened body. Flatworms are mostly endoparasitic in nature found in animals including human beings. There are about 6500 species known which belong to this Phylum.
Characteristics of Platyhelminthes:
- They have a bilaterally symmetrical body which means if you cut the organism it will divide into 2 equal halves. They are also triploblastic and acoelomated animals that have an organ level of organization.
- Parasitic forms of Platyhelminthes possess hooks and suckers. Some of them absorb nutrients directly from the host by attaching them to the surface of their body.
- For osmoregulation and Excretion, specialized cells are presently called flame cells.
- There is internal fertilization and development is indirect as it undergoes many larval stages. Some species like Planaria possess high regeneration capacity.
- Some examples of Platyhelminthes are Taenia (Tapeworm), Fasciola (Liver fluke); etc.
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Phylum Cnidaria/ Coelenterata
Characteristics of Cnidaria/ Coelenterata:
- Cnidarians are aquatic, mostly marine, free swimming and are radially symmetrical in nature.
- The name “Cnidaria '' is derived from Cnidoblasts or Cnidocytes which is stinging capsules or nematocysts seen on the tentacles and the body.
- Cnidoblast performs functions like defence, anchorage and the capture of prey. The tissue level of organization is shown by Cnidarians and they are diploblastic.
- They possess a gastro-vascular cavity. Both intracellular, as well as extracellular digestion, is seen.
Cnidarians exhibit 2 basic body types, they are Polyp and Medusa.
- Polyp: Sessile and cylindrical form e.g. Hydra, Adamsia, etc.
- Medusa: Umbrella-shaped body and is free- swimming in nature e.g., Aurelia or jellyfish.
Both forms show Metagenesis (alternation of generation).
Some examples of Cnidarians are Physalia( Portuguese man-of-war), Adamsia(Sea-anemone) Gorgonia.
Phylum- Ctenophora
They are commonly called sea walnuts or comb jellies. Exclusively marine and have radial symmetry. Ctenophora are diploblastic with tissue level of organisation.
Characteristics of Ctenophora:
- The body of Ctenophora has 8 external rows of ciliated comb plates, which help in the locomotion of animals.
- Digestion is both extracellular and intracellular. They exhibit a well-marked property of Bioluminescence (living organisms that produce light).
- Fertilization is external with indirect development.
- Some examples of Ctenophora are Hormiphora (Sea-walnut), Ctenoplana, Cestum(The Venus’ girdle), Pleurobrachia.
Phylum Aschelminthes
The body of aschelminthes is circular in cross-section, hence named as roundworms. There are 10000 species well known of Aschelminthes.
Characteristics of Aschelminthes:
- They possess an organ- system of organization
- Body is bilaterally symmetrical and triploblastic.
- A well developed muscular pharynx is present.
- Roundworms are dioecious i.e., sexes are distinct.
- Fertilization is internal and development is direct.
Some examples of Aschelminthes are Ascaris, Ancylostoma (Hookworm), Wuchereria( Filaria worm).
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Things to Remember
- The species of the phylum Porifera is commonly called sponges.
- The species belonging to Platyhelminthes are commonly called Flatworms due to their dorso-ventrally flattened body.
- Ctenophora are called sea walnuts or comb jellies.
- The body of aschelminthes is circular in cross-section, hence named as roundworms.
- Dioecious means sexes are distinct.
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Sample Questions
Ques. Write a short note on lower invertebrates. (5 Marks)
Ans. The kingdom Animalia is divided into two groups i.e., Vertebrates and Invertebrates. The vertebrates can be distinguished from the invertebrate as the former has a vertebral column that the latter do not possess. Again, the phyla Invertebrates and divided into lower and higher invertebrates.
The lower invertebrate includes the following phyla:
- Phylum– Porifera
- Phylum- Platyhelminthes
- Phylum- Cnidaria/ Coelenterata
- Phylum- ctenophora
- Phylum– Aschelminthes
Ques. How do vertebrates differ from invertebrates? (5 Marks)
Ans. Below listed are some differences between the vertebrates and invertebrates:
| Invertebrates | Vertebrates |
|---|---|
| They do not possess a backbone. | They possess a backbone. |
| They have an exoskeleton. | They do not possess an exoskeleton. |
| Almost all invertebrates possess an open circulatory system. | All vertebrates have a closed circulatory system. |
| They have radial or bilateral body symmetry. | All the vertebrates have bilateral symmetry only. |
| They have a simple and unorganized nervous system. | They have complex and highly specialized nervous systems. |
Ques. List some of the characteristics of Phylum Porifera. (5 Marks)
Ans. Below listed are some of the characteristics of Phylum Porifera:
- They are typically marine and are mostly asymmetric.
- These are multicellular animals i.e., have many cells and possess a cellular level of organization.
- Porifera possesses a system of transporting water or the canal. Water generally enters through a central cavity which is called spongocoel through a very minute pore called an ostiole in the body wall, from which it flows through the osculum. This water transport mechanism in Porifera is of great importance as it is useful in gathering food, gaseous exchange and removing waste.
- The sponogocoel is lined with collar cells called Choanocytes and the canals.
- Digestion occurs intracellularly.
- The skeleton which is made up of spicules or spongin fibre supports the body.
- They are hermaphrodite i.e., eggs and sperms are produced by the same individual.
- Reproduction occurs as asexually via division and sexually through gamete formation. Fertilisation is internal and indirect development is observed which is marked by the larval stage.
Some examples of Porifera are Euplectella, Sycon, Spongilla.
Ques. Write a short note on Cnidaria. (3 Marks)
Ans. Cnidarians are aquatic, mostly marine, free swimming and are radially symmetrical in nature.
- The name “Cnidaria” is derived from Cnidoblasts or Cnidocytes which is a stinging capsule or nematocytes seen on the tentacles and the body.
- Cnidoblast performs functions like defence, anchorage and the capture of prey. The tissue level of organisation is shown by Cnidarians and they are diploblastic.
- They possess a gastro-vascular cavity. Both intracellular, as well as extracellular digestion, is seen.
Ques. Differentiate between Platyhelminthes and Aschelminthes. (5 Marks)
Ans.
| Feature | Platyhelminthes | Aschelminthes |
|---|---|---|
| Body structure | Flat body and have bilateral symmetry. | Cylindrical body. |
| Coelom | They are acoelomated as they do not possess coelom. | They are pseudocoelomates. |
| Development of organs | Restricted | Complex organ systems development is been observed. |
| Excretory structure | By Flame cells | By Renette cells |
| Gut | Incomplete and have a single opening. | Complete – with mouth and anus. |
| Other name | Flatworm | Roundworm |
Ques. Write a note on Polyp and medusa. (5 Marks)
Ans. Cnidarians exhibit 2 basic body types; they are Polyp and Medusa.
- Polyp: Sessile and cylindrical form e.g. Hydra, Adamsia, etc.
- Medusa: Umbrella-shaped body and is free- swimming in nature e.g., Aurelia or jellyfish.
Both forms show Metagenesis (alternation of generation).
Ques. In which Phyla will you classify the Comb jellies? (5 Marks)
Ans. Comb jellies belong to the class Ctenophora. They are also known as Sea walnuts. They are exclusively marine. Some more characteristics of class Ctenophora can be listed as follows:
- The body of Ctenophora has 8 external rows of ciliated comb plates, which help in the locomotion of animals.
- Digestion is both extracellular and intracellular.
- They exhibit a well-marked property of Bioluminescence.
- Fertilisation is external.
- Development is indirect.
- Examples: Hormiphora(Sea-walnut), Ctenoplana, Cestum(The Venus’ girdle), Pleurobrachia.
Ques. What is the name of those cells that helps Porifera in water circulation? (2 Marks)
Ans. Chanocytes that line the sponogcoel are the collared, flagellated cells that cover large parts of the inner chambers of the sponges or the Porifera that helps in water circulation.
Ques. What are Flame cells? (2 Marks)
Ans. Flame cells are found in Platyhelminthes. These are specialized cells and function as excretory organs in Platyhelminthes like flatworms. These are the simplest organisms that have a dedicated excretory system. The group of flame cells forms a bundle which are known as protonephridia.
Ques. What is a nematocyte? (3 Marks)
Ans. Nematocytes are found in the phylum Cnidaria like jellyfish and sea anemones. These are also called cnidocytes or cnidoblasts. These are highly specialized cells present in the tentacles of jellyfish and are capable of projecting a threadlike structure as a form of self-defence or to capture the prey.
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