Animal Animalia Lower Invertebrates

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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.

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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).

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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. 

  1. Polyp: It is a sessile and cylindrical form example: Hydra, Adamsia, etc.

  2. 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).

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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.

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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).

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FAQ (Frequently Asked Questions)

Q1. What are some Lower Invertebrates?

Ans. Lower invertebrates include species which belong to phylum Porifera, Cnidaria, Ctenophora, Platyhelminthes, Aschelminthes.

Q2. What are Lower Animals?

Ans. Lower animals include fishes and invertebrates and they are the dominant species on the planet, both in numbers and diversity.

Q3. What is the difference between Cnidaria and Ctenophora?

Ans. Cnidarians and Ctenophores are diploblastic animals.

Cnidarians show radial symmetry while ctenophores show biradial symmetry. They both have tentacles, covering their mouth. Its body symmetry is the key difference between cnidarians and ctenophores.