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The Invertebrates unit explores six groups of invertebrates— poriferans (sponges), cnidarians (such as sea jellies and corals), echinoderms (such as sea urchins and sea stars), mollusks (such as octopuses, snails, and clams), annelids (worms), and arthropods (such as insects, spiders, and lobsters).

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K-An arthropod has an open circulatory system
-An arthropod's body can be divided vertically into two mirror images.
-Arthropods have bodies that are internally and externally segmented.
-Arthropods are invertebrates, which means their bodies do not have internal bones for support. To compensate for this, they produce a hard exoskeleton made of chitin, a mixture of lipids, carbohydrates and protein, which covers and protects their bodies like a suit of armor.

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A hard exoskeleton made of calcium - no internal skeleton.

The head has two compound eyes, two pairs of antennae, and three pairs of mouthparts.

A pair of green glands excrete wastes near the base of antennae.

The abdominal segments have swimmerets (swimming legs)

The sexes are separate. Eggs are attached to the swimmerets (swimming legs) of the female. The first pair is enlarged in the male (it is used to pass sperm to the female).

The tail is fan-shaped, and ends in uropods and a telson.

The circulatory system is open; there is no heart and the "blood" is pumped by vessels into sinuses, and does not flow in a closed loop).

The nervous system consists of a primitive ventral nerve cord and ganglia system (similar to those of an earthworm).

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Insects belong to the phylum arthropoda, which means literally ''jointed legs''. As you may have guessed, this is one trait seen in all arthropods, including insects as well as animals like arachnids (spiders) and crustaceans (crabs).

Before delving into the characteristics of insects, it is important to clarify one point. People often refer to insects as bugs, but this is not always true. While it would be correct to say that all bugs are insects, we cannot say that all insects are bugs. True bugs belong to an order of their own, which we will discuss in a bit.

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Arachnids have paired, jointed appendages (parts that are attached to the main body), a hardened exoskeleton (exo means "outer"), a segmented (divided into parts) body, and a well-developed head. Their body consists of two main parts: a fused head and thorax, and an abdomen.

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-The four recognized classes of Cnidaria are: Hydrozoa, Cubozoa, Scyphozoa and Anthozoa.
-The majority of Cnidarians share the characteristic of radial symmetry. Radial symmetry means symmetry around a center point such that any line drawn through the center of the organism divides the body into mirror images.
-All Cnidarians share particular trait: tentacles with stinging nematocysts
-Cnidarians have complex reproductive cycles. Many cnidarians have an asexual stage, usually in the form of asessile polyp which asexually produces other polyps and medusae.

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All worms are bilaterally symmetrical, meaning that the two sides of their bodies are identical. They lack scales and true limbs, though they may have appendages such as fins and bristles. Many worms have sense organs to detect chemical changes in their environments, and some have light-sensing organs.

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1. NUnsegmented soft body with bilateral symmetry

2. Presence of an internal or external shell

3. A toothed tongue (made mostly of chitin) called the radula

4. A mantle which is a fold in the body wall that lines the shell

5. Muscular foot (and/or tentacles in some).

6. Possession of a fluid-filled cavity called coelom

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These are exclusively marine animals.

The larval forms show bilateral symmetry and adult forms show radial symmetry.

They are triploblastic.

It exhibits organ system grade of organisation.

They have a true coelom.

The body is uniquely shaped. It can star like, elongated or spherical.

The body is unsegmented without a head.

The body surface is covered with calcareous spicules.

Body cavity has the distinguishing water vascular system.

Tube feet help in locomotion.

The brain is absent but a nervous system is present with a nerve ring and radial nerve cords.

Respiration occurs through tube feet and gills.

Sense organs are poorly developed and include tactile organs, chemoreceptors, terminal tentacles etc.

Sexes are separate.

Fertilisation is external.

Lost parts can be regenerated.

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Poriferans are commonly called sponges.

These are multicellular organisms which are sessile/sedentary in nature.

Most of them are marine while a few are fresh water forms.

Body is cylindrical, asymmetrical or has radial symmetry.

Body wall is diploblastic which consists of an outer layer called pinacoderm and an inner layer called choanoderm.

The middle layer is called mesenchyme which has various kinds of wandering amoebocytes.

Body wall contains numerous pores called ostia through which water enters in the body through a canal system into the central body cavity, called spongocoel.

The canal syytem mainky comprise of incurrent and excurrent canals which are lined with pinacocytes; and radial canals which are lined with choanocytes (flagellated cells).

Body also contains one or more openings called oscula (-um/singular) through which water passes out from the body.

Sponges feed on minute organisms and small organic particles which enter the body through water current and are ingested by the choanocytes.

Respiration takes place by diffusion of oxygen from water the flowing into the body.

Sponges have an exoskeleton which is made up of either spongin fibers or calcareous/siliceous spicules or a combination of both.

Excretory product, mainly ammonia, is released from the body through outgoing water current.

Reproduction may be asexual or sexual.

Asexual reproduction takes place by external or internal budding (formation of gemmules) or by disintegration of body into reduction bodies.

Sexual reproduction involves internal fertilization. Male gametes enter the sponge body through water current and fertilize with ovum with the help of choanocytes.