## Basis of Classification of Invertebrates: Symmetry, Coelom, Segmentation, and Embryogeny
These four characteristics are fundamental in classifying invertebrates. Let's break down each:
### Symmetry
* **Asymmetry:** No definite shape or pattern. Example: Sponges.
* **Radial symmetry:** Body parts arranged around a central axis. Example: Jellyfish.
* **Bilateral symmetry:** Body can be divided into two equal halves along a single plane. Example: Insects, humans.
### Coelom
* **Acoelomate:** No body cavity. Example: Flatworms.
* **Pseudocoelomate:** Body cavity partially lined with mesoderm. Example: Roundworms.
* **Coelomate:** Body cavity completely lined with mesoderm. Example: Earthworms, insects.
### Segmentation
* **Metamerism:** Body divided into segments with repeated structures. Example: Earthworms, insects.
* **No segmentation:** Body not divided into segments. Example: Jellyfish, flatworms.
### Embryogeny
* **Germ layers:** Number of tissue layers formed during embryonic development.
* Diploblastic: Two germ layers (ectoderm and endoderm). Example: Jellyfish.
* Triploblastic: Three germ layers (ectoderm, mesoderm, and endoderm). Example: Flatworms, insects.
* **Protostomes:** Blastopore develops into the mouth. Example: Mollusks, arthropods.
* **Deuterostomes:** Blastopore develops into the anus. Example: Echinoderms.
[Image of germ layers and protostome vs. deuterostome development]
### Combined Classification
These characteristics are often used in combination to classify invertebrates into different phyla. For example:
* **Phylum Porifera** (sponges): Asymmetrical, acoelomate, no segmentation, diploblastic.
* **Phylum Cnidaria** (jellyfish): Radial symmetry, acoelomate, no segmentation, diploblastic.
* **Phylum Platyhelminthes** (flatworms): Bilateral symmetry, acoelomate, no segmentation, triploblastic.
* **Phylum Annelida** (earthworms): Bilateral symmetry, coelomate, segmented, triploblastic.
* **Phylum Arthropoda** (insects, spiders): Bilateral symmetry, coelomate, segmented, triploblastic.
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