Homologous Structures in Other Phyla
Discussions of homology commonly limit themselves to the limbs of tetrapod vertebrates, occasionally touching on other structures, such as modified teeth as in whales and elephants. Homologies provide important insights into classification elsewhere in the animal kingdom, although some of them may be highly counterintuitive. For example, within the arthropods, Brusca and Brusca provide the following homologies for the first 10 somites (embryonic segments) in several groups of arthropods, but add that "...the subject of head appendage homology among the arthropods is quite unsettled and highly controversial..."
| Somite | Trilobite | Spider (Chelicerata) |
Centipede (Uniramia) |
Insect (Uniramia) |
Shrimp (Crustacea) |
|---|---|---|---|---|---|
| 1 | antennae | chelicerae (jaws and fangs) | antennae | antennae | 1st antennae |
| 2 | 1st legs | pedipalps | - | - | 2nd antennae |
| 3 | 2nd legs | 1st legs | mandibles | mandibles | mandibles (jaws) |
| 4 | 3rd legs | 2nd legs | 1st maxillae | 1st maxillae | 1st maxillae |
| 5 | 4th legs | 3rd legs | 2nd maxillae | 2nd maxillae | 2nd maxillae |
| 6 | 5th legs | 4th legs | collum (no legs) | 1st legs | 1st legs |
| 7 | 6th legs | - | 1st legs | 2nd legs | 2nd legs |
| 8 | 7th legs | - | 2nd legs | 3rd legs | 3rd legs |
| 9 | 8th legs | - | 3rd legs | - | 4th legs |
| 10 | 9th legs | - | 4th legs | - | 5th legs |
Read more about this topic: Homology (biology)
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