Crustaceans Notes

17. Crustaceans Evolutionary Relationships Again, in the pancrustacean hypothesis, insects are shown as being derived from ancestral crustaceans. The stem group arthropods include extinct anomalocaris and opabinia but share characteristics with extant crown group arthropods. Key thing is according to sequencing DNA from 62 genes is that hexapods are nestled within the overall crustaceans, meaning crustaceans are paraphyletic. Also remipedes and cephalocarids (thought of as crustaceans are thought of as sister group to insects). Crustaceans came onto land and turned into insects, which never went back to the sea. Ecological division. Started with a homonomous annelid ancestor and in insects turned into thorax and abdomen but in crustacean there are a series of leg (pereon) and then gill segments (pleon) and a cephalon. Appendages are being adapted to niche. If you compare insect hox genes with crustacean, insects have gained extra hox genes to confer even more specialisation. Contain appendages that have been lost of never occurred in crustaceans. Variety Part of problem is that juvenile forms are completely bizarre, on top of huge morphological diversity. Substantial changes during development. Ontogeny recapitulates phylogeny - in development can see previous evolutionary stages. Notice that there is an 

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