The Head of Onychophorans
The brain of onychophorans has been recently re-investigated and shown to possess two unusual features. First, although the mouth is ventral, as is the case in euarthropods, it is innervated from three different places; the sides, the posterior, and by a nerve that originates dorsally, and passes anteriorly down to curve back to the front of the mouth. This set of innervation makes sense if the mouth of onychophorans was originally terminal and has been bent downwards. Second, the antennae of the onychophorans appear to be innervated from in front of the eyes; which in euarthropod terms implies a protocerebral (or potentially even more anterior) innervation. This is supported by gene expression data, which show that the jaws too are derived from a protocerebral or dueterocerebral segment. As all euarthropod antennae are deuterocerebral or tritocerebral, this implies that the onychophoran antennae are not homologous to any euarthropod ones.
The tritocerebrum in arthropods is homologous to the third head segment in onychophora, which bears the slime glands (a pair of highly modified appendages).
Read more about this topic: Arthropod Head Problem
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