It is mentioned that gill slits are absent in non chordates. But it is also mentioned that arthropods respire through gills ( both these things are mentioned in the textbook itself)

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Please find below the solution to the asked query

Gill slits or pharyngeal gill slits are the openings which lead to gills and are present between pharynx and the outside. These are at some stage found in all chordates.  The chordates mainly possess paired pharyngeal gill slits.
However, gill like structures are also found in some aquatic invertebrates like arthropods etc. They have gills which are mainly modified appendages. The structure and opening of these gills is different from the ones found in chordates. 
Hope this information will clear your doubts about the topic.

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