An arachnid was type of multi-legged invertebrate arthropod. Varieties of these species have been found on many planets, indicating a similarity of evolution among lifeforms.
When Reginald Barclay showed fear of transporters while working with Miles O'Brien in 2369, O'Brien comforted him by explaining that he knew how Barclay was feeling. When Barclay inquired if O'Brien was fearful of transporters as well, O'Brien explained, "No. Arachnids. Sickening, crawly little things, don't you think? All those legs." (TNG: "Realm Of Fear")
The following year, after inadvertently being patient zero of "Barclay's Protomorphosis Syndrome", Barclay was discovered in main engineering by Captain Jean-Luc Picard and Data, after having been de-evolved, or partially transformed, into an arachnid. (TNG: "Genesis")