The spinnerets of spiders are carried on the fourth and fifth segments of the abdomen (opisthosoma). Primitively there are two pairs, anterior lateral (al) and anterior median (am) on the fourth segment, and two pairs, posterior lateral (pl) and posterior median (pm) on the fifth,amare present inLiphistiusbut are never functional. In mygalomorphsamare invariably absent, and usuallyalalso. In araneomorphsamare either reduced to a function‐less colulus, perhaps absent altogether, or represented by the cribellum, which is a specialized spinning organ.It seems unlikely that the lateral and median spinnerets correspond to the exopodites and endopodites of a biramous limb, which limbs are characteristic of the Crustacea, a group having no close relationship to the Arachnida. From embryology it seems clear that the lateral spinnerets are the segmental appendages. Glands, here described as the epiandrous glands, very similar to spinning glands, occur on the second abdominal segment of most male spiders. It is suggested that these may be serially homologous with the median spinnerets, which would then not be appendicular in origin but would be modifications of ventral glandular structure