In nearly all the Theraphosae the anterior pair are absent, while the posterior spinnerets are largely developed. In the Palpimanidae only the anterior spinnerets are present. When all six are found, the usual arrangement is in the form of a rosette, the median spinnerets being hidden by the others in repose, but this disposition is widely departed from. In _Hahnia_ (Agelenidae), for instance, they are ranged in a transverse row at the end of the abdomen, the posterior spinnerets occupying the extremities of the row, and the median ones the centre. These spinnerets are highly mobile appendages, and additional play is given to their action by the presence of articulations, much resembling the “false” joints sometimes found on the legs, on the posterior and anterior pairs. They are always at least bi-articulate, and sometimes present three or four joints. They are movable turrets on which are mounted the “fusulae” or projections where the tubes from the spinning glands open. These are often very numerous, especially in the orb-weaving spiders, where the spinning powers are most highly developed. They consist of two portions, a cylindrical or conical basal part, succeeded by a very fine, generally tapering tube. In some spiders the fusulae are all much alike, but usually a few very much larger than the rest are noticeable under the microscope, and these are often alluded to as “spigots.” The smaller ones are also divisible into two kinds, a few short conical fusulae being noticeable amongst the much more numerous cylindrical tubes. We shall treat of the functions of the various fusulae later (see pp. 335 and 349). Simon remarks that though the battery of fusulae is most complicated in those spiders which possess the greatest spinning powers, it is by no means among them that extremely long spinnerets are developed. The posterior spinnerets of some of the Hersiliidae are of great length, but these spiders spin very little except in forming their egg-cocoons. [Illustration: FIG. 182.—=A=, Spinnerets of _Amaurobius similis_ ♀. Much enlarged. _a_, Anus; _cr_, cribellum; _i.s_, inferior spinneret; _m.s_, median spinneret; _s.s_, superior spinneret. =B=, Part of the 4th leg of the same Spider, showing the calamistrum (_ca_) on the metatarsus. ] In addition to the six spinnerets, and just in front of them, there is to be found in some spiders an extra spinning organ in the form of a double sieve-like plate, the “cribellum.” This is always correlated with a comb of curved bristles on the metatarsi of the fourth pair of legs, the “calamistrum.” Such importance is assigned to these organs by Simon, that the Araneae Veraeare divided by him according to whether they are present or absent, into CRIBELLATAE and ECRIBELLATAE. This is probably an exaggerated view of the importance of these organs, and the spiders possessing them certainly do not seem to form a natural group.