On the other hand, in the reverse case, where the structure of the same joint is very similar, the nests may be wholly unlike, as in _Nemesia Eleanora_, Plate XII., p. 106, and _N. cæmentaria_, Plate VIII., where the nest of the former is of the double-door unbranched type, and that of the latter of the single-door cork type. It is probable however that a fuller and closer comparison of, and a more exact acquaintance with the several parts and their functions might show us that all spiders which spin similar webs are furnished with equivalent instruments, so that what one leg lacks another may possess in some shape or another; brushes of stiff hairs in one place, compensating for a toothed claw, or for a row of moveable spines in another.[59] [Footnote 59: The claws are probably of first-rate importance in this respect and should be most carefully studied. M. Lucas has stated that the claws of _Mygale Blondii_, and _M. nigra_ from Algiers, and of _M. nigra_ and _M. avicularia_ from Brazil, are retractile like those of a cat! Unfortunately the dwellings of these spiders have not been described. See Lucas (H.) in Rev. et Mag. de Zoologie, sér. 2, tom. ix. 1857, p. 587, and Ann. de la Soc. Entom. de France, ser. 3, tom. v. p. cxx., and vi. p. clxxi. Another curious point in which spiders differ is the presence or absence of viscidity in the hairs which clothe their feet and palpi. Mr. Blackwall states (Spiders of Great Britain and Ireland, vol. i. p. 13), that by far the greater number of the sub-order _Territelariæ_, or _Mygalidæ_ as he terms them, "have the inferior surface of their biungulate tarsi, and of the digital joint of their pediform palpi, in the females, densely clothed with compound, hair-like papillæ, constituting an apparatus which, by the emission of a viscous secretion, enables them to traverse the perpendicular surfaces of dry, highly polished bodies; others have three pairs of spinners and are destitute of hair-like papillæ on the legs and palpi." The four species of trap-door spider on the Riviera, here described, appear to form exceptions to this rule, however, for they all remained helpless prisoners when placed in a glass tumbler, though struggling vigorously for freedom. When, however relying on this experience, I placed a number of smaller spiders of different kinds in glasses for examination some walked straight out without any difficulty, while others were unable to climb up the sides.] It would be interesting, from this point of view, to draw all the parts which may be supposed to aid in the act of weaving, and so to contrast the corresponding limbs of different spiders, the nests of which are known, that one might see at a glance in what they differed and agreed. I have done this for the falces and the last joint of the foremost right foot of the four spiders figured in Plates VII., VIII., IX., and XII., but to make the case complete all the limbs should be represented in the same way.