2. Tracheal gills are not always homodynamous or morphologically equivalent. In Ephemeridæ, some are dorsal in position, some ventral (first abdominal pair in _Oligoneuria_ and _Rhithrogena_); they may be cephalic, springing from the base of the maxilla, as in _Oligoneuria_ and _Jolia_; _Jolia_ has a branchial tuft at the insertion of each of the fore legs.[78] In Perlidæ the tracheal gills may have a tergal, pleural, sternal, or anal insertion. In some Libellulidæ also, anal leaflets occur.[79] [78] We take these instances from Eaton, Monograph of Ephemeridæ, Linn. Trans., 1883, p. 15. [79] Charles Brongniart has lately described a fossil Insect from the Coal Measures of Commentry, which he names _Corydaloides Scudderi_, and refers to the Pseudo-Neuroptera. In this Insect every ring of the abdomen carries laminæ, upon which the ramified tracheæ can still be made out by the naked eye. Stigmata co-existed with these tracheal gills. (Bull. Soc. Sci. Nat. de Rouen, 1885.) Some Crustacea are furnished with respiratory leaflets, curiously like those of Tracheates, with which, however, they have no genetic connection. In Isopod Crustacea the exopodites of the anterior abdominal segments often form opercula, which protect the succeeding limbs. In the terrestrial Isopods, _Porcellio_ and _Armadillo_, these opercula contain ramified air-tubes, which open externally, and much resemble tracheæ. The anterior abdominal appendages of _Tylus_ are provided with air-chambers, each lodging brush-like bundles of air-tubes, which open to the outer air. Lamellæ, projecting inwards from the sides of the abdominal segments, incompletely cover in the hinder part of the ventral surface of the abdomen, and protect the modified appendages. (Milne Edwards, Hist. Nat. des Crustacés, Vol. III.) 3. Tracheal gills never perfectly agree in position and number with the stigmata throughout the body. Sometimes they occur on different rings, sometimes on different parts of the same ring. Gegenbaur’s statements on this point are incorrect. 4. Tracheal gills may co-exist with stigmata. In Perlidæ the tracheal gills persist in the imago, and may be found, dry and functionless, beneath the stigmata. In Trichoptera they gradually abort at successive moults, and in some cases remain after the stigmata have opened. 5. Stigmata do not form by the breaking off of tracheal appendages, but by the enlargement of rudimentary tracheal branches, which open into the main longitudinal trunks. In larvæ with aquatic respiration these branches exist, though they are not functional.