362 NATURAL ARRANGEMENT OF INSECTS. 



such instance, in fact, can be cited. Cuvier, indeed, has 

 done this in the case of the Syngnathida' among fishes, — 

 a single genus, which, because its branchia? are in tufts, 

 instead of being pectinated, as in most of the other 

 osseous fishes, he detaches as the type of a distinct 

 order; and yet the genus Heterobranchus, which has two 

 sorts of branchiae*, (a structure unexampled among 

 the whole class of Pisces,) is left with its natural 

 allies, and is only distinguished as a genus in the fa- 

 mily of SiluridcB. Among the testaceous Mollusca, 

 again, the variations in the mode of breathing are still 

 more remarkable ; and yet they are considered but of 

 very inferior moment, when compared with other points 

 of structure. Still more is it apparent in the class of 

 Amphibia, where we find the branchia? external in one 

 group, internal in another, and absent in a third ; while, 

 among the salamanders ( Urodelci), the young have ex- 

 ternal branchiae, and the adult is furnished with cellular 

 lungs. The frogs (^Anoura), when tadpoles, have ex- 

 ternal branchiae, which disappear in the perfect animal, 

 and become internal. The whole of the Animal King- 

 dom, in short, exhibits instances of this. It has been 

 observed by a well-known writer, that Cuvier never 

 made a more palpable mistake, than when he attempted 

 to arrange the Mollusca according to their several systems 

 of generation and — respiration. 



(321.) Reverting, then, to the enlarged views of the 

 early writers relative to the Aptera, we shall at once look 

 to the class before us as composed of the five following 

 orders, whose relations to the corresponding group of 

 Ptilota will be subsequently noticed : — I. The Arach- 

 nid a, or spiders, consisting of insects whose head is 

 confounded with the thorax, and whose mandibles assume 

 the appearance of fangs, or hooks, from which they are 

 capable of ejecting a poisonous liquor : the body is ge- 

 nerally short, oval, and pedunculated ; and the legs are 

 eight. — II. The Mykiapoda, or centipedes, having the 



• Classification of Fishes, i. 359. 



