Or Axolotl of Mexico were brought to him by Humboldt in the beginning of the 10th century, that these Batrachians were not really related to the Perennibranchiates, such as Siren and Proteus, with which he was well acquainted, but represented the larval form of some air-breathing salamander.
Remains of air-breathing insects, myriapods and arachnids show that these forms of life were both well developed and individually numerous.
Zoologists are familiar with many instances (fishes, crustaceans) in which the protective walls of a water-breathing organ or gill-apparatus become converted into an air-breathing organ or lung, but there is no other case known of the conversion of gill processes themselves into air-breathing plates.
Their appearance in the air-breathing Arachnids does not separate those forms from the water-breathing Arachnids which are devoid of them, any more than does their appearance in certain Amphipoda separate those Crustaceans from the other members of the class.
Bearing in mind that Linnaeus, in his use of the name Amphibia, was not alluding to the gill-breathing and air-breathing periods through which most frogs and newts pass in the course of their existence, but only wished to convey the fact that many of the constituents of the group resort to both land and water (e.g.
Though the aquatic members of a class of animals are in some instances derived from terrestrial forms, the usual transition is from an aquatic ancestry to more recent land-living forms. There is no doubt, from a consideration of the facts of structure, that the aquatic water-breathing Arachnids, represented in the past by the Eurypterines and to-day by the sole survivor Limulus, have preceded the terrestrial air-breathing forms of that group. Hence we see at once that the better-known Arachnida form a series, leading from Limulus-like aquatic creatures through scorpions, spiders and harvest-men, to the degenerate Acari or mites.
Among the remaining divisions of Invertebrata special mention may be made of the air-breathing Arthropoda - on the whole the most important and interesting group. About one-third of the animals belonging thereto that occur in the higher regions are exclusively alpine (or alpine and northern); these characteristically alpine forms being furnished chiefly by the spiders, beetles and butterflies.