168 PRIMITIVE FORMS OF LIFE 



envelope about as thick as that of the Arthropods. They are 

 very archaic organisms, belonging unquestionably to the first 

 land immigration of the segmented members of the animal 

 kingdom, for they are found at widely separated localities 

 which could only have been connected during the existence of 

 the former Gondwana continent, e.g. the Cape of Good Hope, 

 New Zealand, the Amazon Valley, etc. Their respiratory 

 apparatus consists of numerous invaginations of the thinner parts 

 of the integument arising indifferently from the dorsal or the 

 ventral surface of the body ; they are even seen on the surface 

 of the membranous feet, constituting as many internal tubes, 

 which, after being expanded into an umbrella, give rise to a 

 bunch of slender tubes spreading from the centre thereof 

 and terminating in a cul-de-sac without any rami- 

 fication. These structures are known as trachea, and this 

 same term tracheae is applied to all the internal respiratory 

 tubes of the Arthropods, whatever their form and origin. 

 No connexion is to be seen between these very numerous 

 respiratory tubes without any fixed morphological position, 

 and the so-called lungs of the Arachnida. MacLeod has 

 propounded an interesting hypothesis for the origin of these 

 last-named organs, which does not, however, destroy the 

 validity of Marie Pereyaslawzeva's x observations. For him, 

 in short, the lungs of Scorpions are nothing but a slight 

 modification of the branchial apparatus of Limuhis. The 

 abdomen of these creatures, the earliest known Arthropods, 

 since they are found in the Silurian deposits, possess 

 flattened feet in the form of large chitinous lamellae, in the 

 rear of which are sheltered a whole series of thin leaves, super- 

 posed like those of a book. If that portion of the integument 

 which supports these leaves were to be invaginated interiorly 

 to the body, drawing them with it, while the protective plate 

 constituted by the foot became shorter, a pocket would 

 necessarily be produced, having on its inner side a series of 

 leaves or lamellae and opening externally by a slit — that is to 

 say, a lung of the Scorpion type with its respiratory orifice. 



1 These objections are founded on a lack of agreement between the actual 

 order of appearance of parts in the lungs of the embryo and the theoretical 

 order that ought to occur in the formation of these parts according to the 

 hypothesis of MacLeod. However, we know that such reversals are frequent 

 in embryogenetic development, and are the result of tachygenesis. 

 (LXXII, 247.) 



