452 Dr. A. Dolirn on Eugereon Boeckingi 



poda, and Insecta must have been a Zoepod, Avhich accus- 

 tomed itself to living on the land and to direct aerial respira- 

 tion, and thus gradually, in the course of a long series of 

 generations, acquired the very characteristic tracheal respira- 

 tion. It must have been developed in the time between the 

 Silurian and Carboniferous periods ; for in the Silurian period 

 there were as yet (at least so far as we know at present) no 

 terrestrial organisms, but in the Carboniferous period, and 

 even in the Devonian (according to the most recent publica- 

 tions of S. Scudder), the earliest developed Tracheata, both 

 Insects and Arachnida, had already made their appearance. 



The primitive forms of the three sections Arachnida, My- 

 riopoda, and Insecta, as to which we can now only form ana- 

 logical conclusions, are named Frotracheata [Urkerfe, Primi- 

 tive Insects) by Hackel, who characterizes them as follows : — 

 " Of these primitive forms of the Tracheata, developed from 

 the Zoepoda between the Silurian and Carboniferous periods, 

 no fossil remains are knoAvn to us. Nevertheless the com- 

 parative ontogeny of the Malacostraca, Arachnida, Myriopoda, 

 and Insecta enables us to arrive, with tolerable certainty, at 

 definite conclusions as to their form. Like many Zoepoda 

 (which are still preserved to us in Zoea-states) and like the 

 true insects, between which they occupy an intermediate posi- 

 tion, the Protracheata, as the type of which we may establish 

 the hypothetical genus Zoentomon^ must have possessed three 

 pairs of jaws and three joairs of locomotive extremities. From 

 these hexapod Zoentomidae, in all probability, the Insecta have 

 been developed as the direct branch, and the Arachnida as a 

 weaker lateral branch. The Myriopoda constitute only an in- 

 considerable lateral branchlet of the Insecta. Whether any 

 Protracheata are still in existence is doubtful. The SolifugcB 

 might, perhaps, be placed in this category, and perhaps also 

 those ' apterous insects ' (if there be any such among existing 

 insects) in which the want of wings is ahoriginal^ and has not 

 been acquired by adaptation." 



The Arachnida I likewise leave out of the question here, 

 and will only mention the one highly remarkable form which 

 alone in this class has still retained the old type, and which 

 allows us to arrive at a certain conclusion as to the original 

 community of ancestry of the Insecta and Arachnida — that of 

 the Solifugce. In this family we find no amalgamation of the 

 head and thoracic segments to form a cephalothorax, but three 

 perfectly separated regions of the body — head, thorax, and 

 abdomen. The head bears the pair of eyes, the pair of an- 

 tenna?, and two pairs of maxillary palpi. The three segments 

 of the thorax bear the three pairs of true legs. The abdomen, 



