452 Dr. A. Dohrn on Eugereon Boeckingi 
poda, and Insecta must have been a Zoépod, which accus- 
tomed itself to living on the land and to direct aérial 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 8. 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 Protracheata See Primi- 
tive Insects) by Hiickel, who characterizes them as follows :-— 
“Of these primitive forms of the Tracheata, developed from 
the Zoépoda between the Silurian and Carboniferous periods, 
no fossil remains are known 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 Zoépoda 
(which are still preserved to us in Zoéa-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 pairs of locomotive extremities. From 
these hexapod Zoentomide, 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 Solifuge 
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 aboriginal, 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 Solifuge. 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- 
tenn, and two pairs of maxillary palpi. The three segments 
of the thorax bear the three pairs of true legs.. The abdomen, | 
