e with somite in Limulus and
Scorpio seemed to be threatened by this discovery. But in 1896 Dr
August Brauer of Marburg (9) discovered in the embryo of Scorpio a
seventh prosomatic somite (see VII PrG, figs. 17 and 18), or, if we
please so to term it, a _praegenital_ somite, hitherto unrecognized.
In the case of Scorpio this segment is indicated in the embryo by the
presence of a pair of rudimentary appendages, carried by a well-marked
somite. As in Limulus, so in Scorpio, this unexpected somite and its
appendages disappear in the course of development. In fact, more or
less complete "excalation" of the somite takes place. Owing to its
position it is convenient to term the somite which is excalated in
Limulus and Scorpio "the praegenital somite." It appears not
improbable that the sternal plates wedged in between the last pair of
legs in both Scorpio and Limulus, viz. the pentagonal sternite of
Scorpio (fig. 10) and the chilaria of Limulus (see figs. 13 and 20),
may in part represent in the adult the sternum of the excalated
praegenital somite. This has not been demonstrated by an actual
following out of the development, but the position of these pieces and
the fact that they are (in Limulus) supplied by an independent
segmental nerve, favours the view that they may comprise the sternal
area of the vanished praegenital somite. This interpretation, however,
of the "metasternites" of Limulus and Scorpio is opposed by the
coexistence in Thelyphonus (figs. 55, 57 and 58) of a similar
metasternite with a complete praegenital somite. H.J. Hansen (10) has
recognized that the "praegenital somite" persists in a rudimentary
condition, forming a "waist" to the series of somites in the Pedipalpi
and Araneae. The present writer is of opinion that it will be found
most convenient to treat this evanescent somite as something special,
and not to attempt to reckon it to either the prosoma or the mesosoma.
These will then remain as typically composed each of six
appendage-bearing somites-the prosoma comprising in addition the
ocular prosthomere.[1] When the praegenital somite or traces of it are
present it should not be called "the seventh prosomatic" or the "first
mesosomatic," but simply the "praegenital somite." The first segment
of the mesosoma of Scorpio and Limulus thus remains the first segment,
and can be identified as such throughout the Eu-arachnida, carrying a