oma have disappeared in Limulus, but are
represented by the unsegmented prae-anal region. It is probable that
we have in the metasoma of Limulus a case of the disappearance of once
clearly demarcated somites. It would be possible to suppose, on the
other hand, that new somites are only beginning to make their
appearance here. The balance of various considerations is against the
latter hypothesis. Following the metasoma in Limulus, we have as in
Scorpio the post-anal spine--in this case not a sting, but a powerful
and important organ of locomotion, serving to turn the animal over
when it has fallen upon its back. The nature of the post-anal spine
has been strangely misinterpreted by some writers. Owen (7) maintained
that it represented a number of coalesced somites, regardless of its
post-anal position and mode of development. The agreement of the
grouping of the somites, of the form of the parapodia (appendages,
limbs) in each region, of the position of the genital aperture and
operculum, of the position and character of the eyes, and of the
powerful post-anal spines not seen in other Arthropods, is very
convincing as to the affinity of Limulus and Scorpio. Perhaps the
most important general agreement of Scorpio compared with Limulus and
the Eurypterines is the division of the body into the three regions
(or tagmata)--prosoma, mesosoma and metasoma--each consisting of six
segments, the prosoma having leg-like appendages, the mesosoma having
foliaceous appendages, and the metasoma being destitute of appendages.

[Illustration: FIG. 9.--Ventral view of the posterior carapace or
meso-metasomatic (opisthospmatic) fusion of _Limulus polyphemus_. The
soft integument and limbs of the mesosoma have been removed as well as
all the viscera and muscles, so that the inner surface of the terga of
these somites with their entopophyses are seen. The unsegmented dense
chitinous sternal plate of the metasoma (XIII to XVIII) is not
removed. Letters as in fig. 7.

(After Lankester, _loc. cit._)]

In 1893, some years after the identification of the somites of Limulus
with those of Scorpio, thus indicated, had been published, zoologists
were startled by the discovery by a Japanese zoologist, Kishinouye
(8), of a seventh prosomatic somite in the embryo of Limulus
longispina. This was seen in longitudinal sections, as shown in fig.
19. The simple identification of somit