arged first metasomatic
somite.
Per, Pericardium.
VPM^1 to VPM^7, The series of seven pairs of veno-pericardiac
muscles (labelled pv in fig. 31).

There is some reason to admit the existence of another more anterior
pair of these muscles in Scorpio; this would make the number exactly
correspond with the number in Limulus.

(After Lankester, _Trans. Zool. Soc._ vol. xi, 1883.)]

_Conclusions arising from the Close Affinity of Limulus and
Scorpio._--When we consider the relationships of the various classes of
Arthropoda, having accepted and established the fact of the close
genetic affinity of Limulus and Scorpio, we are led to important
conclusions. In such a consideration we have to make use not only of the
fact just mentioned, but of three important generalizations which serve
as it were as implements for the proper estimation of the relationships
of any series of organic forms. First of all there is the generalization
that the relationships of the various forms of animals (or of plants) to
one another is that of the ultimate twigs of a much-branching
genealogical tree. Secondly, identity of structure in two organisms does
not necessarily indicate that the identical structure has been inherited
from an ancestor common to the two organisms compared (homogeny), but
may be due to independent development of a like structure in two
different lines of descent (homoplasy). Thirdly, those members of a
group which, whilst exhibiting undoubted structural characters
indicative of their proper assignment to that group, yet are simpler
than and inferior in elaboration of their organization to other members
of the group, are not necessarily representatives of the earlier and
primitive phases in the development of the group--but are very often
examples of retrogressive change or degeneration. The second and third
implements of analysis above cited are of the nature of cautions or
checks. Agreements are not _necessarily_ due to common inheritance;
simplicity is not _necessarily_ primitive and ancestral.

On the other hand, we must not rashly set down agreements as due to
"homoplasy" or "convergence of development" if we find two or three or
more concurrent agreements. The probability is against agreement being
due to homoplasy when the agreement involves a number of really separate
(not correlated) coincidences. Whilst the chances are in favour of some
_one_ homoplastic coincidence or structural ag