oda, that
Apus and Branchipus really come very near to the ancestral forms which
connected those two great branches of Appendiculate (Parapodiate)
animals. On the other hand, the land crabs are at an immense distance
from these simple forms. The record of the Crustacean family-tree is, in
fact, a fairly complete one--the lower primitive members of the group
are still represented by living forms in great abundance. In the case of
the Arachnida, if we have to start their genealogical history with
Limulus and Scorpio, we are much in the same position as we should be in
dealing with the Crustacea, were the whole of the Entomostraca and the
whole of the Arthrostraca wiped out of existence and record. There is no
possibility of doubt that the series of forms corresponding in the
Arachnidan line of descent, to the forms distinguished in the Crustacean
line of descent as the lower grade--the Entomostraca--have ceased to
exist, and not only so, but have left little evidence in the form of
fossils as to their former existence and nature. It must, however, be
admitted as probable that we should find some evidence, in ancient rocks
or in the deep sea, of the early more primitive Arachnids. And it must
be remembered that such forms must be expected to exhibit, when found,
differences from Limulus and Scorpio as great as those which separate
Apus and Cancer. The existing Arachnida, like the higher Crustacea, are
"nomomeristic," that is to say, have a fixed typical number of somites
to the body. Further, they are like the higher Crustacea,
"somatotagmic," that is to say, they have this limited set of somites
grouped in three (or more) "tagmata" or regions of a fixed number of
similarly modified somites --each tagma differing in the modification of
its fixed number of somites from that characterizing a neighbouring
"tagma." The most primitive among the lower Crustacea, on the other
hand, for example, the Phyllopoda, have not a fixed number of somites,
some genera--even allied species--have more, some less, within wide
limits; they are "anomomeristic." They also, as is generally the case
with anomomeristic animals, do not exhibit any conformity to a fixed
plan of "tagmatism" or division of the somites of the body into regions
sharply marked off from one another; the head or prosomatic tagma is
followed by a trunk consisting of somites which either graduate in
character as we pass along the series or exhibit a large variety in
different g