enera, families and orders, of grouping of the somites. They
are anomotagmic, as well as anomomeristic.

[Illustration: FIG. 33.--The alimentary canal and gastric glands of a
scorpion (A) and of Limulus (B).

From Lankester, "Limulua an Arachnid."

ps, Muscular suctorial enlargement of the pharynx.
sal, Prosomatic pair of gastric caeca in Scorpio, called salivary
glands by some writers.
c^1, and c^2, The anterior two pairs of gastric caeca and ducts of
the mesosomatic region.
c^3, c^4 and c^5. Caeca and ducts of Scorpio not represented in
Limulus.
M, The Malpighian or renal caecal diverticula of Scorpio.
pro, The proctodaeum or portion of gut leading to anus and formed
embryologically by an inversion of the epiblast at that orifice.]

When it is admitted--as seems to be reasonable--that the primitive
Arachnida would, like the primitive Crustacea, be anomomeristic and
anomotagmic, we shall not demand of claimants for the rank of primitive
Arachnids agreement with Limulus and Scorpio in respect of the exact
number of their somites and the exact grouping of those somites; and
when we see how diverse are the modifications of the branches of the
appendages both in Arachnida and in other classes of Arthropoda (q.v.),
we shall not over-estimate a difference in the form of this or that
appendage exhibited by the claimant as compared with the higher
Arachnids. With those considerations in mind, the claim of the extinct
group of the trilobites to be considered as representatives of the lower
and more primitive steps in the Arachnidan genealogy must, it seems,
receive a favourable judgment. They differ from the Crustacea in that
they have only a single pair of prae-oral appendages, the second pair
being definitely developed as mandibles. This fact renders their
association with the Crustacea impossible, if classification is to be
the expression of genetic affinity inferred from structural coincidence.
On the contrary, this particular point is one in which they agree with
the higher Arachnida. But little is known of the structure of these
extinct animals; we are therefore compelled to deal with such special
points of resemblance and difference as their remains still exhibit.
They had lateral eyes[5] which resemble no known eyes so closely as the
lateral eyes of Limulus. The general form and structure of their
prosomatic carapace are in many striking features identical with that of
Limulus. The tril