ool. Soc._ VOL. xi., 1883.

d, Chelicera.
ch, Chela.
cam_, Camerostome.
m, Mouth.
ent_, Entosternum.
p, Pecten.
stig^1, First pulmonary aperture.
stig^4, Fourth pulmonary aperture.
dam, Muscle from carapace to a praeoral entosclerite.
ad, Muscle from carapace to entosternum.
md, Muscle from tergite of genital somite to entosternum (same as dpm
in fig. 30).
dv^1 to dv^6, Dorso-ventral muscles (same as the series labelled tsm
in fig. 30).
pv^1 to pv^7, The seven veno-pericardiac muscles of the right side
(labelled VPM in fig. 30).]

[Illustration: FIG. 32.--Diagram of a lateral view of a longitudinal
section of Limulus.

After Benham, _Trans Zool. Soc._ vol. xi, 1883.

Suc, Suctorial pharynx.
al, Alimentary canal.
Ph, Pharynx.
M, Mouth.
Est, Entosternum.
VS, Ventral venous sinus.
chi, Chilaria.
go, Genital operculum.
br^1 to br^5, Branchial appendages,
met, Unsegmented metasoma.
Entap^4, Fourth dorsal entapophysis of left side.
tsm, Tergo-sternal muscles, six pairs as in Scorpio (labelled dv in
fig. 31).
VPM^1 to VPM^8, The eight pairs of veno-pericardiac muscles (labelled
pv in fig. 31). VPM^1 is probably represented in Scorpio, though not
marked in figs. 30 and 31.]

Leaving that question for consideration in connexion with the systematic
statement of the characters of the various groups of Arachnida which
follows on p. 299, it is well now to consider the following question,
viz., seeing that Limulus and Scorpio are such highly developed and
specialized forms, and that they seem to constitute as it were the first
and second steps in the series of recognized Arachnida--what do we know,
or what are we led to suppose with regard to the more primitive
Arachnida from which the Eurypterines and Limulus and Scorpio have
sprung? Do we know in the recent or fossil condition any such primitive
Arachnids? Such a question is not only legitimate, but prompted by the
analogy of at least one other great class of Arthropods. The great
Arthropod class, the Crustacea, presents to the zoologist at the present
day an immense range of forms, comprising the primitive phyllopods, the
minute copepods, the parasitic cirrhipedes and the powerful crabs and
lobsters, and the highly elaborated sand-hoppers and slaters. It has
been insisted, by those who accepted Lankester's original doctrine of
the direct or genetic affinity of the Chaetopoda and Arthrop