40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IIQ

NO. 10

FnEDING ORGANS

^01-"

AKACH NIUA— SNODGRASS

4I surface overlianging the

mouth and

the inner end of the prcoral food cavity.

The mouth

(E, G, Mtli) is a small

unguarded

aperture be- neath the base of the labrum. I->om below the

mouth

there projects forward the broad, basinlike floor of the preoral cavity

(E) formed

of the endite lobes of the first leg coxae (IIIEiidt).

Between

the mesal edges of these coxal lobes is a groove,

which

is closed below by the closely applied endites of the second leg coxae (D, IVllndt),

and

proximally runs into the

mouth

(E,

Mth). The

pedipalp coxae are entirely separated

from

each other,

and

there is

no

recognizable

remnant

of the pedipalp

sternum

in the scorpion.

The

broad, flat,

mesal surfaces of the coxae are mostly

membranous

(O, IICx),

and form

the lateral walls of the preoral food cavity (D,

PrC). An-

teriorly the coxae are

produced

into short coxal processes (cxp)

mesad

of the trochanters.

The

coxal bases are connected by

two

thick bundles of transverse muscle fibers, which pass through the

labrum

(G, tnicl).

These

muscles occur in other arachnids,

and

^are labral muscles; in the scorpion they appear to be operative on the pedipalp coxae by reason of a union of the base of the

labrum

with the coxal walls.

The

lobes of the first

and

second leg coxae (fig. 13 D, IIIEudt,

IVEndt)

that

form

the lower lip of the scorpion, or floor of the preoral cavity, are here termed cnditcs because they arise

from

the l)asesof the coxae

(F) and

not frotn their distal endsas

do

thecoxal processes {cxp) of thepedipalpcoxae. Coxal lobesof the

same

nature occur also in the Phalangida.

There

is

no

prominent epislomal plate in the scorpion, but at the base of the dorsal wall of the

labrum

^is a strong, irregular scleroti- zation (fig. 13 G, Epst),

which

clearly represents the epistome, since

on

it are attached the dorsal dilators of the

pharynx

(did),

and from

it is given off a pair of

apodemal arms {cAp).

The pharynx

of the scorpion is asmall, pear-shapedsack enlarging

upward and

posteriorly

from

^its

narrowed

etitrance at the

mouth

(\\f^. 13G, riiy). It is .somewhat comjjresscd ^laterally,

rounded

at

the inner end; the slender oesophagus

(Oc)

departs

from

the lower wall at the end of a ventral channel

from

themouth.

The

dorsal wall of the sack is deeply infolded

and

the trough of the invagination is

strengthened by an elastic rod. Dilator muscles attached on the con- cave dorsal wall (did) arise on the epistomal sclerotization (Epst)

at the base of the labrum,

and

lateral dilators take their origins on theepistomal

apodemes (cAp). Compressor

muscles coverthe lateral walls of thepharynx.

The

sting of the scorpion appears to be an ai^pcndagc of the last

42 SAIITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. IIO abdominal

segment

(fig. 14B),

and

not the terminal

segment

itself.

The

end segment, or telson, of

an

arthropod contains the anus; in the scorpion the anus lies before the base of the sting in the end of the sting-supporting

segment

(A,

An). The

scorpion sting in its relation to the end of the

abdomen

is comparable with the flagellum of the Thelyphonidae

and

the tail spine of Limulns.

The

baseof the sting is articulated

on

the end of the supporting

segment

in such a

manner

that its principal

movement

is in a vertical plane, but because

Fig. 14.

—

Tlie sting of a scorpion.

A, Paiidiniis sp., end of last postabdominal segment, with the sting, ventral, showing the anus (An) in last segment at base of sting. B, Ccntruroides sp., last two postabdominal segments and the sting, lateral. C, same, the sting with

its muscles arising in the last segment. D, same, cross section through base of sting, showing the venom glands and enclosing muscles. E, same, terminal part of sting, showing aperture of left venom duct. F, same, right half of base of sting, mesal view, showing muscles covering right gland, and duct.

of the amplitude of the articular

membrane

it is capable also of lateral

movements. On

its base are attached four long muscles,

two

dorsal,

and

one

on

each side (C).

The

dorsal muscles are widely divergent but both are inserted

on

a strong dorsal process of the sting base.

The

lateral muscles areattached belowthearticularpoints of the sting,

and

are hence depressors, but probably, acting as antag- onists, they produce also lateral

movements

of the sting; each lateral muscle separates into

two

distinctbundles of fibers.

The

segments of

NO. 10

IKKDIXC

OK(..\N> OK AKAi 11NIIiA -SNOIK'.RASS

43

the

postabdomen

are likewise strongly niusculated,each beingprovided with asingle wide dorsal musclewith an axial tendon, a lateral muscle on each side, and in addition a large

median

ventral muscle.

The venom

of the scorpion is

produced

in

two

sacklikc glands contained within the swollen basal part of the sting (fig. 14D, Gld).

'i'he glands have individual ducts opening separately near the point of thesting through

two

lateral pores (E, I'Pr),

from which

grooves extendto thetip.

Each

glandisclosely invested alongthe entirelength of its mesal

and

dorsal surfaces by a thick muscular sheath

made

uj) of several layers of semicircularfibers (D, F, nicls), attacheddorsxilly on the upper part of the lateral wall of the containing capsule,

and

ventrally

on

the lower wall. Contraction of the muscles evidently compresses the gland sacks against the rigid capsular walls.

The

scorpion is certainly not a primitive arachnid, though an an- cient one. //, therefore, the scorpions have

any

relationship to the extinct Eurypterida, the theor}- of \'ersluys

and Demoll

(1920), ^in- sofar as it derives the eurypterids

from

primitive scorpions,

would

appearto be

more

reasonable thanthe reverse.

That

the Xiphosurida have a scorpion ancestry, however, is difficult to believe, considering their primitive

method

of feeding and their evident relation to the trilobites.

VIII.

THE

PH.\L.\XGID.\,

OR OPILIONES

The

Phalangida are characterized by the presence of lobes arising

from

the bases of the coxae of the pedipalps

and

the first

two

pairs of legs. Because of their position

on

the coxal bases these lobes of the phalangiids are analogous to the lobes on the second

and

third leg coxae of the scorpions, which, as explained in the last section.

are here

termed

cnditcs to distinguish

them from

thedistal processes of the pedipalp coxae in other arachnids.

The

jiedipalp endites of the Phalangidaare alwayscloselyassociated withthe mouth,

and

have the appearance of a pair of jaws; they

may

be prehensile, but they have

no

masticatory function,

and

henceare not appropriately termed

"manducatory"

^lobes.

The

^first leg endites in the Phalangiidae re- semblethe pedipalp endites

and

are likewise associatedwiththemouth, but the endites of the second legs never have a direct relation to feeding. In the Cyphophthalini

and

the Laniatores the coxal endites, whether hard or soft in texture, are

immovably

fixed on the coxae, but in the Palpatores they are flexibly attached to the coxae,

and

become

indei)endently niovabk- Ijecause

some

of the

body

muscles of the coxae are attached on liu-ir bases. .V

labrum

is always present,