30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 are more closely related to the Arachnida than to the Xiphosurida

Aside

from

the adaptations to terrestriallife bythe scorpions, such asthe substitution of ^"lungbooks^" forgills andthe correlatedsup- pression of the abdominal appendages or their incorporation in the sternal plates, the principal structural difference between the

two

groupsmightbereducedtotheelimination ofonegill-bearingsegment inthe eurypterids.

VI.

ARACHNIDA

The number

of postoral body segments present in adult or

em-

bryonic stages of Arachnidavaries

from

a

maximum

of 19in Scor- pionida to a

minimum

of 13 in Araneida (not considering acarid forms in which the body segmentation is obscure).

However,

since thescorpions aresaid tohaveonly 18embryonicpostoralneuromeres,

it

may

be true as Petrunkevitch (1922) suggests, that

two

of the adult segments of the scorpion ^"tail" represent a single primitive somite,and thatthe

maximum

arachnid segmentation, therefore,

may

includeonly 18 postoral somites, as inEurypterida, Malacostraca,and

Hexapoda. On

the other hand, as already noted, the fact that the lastlung books of the scorpions and thelast gills of Xiphosuraare on segment

XIII

,though^{thelast gills} of Eurypterida are saidto be on segment XII,issuggestive that the eurypteridshavelostasegment inthe gill-bearing regionof the body.

A

terminal spine,poison claw, or flagellum, which is presumal)ly the telson, is not counted in the above enumerationof segments.

The

bodydivision into

prosoma

and opisthosoma in Arachnida is

between segments

VI

and VII, as in Eurypterida, butsegment

VII

is always reducedandis often suppressed, so that theapparentdivi- sionis usuallybetween segments

VI

and VIII.

A

differentiation of theopisthosomaintoawideranterior

preabdomen

andataillikepost-

abdomen

isconspicuousinScorpionida(fig. 10

D),

Palpigradida,and Pedipalpida (fig. 11A). In thefirsttwo ofthese orders the division lies between segments

XI]'

and

XV,

butin the third it is between segments

XV

and

XVI.

In nocasedoes the arachnid subdivision of the opisthosoma correspond withthe eurypterid division if thelatter isactuallybetween segments

XIII

and

XIV

,asitappearstobe ^(fig.

10 B, C), but, asabove noted, if it is

assumed

that the eurypterids have lost a segmentin the preabdomen, the segmentation and body divisionof the eurypteridsand scorpionsbecomesthe same.

The

Arachnida have a singlemedian genital openingineach sex.

which (exceptin

some

of theAcarina) alwayspertainstothe second o]:)isthosomalsegment, or segment VIII, andthus has the

same

seg-

NO. 14 INSECT

ABDOMEN

SNODGRASS 31 mental positionasthejiaired gonoporesofXiphosurida;but theposi- tionof theopeningisvariable with respecttotheventral plateof the genital segment, since it is sometimes behind and sometimes before the genital sternum.

The

arachnid genital aperture is a gonotreme

(fig. 4 G-J, Gtr), inasmuch as it is the immediate opening of an

Fk;. II.—PedipalpidaandSolpugida^:bodysegmentationandexternalgenitali A, Mastigoproctusgigantcus, ventral view. B, Tarantula palmata (Ambly- pygi), baseofabdomenofmale, ventral surface,with endofgenitalorganpro- jectingfromgenital chamber (from Borner, 1904). C, Tarantula juscimanns, malegenital organ, dorsal view,exposedby partial removal ofwallsof genital chamber (fromBorner, 1904). D,asolpugid, lastprosomalsegment and base ofopisthosoma with genital opening, ventral view (sex undetermined).

a,b,wallsof genitalchamber; An,anus;c,genitalducts; d,spatulateappen- dages oflast legs; Fl,flagellum; GO,^{genital organ;} Gtr, gonotreme; 1L-4L, legs; Pdp, pedipalp; Sp, respiratory apertures; I'l-XVIII, postoral somites.

ectodermalgenital

chamber (GC)

thatcontains the truegonoporesor gonopore {Gpr).

The

gonads of theArachnidaliein the opisthosoma.in

some

cases above the alimentary canal, in others below^ it.

Each

organ in its

fundamental structure isa mesodermal tube containing thegerminal

cells in its epithelial walls, and is continued anteriorly into a cor-

32

SMITHSONIAN MISCELLANEOUS

COLLECTIONS VOL. 95 respondinggonoduct. Boththegonad andtheduct areensheathedin a muscularcoat.

The

tubular

form

of thegonad

may

be retained in theadult,but generally the primarytube becomes branched, produc- ingeither a pair of lengthwisetubes,or

more commonly

a

number

of lateral diverticula. In

some

cases the

two

gonads of opposite sides are united inasingletube or elongatesac, whichis eithersimple or branched. IntheScorpionida mesal branchesof theprimarygonadial tubes are generally unitedwitheach other formingaseriesofloops, which

may

join withthose

from

the opposite side,particularlyinthe female, to produce a composite

gonad

having the

form

of a

wide- meshed

net (seePavlovsky, 1924, 1924a).

The

gonadof thenet type inthescorpionsresembles thereticulate

gonad

of XipJwsitra(fig.8

B)

except that the latter has a

much

finer mesh,but the union of the gonadial branches appears to be independently evolved in the Scor- pionida, and is thus no evidence of close relationship between the scorpionsand thexiphosurids. In the arachnid ovaries the eggs are developedsinglyin

numerous

smallfolliculardiverticula of the ovarian tubes,a featurewhichthe arachnidshavein

common

with Xiphosurida and Onychophora.

The embryos

of the viviparous scorpionsdevelop either in swellings of the ovarian tubesbetween the egg follicles or inthefolliclesthemselves.

The

gonoducts, regardless of the

form

of the gonads, are always asinglepairof tubesextendingusually forward

from

the ovaries or testes. Inbothsexes of Scorpionida, in themales of Solpugidaand Phalangida, andin the females of Chelonethida, thegonoducts open separatelyintothegenital

chamber

(fig.4 G),butinmostother cases they uniteina

common

duct

(H)

oraninner sac (I) thatdischarges intothe externalgenitalchamber. Since neither the

common

ductnor the inner

chamber

isever foundto havea chitinous cuticularlining, the

two

appeartobe structures of

mesodermal

originprobably formed bythe union of the distal parts of the primary

mesodermal

ducts.

The common

duct in thiscase is not strictlyequivalent tothe usual ectodermal ductus ejaculatorius or the oviductus communis, and is

henceheretermedaductusconjunctus (fig.4

H,

_J, Den).

The

inner sac (I, Si), which is evidently an enlargement of the ductus con- junctus, is

commonly

called the " uterus internus " in arachnid anatomy, the

name

being applied alike in both the female and the male, but since the organ is not functionally a uterus even in the female,^it ishere termedthesaccus internus (Si).

The

wallsof the inner sachavea strong sheath of musclefibers,and boththe sacand theducts

may

beprovided with glandularandother kinds of diver- ticula, including the complex " paraxial organs

"of

male scorpions describedby

Pavlowsky

(1924).

NO. 14 INSECT

ABDOMEN

SNODGRASS 33

The

external genitalchamber,or bursagenitalis (fig.

4G-I, GC), commonly known

in arachnology as the " uterus externus", is un- doubtedly an ectodermal pouch of the bodywall since it always has a thickcuticular lining. Into its anteriorend open either thepaired gonoducts (G),or themedianductus conjunctus

(H)

or saccusinter- nus (I). Diverticula of the bursal wall

form

accessorygenitalglands of various kinds, often of largesize,andother structures of

unknown

function,which in

some

casesare eversible. In the femalea pair of lateralpouches

may

serve as seminalreceptacles.

The

externalopen- ingof thegenitalchamber,orgonotreme (Gtr), as already observed,

is alwaysassociatedwith the venter of thesecond opisthosomal seg- ment, which is segment

VIII

(figs. 10D, E, 11A, D, 13A, Gtr).

In the

more

generalizedcondition thegonotremeliesbehindthester- nal plateof thegenitalsegment (figs. 10E, 11 A, B), but

when

the genital regionis displaced forward between ^the legs,as in the Pha- langida (fig. 13

A),

thegenitalsternum

may become

reducedor con- cealedbyinvaginationwithin the bursa.

A

comparisonof the externalgenitalstructure of the

more

general- ized Arachnida withthat of Xiphosura showsthat the lumenof the genital chamberin the former,intowhich thegonoducts open,

must

correspond withthe external cavitybehindtheoperculumofthelatter,

whichcontains the gonopores.

The

relationof the arachnid structure to the

more

primitive conditionin Xiphosura is particularly evident in the scorpions,

where

the genital

chamber

opens behind a small opercularplateor pair ofplates (fig.10D,E,Opl) onthe venter of theeighthsegment,andthegonoductsopenseparatelyintothe anterior endof the enclosedcavity.

The

operculumof Xiphosura,as

we

have seen, clearly includes the appendages of the eighth postoral somite and amedian fold of the venter of the

same

segment ^(fig.9^B, V), on the posterior surface of which are located the genital papillae (Pen). In a truly primitive stage of the Chelicerata (except Pycno- gonida), therefore, the pairedgonoductsof each sex

must

haveopened onthe venter of the eighth postoral segment

mesad

of the bases of theappendages.

Most

of the Arachnida do nothave an organ for intromission of thespermassociateddirectlywiththegonotreme.

The

matinghabits of the arachnids are well

known

only ina

few

groups, but generally ifthemalepersonallyinsertsthespermatozoaorspermatophoresinto thespermreceptacleof the female,heuses forthispurposea pair of the body appendages, suchas the chelicerae in the Solpugida, or the pedipalpsintheAraneida.

The

female,however,

may

beinseminated in

some

indirect manner, as with the Chelonethida.

At

thetime of

34

SMITHSONIAN MISCELLANEOUS

COLLECTIONS VOL. 95