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Published by Field Museum of Natural History 

Volume 39, No. 4 September 25, 1978 

Arthropods: A Convergent Phenomenon 
Frederick R. Schram- 

Eastern Illinois University 
Charleston, Illinois 


Research Associate 
Field Museum of Natural History 


"In the absence of precise knowledge of the meaning of animal 
shapes it has been safe to put forward many unsound comments on 
arthropod evolution" (Manton, 1958, p. 64). Thus we can charac- 
terize the nature of the revolution in our understanding of arthropod 
functional morphology and embryology which has taken place in the 
last generation. We have moved from a condition in which many 
gaps in our knowledge of basic arthropod biology existed, but which 
never deflected people from engaging in extensive phyletic specula- 
tion. We are now arriving at a phase in which a wide range of inves- 
tigations of basic form and function, what Beklemishev (1969) refers 
to as "promorphology," are forcing us to re-evaluate traditional 
views of arthropod evolution. A long-established monophyletic 
viewpoint is giving way to a polyphyletic one. However, as the his- 
torical review of Tiegs and Manton (1958) reveals, the pendulum of 
debate of monophyly versus polyphyly in arthropods has been 
swinging for some time. 

Recent functional morphological and comparative embryological 
studies on modern groups of arthropods reveal that we are dealing 
with several coherent taxonomic groups of the status of phyla. 
Using this information, as well as paleontological evidence, integra- 
tion of the trilobitomorphs and pycnogonids into this scheme is 

•Present address: San Diego Natural History Museum 
Library of Congress Catalog Card No.: 78-52663 
ISSN 0096-2651 

Publication 1287 61 

rrnin»w iibdj 


attempted. There are seen to be at least three, probably four, phyla 
which have reached the arthropodous grade of organization: Uni- 
ramia, Crustacea, Cheliceriformes, and probably Trilobitomorpha. 

It is my intent to review and summarize some of these recent dis- 
coveries in comparative arthropod biology which have occurred 
since Tiegs and Manton (1958). I will also evaluate what is known of 
some of the "peripheral" groups which are not as well understood as 
the major living groups (trilobites and pycnogonids) to see where 
they might fit in any phyletic arrangement of the arthropods. 


The central dominating theme of arthropod evolution is the multi- 
plicity of convergent development. No phyletic scheme, monophy- 
letic or polyphyletic, can escape this (Tiegs and Manton, 1958). In 
the late 1800's the arthropods were viewed as a tightly organized 
group. The merostomes were considered to be Crustacea. The arach- 
nids were aligned with the myriapod-hexapods in the Tracheata, 
based on the possession of tracheae in many of these animals. The 
presence of Malpighian tubules in insects and many arachnids 
served to strengthen the links between these two groups. But Lan- 
kester (1881) effectively demonstrated the affinities of Limulus with 
scorpions and thus all arachnids. The Chelicerata were established 
as a group separate and distinct from all others. The arachnids, with 
their tracheae and Malpighian tubules, were seen to be derived from 
marine merostome types and not myriapodous forms. Later, study 
of Malpighian tubule development revealed that these structures 
were evaginations of the proctodeum or hindgut, i.e., ectodermally 
derived, in the insects, and outgrowths of the midgut, i.e., endoder- 
mally derived, in arachnids. The original convergence of these excre- 
tory structures demanded by taxonomic considerations of external 
anatomy of Chelicerata were eventually reinforced by embryonic 

Subsequent to the elucidation of the Chelicerata, the crustaceans 
and myriapod-hexapod groups were allied in the subphylum Man- 
dibulata (Snodgrass, 1938), but the recent work of Manton (1964) on 
the functional morphology of arthropod jaws has completely altered 
our understanding of mandible evolution. 

There are two basic jaw types in arthropods: a gnathobasic type, 
in which only the modified coxa is used for biting; and the whole- 
limb type, in which the entire appendage is employed, the biting sur- 


face being the tip of the distalmost segment or distalmost part of 
the whole limb jaw. The gnathobasic type is found in the crustacean 
mandible (and on the prosomal appendages 2 to 6 in Limulus). The 
whole-limb jaw is used in the onychophorans, myriapods, and hexa- 
pods. Man ton (1964) concluded that these jaw types are so distinc- 
tive that neither one can have given rise to the other. 

Within each basic jaw type there are different modes of action. 
The gnathobasic jaw of Crustacea primitively employs the coxal 
promotor-remotor muscles to produce the antero-posterior rolling 
action of the molar process around a dorso-ventral axis. In some 
eumalacostracans the development of an incisor process posterior to 
the molar process produces a secondary transverse action using the 
incisor process. Powerful remotor muscles produce the grinding 
(rolling) action of the molar process and the biting (transverse) 
action of the incisor. The weaker promotor muscles part the molar 
and abduct the incisor processes. 

The living merostomes, sometimes placed in three separate gen- 
era (Limulus, Carcinoscorpius, and Tachypleus), use prosomal ap- 
pendages 2 to 6 as both walking limbs and biting limbs. The coxae 
on these appendages are capable of two different actions. A series of 
promotor-remotor muscles moves the coxa antero-posteriorly when 
the animal walks. Another set of special abductor-adductor muscles 
moves the coxa transversely when the coxa is used for biting. Man- 
ton claims that the muscles used and the modes of action of the 
crustacean and limulid gnathobasic jaws are so different that there 
is little possibility that they are related in any way to each other and 
are therefore only analogous. 

The whole-limb jaw type also exhibits different modes of action. 
The onychophoran mandible has an antero-posterior slicing action, 
each jaw frequently moving in opposite phase to the other, like the 
onychophoran walking legs. Some hexapods have a rolling, grinding 
action similar to crustaceans. The basic promotor-remotor action in 
hexapods can also be converted in some forms to a transverse action 
with the development of incisor processes. As occasionally happens 
in the eumalacostracans, the hexapods, especially pterygote insects, 
greatly reduce the anterior molar process and completely convert 
the mandible to a transverse type while still employing promotor- 
remotor muscles. 

The segmented mandibles of myriapods have a transverse action 
with the musculature being largely adductor, the abduction being 



adductor mandibulae from cardo (n') 

r from traniverae mandibular ttndon (i) 
adductor mandibulae from itipet (n) 

gnathal lobe tele rue 

levator mandibulae (k) 

intnnuc levator from giuchaJ lobe (o) 

gnathal lobe tendon 

middle line 
lateral chitinous plate of nypopharyn 


| lobe muscle to sclerite 

FlG. 1. Anterior view of the mandibles of the diplopod, Poratophilus punctatus. 
Left: the musculature is entire and the mandible is in a position of maximum adduc- 
tion. Right: muscles are removed from the mandibular cuticle, with the exception of 
one intrinsic muscle, and the mandible is in a position of extreme abduction. (From 
Manton, 1964.) 

achieved indirectly. Powerful adductor muscles inserting on the 
cardo, stipes, and gnathal segments of the mandible exert a strong 
force (fig. 1). Abduction is achieved by the endoskeletal tentorium 
which swings downward and forward from the inner surface of the 
cranium and pushes the jaws apart. This is a basic arrangement for 
all myriapod groups and is so different from that of the hexapods 
that neither can have given rise to the other. Manton (1964, 1969) 
thus disproves the view that insects have been derived from sym- 
phylans as has been commonly suggested in the past; and Rasnitsyn 
(1976) agrees based on a consideration of fossil evidence. 

The old arthropod subphylum Mandibulata is thus seen to have 
no basis (fig. 2). Similarities between crustaceans and myriapod- 
hexapods are merely convergent. Though the variations in jaw func- 
tion between Myriapoda and Hexapoda are striking, there remain a 
great enough number of similarities shared by all classes, such as 
the labium, derivation of the jaw, a single pair of antennae, trache- 
ae, and manner of potential entognathy (though this has arisen sev- 
eral times within the myriapod-hexapod group), that it appears that 
the onychophorans-myriapods-hexapods form a fairly coherent unit. 



trtnivarMty biting |»wt 


nnivtmly biting |awi 

iriruver»«l)r bmnf |jwi 



rollin| |«>»5 



rolling |j~i 

unjolntcd jiwt 


• i i . f - .t . biting fawn 

|Oinic4 |*wi 


Fig. 2. Diagram showing the conclusions reached concerning the distribution of 
the principal types of mandibles or jaws (below) and the derivation of the jaw mech- 
anism (above). Heavy vertical lines indicate an absence of common ancestry between 
the jaws referred to on either side; an interrupted vertical line indicates separate 
evolutions of jaw mechanisms which probably had a common origin; the shaded area 
indicates jaw mechanisms showing convergent similarities derived from unlike 
origins. (From Manton, 1964.) 

When the concept Mandibulata is discarded, another striking con- 
vergence is revealed: that of compound eyes. Recent work has indi- 
cated differences in the pigment screen between crustacean and 
insect compound eyes (Struwe et al., 1975), and there is a lack of 
homology between insect primary pigment cells and crustacean cor- 
neagen cells (Elofsson, 1970) as has been previously supposed. 
Elofsson (pers. comm.) believes insect and crustacean compound 
eyes to be as different from each other as are vertebrate and cephal- 
opod eyes, i.e., a striking gross superficial similarity exists, but con- 
stituent parts have quite different embryonic histories. Compound 
eyes are also possessed by merostome chelicerates and trilobites. 
The compound eyes of Limulus have a thick cornea covering over all 
the ommatidial units, in the living limulids as well as the fossil 
merostomes, as opposed to the typically distinct cornea over each 


ommatidium in the eyes of crustaceans and insects. Little is known 
about the trilobite eyes that can be effectively compared with the 
detail available in living arthropod eyes. Though trilobite compound 
eyes are composed of several optical units (Clarkson, 1973) some 
may not have been as closely aligned and co-ordinated as those of 
living arthropods (Clarkson, 1966). Several different types of trilo- 
bite eyes are recognized (Clarkson, 1975; Jell, 1975). 

It may be difficult to accept the convergent development of such a 
complex and important structure as the compound eye, yet I think 
this convergence is probably no more profound than that which oc- 
curs between the eyes of some cephalopods and vertebrates. Arthro- 
pods being what they are, with the ocellus representing the simple 
basic optical unit, there is only a certain optimal way to solve the 
problem of visual perception. The ocellus is common to all groups of 
arthropods and given this simple structure as a foundation, natural 
selection has resulted in compound eyes in various sorts of crustace- 
ans, insects, trilobites, and merostomes. 


In addition to a uniformity in jaw apparatus, the onychophorans- 
myriapods-hexapods share a common uniramous appendage. This 
appendage evolved in a condition where a long series of identical 
limbs had to be precisely co-ordinated to achieve locomotion. Lack 
of co-ordination would lead to interference of one limb with an adja- 
cent one and stumbling (Manton, 1969). 

Body shape of the various groups within this assemblage— named 
the Uniramia by Manton (1973a)— is correlated with habit. The 
onychophoran unsclerotized, deformable body, connective tissue en- 
doskeleton, and unstriated muscle enables these animals to squeeze 
through narrow openings and spaces, allowing them to escape pred- 
ators; though some might prefer to continue to use these same char- 
acters as criteria for maintaining Onychophora as a separate phy- 
lum. The diplosegments of the Diplopoda are related to a need to 
develop motive force in burrowing or pushing through leaf litter, 
soil, and decomposing wood. In the Chilopoda the lengthening of the 
legs, variation in body segment size, and special muscle insertions 
enable these animals to exploit a running, carnivorous habit. The 
Sympyhla have divided tergites which allow them to twist and flex 
their bodies in climbing under, over, around, and in between 
obstructions and in executing sharp angle turns to escape and elude 


predators. The Hexapoda have reduced the number of legs, length- 
ened the appendage, and spread out the field of movement of each 
limb (with resultant increase of mechanical advantage) to allow 
them to exploit the resultant versatility of movement and speed. 

The chelicerates developed from long-legged forms with few ap- 
pendages on the body and no more than five postoral limbs on the 
adult prosoma (Manton, 1973a, b). There is no necessity to rigidly 
co-ordinate movements among such a small number of legs and so 
chelicerates consequently execute rather inaccurate stepping move- 
ments in contrast to the Uniramia. In arachnids the stepping move- 
ment does not typically involve a promotor-remotor swing of the 
coxa. Rather, the chelicerates employ a "rocking" action in length- 
ening the stride of the leg to greater effect than any other arthro- 
pods. This motion is so termed from the position of the dorsum of 
the appendage during movement: on the propulsive backstroke, the 
dorsum is directed forward; on the recovery stroke, when the leg is 
brought forward, the dorsum is directed posteriorly. The appear- 
ance of a single isolated leg would then describe a rolling or rocking 
motion. Manton (1973b) is not clear on just how this rocking is 
achieved, but Schram and Hedgpeth (1978) note similar movements 
in pycnogonids and attribute it in part to sets of adjacent but 
separate extensor muscles in the basalmost segments of the legs. 

The primitive living crustaceans have a flat, multi-ramous ap- 
pendage (Hessler and Newman, 1975) which is used in a meta- 
chronal, swimming pattern. The legs are directed ventrally, under 
the body, and locomotion is typically combined with food getting 
and processing. 

This multiplicity of jaw structure and embryonic derivation, com- 
bined with a meticulously detailed analysis of leg movements com- 
pelled Manton (1973a) to view arthropods as a grade of evolutionary 
organization containing three phyla having independently arrived 
at this grade: Uniramia, Crustacea, and Chelicerata. The uniramians 
evolved from multi-legged, uniramous, soft-bodied or flexible forms 
with lobopodial appendages that handled food at the appendage 
tips. Manton is less clear about what the ancestors of the crustace- 
ans and chelicerates were like than she is about the uniramians, but 
we may infer that the crustaceans would have evolved from multi- 
legged, polyramous forms with foliaceous (leaf-like) appendages 
directed ventrally that handled food at the leg bases. The 
chelicerates would be developed from possibly uniramous forms, 
with few legs, possibly processing food at the leg bases. 



An independent set of data from that of Manton (1973a), with per- 
haps more convincing reasons for recognizing three arthropodous 
phyla, comes from the comparative embryological studies of D. T. 
Anderson (1973). His findings (fig. 3) for Annelida, Uniramia, Crus- 
tacea, and Chelicerata are summarized in Table 1. Data on the Pyc- 
nogonida has been added from T. H. Morgan (1891), Dogiel (1913), 
Sanchez (1959), and King (1973). 

The uniramians and annelids are seen to have a basic similarity of 
development. The annelids have spiral cleavage in eggs with little to 
moderate amounts of yolk (while uniramians do not exhibit spiral 
cleavage because of a modification induced by large amounts of yolk 
in the egg). In annelids the presumptive endoderm (midgut) arises 
from the 3 A, 3B, 3C, and 4D cells located along the ventral part of 
the blastoderm. This midgut area is enclosed by an overgrowth 
(epiboly) of cells from the dorsal blastoderm. The stomodeum arises 
from the 2b cell at the time of gastrulation as a solid mass of cells 
which subsequently hollows out and forms a mouth. The presump- 
tive mesoderm arises from the 4d lineage, is located posterior to the 
presumptive endoderm, and becomes internalized during the epiboly 
of the ectoderm. The mesoderm then grows forward as a pair of 
bands from which the somites bud. The presumptive ectoderm of 
the embryo develops from the 2d lineage. 

The uniramians have a developmental pattern like that of the an- 
nelids, although cell lineages cannot be traced because of the loss of 
spiral cleavage. The presumptive endoderm is a group of cells on the 
ventral surface of the blastomeres. (Frequently large anterior and 
smaller posterior midgut sections can be delineated, as in the Ony- 
chophora.) Gastrulation typically occurs with the presumptive 
endoderm migrating inward and becoming vitellophagic. The sto- 
modeum forms a solid mass of cells which then hollows out as a 
tube. The presumptive mesoderm in the onychophorans and chilo- 
pods arises from an area posterior to the presumptive endoderm (in 
the same position as in the annelids) and after involution, grows for- 
ward as a pair of bands. The annelid and uniramian development 
can thus be seen to conform to a basic plan. The uniramians, how- 
ever, are primitively epimorphic in their development, i.e., hatching 
with the adult complement of segments. 

The crustaceans are entirely and strikingly different from the 
above pattern. The cleavage of crustaceans which can be followed 





Fig. 3. Blastoderm fate maps of various types of articulates. A, the annelid, Tubi- 
fex; B, the onychophoran, Peripatus; C, a cirriped crustacean; D, a xiphosuran cheli- 
cerate. s=stomodeum, en=endoderm, m= mesoderm, e= ectoderm, etl= ectodermal 
teloblasts, pne=postnauphar ectoderm, me=mandibular ectoderm, a n e=antennal 
ectoderm, a]e=antennulary ectoderm, pte=protocerebral ectoderm, dee = dorsal ex- 
traembryonic ectoderm, cle=cephalic lobe ectoderm, ppe=pedipalp ectoderm, gze= 
growth zone ectoderm, p= proctodeum. It is important to note the relationships be- 
tween the stomodeum, endoderm, and mesoderm. (From Anderson, 1973.) 

out is spiral and can allow the tracing of cell lineages. The presump- 
tive endoderm arises from the 4D cell only. The presumptive meso- 
derm arises from the 3A, 3B, and 3C lineages and is thus anterior to 
the presumptive endoderm, rather than posterior as in the unirami- 
ans and annelids. The presumptive ectoderm arises from the 3d and 
4d cells and is very early zoned into regions conforming to the nau- 
plius topography. The stomodeum arises from the 2b cell, but does 
so independent of gastrulation. Thus the presumptive area relation- 



Table 1. Comparison of early embryonic conditions in various groups of arthro- 










arises as solid mass 

of cells at time of 


InvaginaLes as solid 

mass at gastrulation, 

seals off, and forms 

tube and mouth 


independent of 



3A, 3B, 3C, 
and 4D 

midventral sheet 
of surface cells 




posterior to 



surface cells pos- 
terior to presump- 
tive endoderm 

3A, 3B, and 3C 

anterior to 

presumptive endoderm 





bilateral, ventral 

bands lateral to 

presumptive endoderm 

3d and 4d 




mesoderm proliferates 

anteriorly as paired 


mesoderm proliferates 
anteriorly as paired 
bands only in primitive 

ships in the crustacean blastoderm are quite different than that 
seen in the uniramians. The Crustacea have anamorphic develop- 
ment, typically undergoing a more or less extensive larval phase 
after hatching. 

Chelicerate embryology has not received the attention that other 
arthropod groups have, but from what is known, the chelicerates do 
not have spiral cleavage. The cleavage is total, however, despite the 
amount of yolk present. The presumptive endoderm is segregated 
internally during cleavage. Typically the egg divides completely. 
Then the nuclei and surrounding cytoplasm "float" to the surface of 




little yolk 

moderate yolk 

copious yolk 

non -spiral 

non-sprial, but with distinct macro- and raicromeree 

associated with presump- 
tive mesoderm at anterior 
end of gastric groove 

associated with 



segregated Internally 

during cleavage- some 

groups with multipolar 

delamlnatlon of 


dorsal cell sinks 

and proliferates, 


multi-polar delam- 

ination from the 


dorsal macromeres 

enveloped by 
ventral mlcromeres 



from blastoderm 

of at least 


aldventral cells which 
form a gastric groove 
that sinks and 

circum- presump- 
tive endoderm 
cells sink and 
proliferate or 

de laminate (?) 

delaminates from 

edge of micromere 


delaminates from 

cir cum s tomodeum 


circum- presumptive 
and zoned 

ventral cells 


micromere forming 

mesoderm sinks at gastric 

groove and proliferates 

outward between ectoderm 

and endoderm 

little work done, and that sometimes 
contradictory in details 

the developing embryo, and begin to divide and develop a cell layer 
around the yolky cells on the inside. The presumptive mesoderm 
exists as a small, elongate area along the ventral midline of the blas- 
todisc. The mesoderm sinks inward as a gastric groove and prolif- 
erates cells that spread outward between the ectodermal and endo- 
dermal layers. The stomodeum is associated with the presumptive 
mesoderm at the anterior end of the gastric groove and forms the 
mouth at the time of mesoderm initiation. The embryonic ectoderm 
occupies the rest of the germinal disc and is zoned in patterns corre- 
sponding to the first four postoral embryonic segments. Nothing 

Fig. 4. Early embryology of pycnogonids. A-D, Phoxichilidium (from Dogiel, 
1913); E, F, Tanystylum orbiculare (from Morgan, 1891); G-J, Nymphon (from 
Dogiel, 1913); K-P, Callipalene empusa (from Morgan, 1891). e=ectoderm, m=meso- 
derm, en=endoderm, ma=macromere, mi=micromere, s = stomodeum. 



can be determined concerning cell lineages in chelicerates (or if they 
indeed ever had any). Certain features of their development allow 
them to be separated from other arthropods, viz., the simultaneous 
separation of presumptive endoderm from other regions with cleav- 
age, the initiation and sinking of mesoderm by delamination along 
the ventral midline of the embryo. Chelicerates have epimorphic 

It is unclear where Pycnogonida fit into this embryonic scheme; 
Anderson (1973) did not deal with them. Little reliable work has 
been done on pycnogonid development. A short review of what is 
known from the literature is attempted here (fig. 4). Their cleavage 
is total and non-spiral. Sanchez (1959) claimed to detect a spiral ar- 
rangement in the eight-cell stage of Callipallene. Morgan (1891), 
however, indicated cell lineages in Callipallene could not be traced 
and that the "micromeres" in such an arrangement are the only cells 
which eventually form the germinal disc. The yolk in pycnogonids 
can range from small to large amounts and the patterns of the pre- 
sumptive areas in the blastoderm vary among forms with different 
amounts of yolk. In the eggs with little yolk, the presumptive mid- 
gut forms from a surface cell which sinks to the interior and prolif- 
erates to form a syncytium (Dogiel, 1913). The mesoderm in such 
forms develops from cells which surround the presumptive midgut 
cell and delaminate mesodermal cells at the time the endoderm dif- 
ferentiates. Morgan (1891) reported multi-polar delamination of the 
endoderm in the pycnogonids with little yolk that he studied, Phoxi- 
chilidium maxillare and Tanystylum orbiculare, and is unclear as to 
how the mesoderm developed in these forms. 

In pycnogonids with moderate amounts of yolk, such as 
Nymphon stromii, the presumptive midgut develops from dorsal 
"macromere cells" of the embryo which come to be enveloped by an 
overgrowth of ventral "micromere cells." The presumptive meso- 
derm arises from some cells around the edge of the micromere "cap" 
which, when the "blastopore" reaches the equator of the embryo, 
migrate in under the micromere cap and proliferate mesoderm. 
Dogiel (1913) is unclear as to whether this is a true migration of cells 
or a marginal delamination from the micromere cap. 

In forms with large amounts of yolk, like Chaetonymphon spino- 
sum or Callipallene empusa, the division of the "macromere" cells 
stops at an advanced stage (cytokinesis is typically incomplete in 
many of these cells). Only the micromeres continue to divide, spread 


over the yolky macromeres, and form a germinal disk (Morgan, 
1891). Cells which form the endodermal tissue arise by multi-polar 
delamination from the disc. The mesoderm arises by multi-polar de- 
lamination of cells from around the region of the involuting stomo- 

The presumptive ectoderm of all pycnogonids is zoned into five 
regions and soon gives rise to the ventral organs corresponding to 
the protocerebral brain and first four postoral embryonic ganglia. 
Many pycnogonids have epimorphic development, though some of 
them are anamorphic with a protonymphon larva. 

The precise affinity of pycnogonids based on what is known of 
their embryology is inconclusive. The early separation of the endo- 
derm during cleavage, the association in at least some of the pyc- 
nogonids of mesoderm formation with developing stomodeum, and 
the zonation of the ectoderm suggests perhaps at least distant rela- 
tionship with the chelicerates. But a great deal more conclusive in- 
formation is needed, especially on the forms with large amounts of 
yolk in the egg, before pycnogonid embryology can be co-ordinated 
with that of other arthropods. 


A modification of the phyletic scheme of Manton and Anderson 
has been proposed by Cisne (1974, 1975) as a result of his examina- 
tion of the internal anatomy of trilobites. Cisne used stereoscopic 
x-ray techniques to study some unusual preservations high in pyrite 
of Triarthrus eatoni. He obtained information on trilobite internal 
soft anatomy unknown before, and was able to discern appendage 
structure, digestive organs with extensive diverticula in the ceph- 
alon, and the body and appendage muscles. Further, he found that 
the body muscles conform to a pattern found in the cephalocarid 
crustaceans (Hessler, 1964) and this reinforced Cisne's (1973) idea 
that trilobites and cephalocarids were structurally and functionally 
similar in their feeding habits. Cisne is very unclear, however, as to 
specific details in this conception. This muscular similarity he takes 
as proof that these two groups are related and thus proposes a di- 
phyletic scheme for the arthropods (Hessler and Newman, 1975, 
concur). On one hand are the Uniramia and on the other are the Tri- 
lobita-Crustacea-Chelicerata. Neither Cisne nor Hessler and New- 
man comment on Anderson's (1973) crucial embryological studies of 
the chelicerates and crustaceans nor on Manton's (1973b) observa- 


tions on chelicerate appendage morphology and derivation. There 
are serious problems concerning Cisne's (1974, 1975) supposed simi- 
larities in trilobite and primitive crustacean feeding habits and with 
his views of embryology which will be taken up below. 

The continuing problem among arthropodologists now seems to 
be not so much that the assemblage may be polyphyletic, but rather 
how many times this arthropodous phenomenon was evolved into, 
especially in regard to the trilobite-crustacean-chelicerate-pycno- 
gonid branch. I will attempt to present below an examination of the 
available evidence that might serve to clarify the inter-relationships 
of these marine arthropods. 


Anderson's (1973) treatise, while doing a superb job on the arthro- 
pod groups he covers, does not discuss the pycnogonids and the 
trilobites. As was stated above, what little is known of pycnogonid 
development is frequently not reliable nor very helpful in revealing 
their relationships, and although trilobites can never be known in 
the detail of living arthropods, some gross factors of trilobite devel- 
opment are known. 

Excellent reviews of trilobite development have been assembled 
by Whittington (1957, 1959). Trilobites are anamorphic with the 
protaspis as the earliest larval stage recognized (fig. 5). Some stu- 
dents have questioned whether the protaspis is indeed the earliest 
phase in the developmental sequence of the trilobites. It is sug- 
gested that some unfossilized pre-protaspis stage may have existed. 
(Gurney suggested in 1942 that nauplii evolved from them.) This is, 
of course, a possibility, though the size of protaspids, 0.25-1.0 mm., 
would suggest that at the very least we are dealing with a stage 
very close to that which hatched from an egg. Apparently not all 
protaspids were fossilized, the protaspids of the primitive olenellid 
trilobites have never been found. Sclerotization of the earliest stages 
in trilobite development may have been a relatively late evolution- 
ary event. Many of the larger types of protaspids, those in excess of 
0.4 mm., have the axis divided into five rings. (Smaller protaspids 
show no sign of this division.) The most anterior ring is the largest, 
is associated with the eyes, and possibly represents an acron. The 
remaining four segments, ending in the occipital, have been inter- 
preted as the basic four segments of the adult trilobite cephalon 
(Henricksen, 1926; Beklemishev, 1969). This axial segmentation of 



Fig. 5. Anamorphic larval stages of the trilobite Shumardia pusilla. A, protaspis; 
B-G, meraspis, degrees 0-5; H, I, holaspis stages. (From Whittington, 1959.) 

the protaspid is frequently lost in the higher trilobites, e.g., in the 
Lichidae (Whittington, 1956) the protaspids have no segmental 
grooves but have five sets of spines thought to correspond to the 
five basic axis segments. 


Trilobites have been frequently linked phyletically to merostomes. 
Iwanoff (1933) suggested the development of Tachypleus indicated 
a phyletic relationship with the trilobites. Iwanoff rejected the so- 
called "trilobite larva" of the limulids as indicating a relationship of 
the two groups (he suggested the larva was more like the Carboni- 
ferous merostome Euproops than a trilobite). His chief reason for 
relating the two groups was that in Tachypleus the initial delinea- 
tion of body segments by the mesoderm is into four somites, and he 
compared these to the four "post-acronal" segments of the trilobite 

This segmental arrangement is similar to that of the pycnogonids. 
In these arthropods the ectoderm produces five ventral organs 
which eventually give rise to the protocerebral (acronal) part of the 
brain and the first four embryonically postoral ganglia. In those 
pycnogonids which have a protonyphon larva, the first three of 
these ganglia are associated with the chelifore, pedipalp, and oviger 
segments respectively— the last ganglion not being associated with 
any appendages at this stage. 

The crustaceans, however, have a different embryonic segment 
pattern all together. In the initial nauplius stage typically three 
ganglia and three sets of appendages appear: the antennules, inner- 
vated by the deutocerebral portion of the brain, and the antennae 
and mandibles, innervated by the first two postoral ganglia. An- 
derson (1965, 1967) and Sollaud (1923) document variations in num- 
bers of ganglia, but it is not clear if these are not secondary. 

It would seem that trilobites, chelicerates, and pycnogonids all 
share early developmental stages with four postoral segments, 
while the Crustacea seem to be quite distinctive with only two. 
Whether the trilobites have the earliest "embryonic" stage with 
four postoral segments, as in the chelicerates and pycnogonids, is 
not at all determinable. Melnikov (1974) feels all arthropods have 
the same number of larval segments, but reasons largely in hypothe- 
sized stages. 


This consideration of embryonic larval segments in arthropods 
brings us to what Manton (1949) calls the "vexed subject" of head 
segmentation. Myriad interpretations by various authors have been 
put forward from time to time (for recent summaries see Manton, 
1949; Bullock and Horridge, 1965; Beklemishev, 1969). Hedgpeth 


(1954) termed these efforts as "the amiable pastime of phylogenetic 

All these efforts to determine the primitive number of head seg- 
ments have been based for the most part on embryos (Manton, 
1960). Manton (1949), however, pointed out some principles of em- 
bryology which are typically ignored in such speculations: 1) Somite 
boundaries are obvious, but two connected or separate coelomic 
sacs are not conclusive evidence of two somites. 2) Somites can be 
united to form one unit, but the early development usually shows 
evidence of such a fusion. 3) Coelomic sacs can arise from several 
mesodermal lacunae in a somite (especially in large embryos) and 
such lacunae do not indicate multiple somites. Her opinion was that 
most of the schemes of head segment homologies were probably not 
to be taken too seriously. 

The homology of head segments is necessarily related to the 
homology of brain regions. Bullock and Horridge (1965), after 
reviewing the literature, opted for the simplest, most parsimonious 
arrangement they could come up with, and which would permit a 
correspondence of nerve roots to similar regions in all arthropod 
groups and the annelids. This sort of approach was developed and is 
used in vertebrate morphology, is certainly more pragmatic, and 
may be more logical. If we are, in fact, dealing with separate phyla, 
it may be unreasonable to demand the homology of all head struc- 
tures. Using this scheme, the anteriormost appendages of arthro- 
pods can be compared (table 2). Manton (pers. comm.) would dis- 
agree in principle with Bullock and Horridge. 

Bullock and Horridge considered the protocerebrum and deuto- 
cerebrum to be two parts of an asegmental anterior neural mass 
(acronal). (There seems to be some embryonic indication that there 
may be true somites in the preoral region (Manton, 1960). Thus the 
term "acronal" here may be misleading.) The tritocerebrum is the 
first in the postoral series of ganglia. Pycnogonids and chelicerates 
do not have a deutocerebrum. The Crustacea and Uniramia are the 
only groups with true preoral (possibly acronal) appendages. 

The trilobite cephalic condition is only beginning to be made clear. 
Cisne (1974) records four somites in the cephalon of Triarthrus, an 
antenna-bearing segment and three leg-bearing segments. He claims 
that the antennal segment is preoral in derivation. Cisne homolo- 
gizes this segment with the cheliceral segment of the chelicerates, 
but he mistakenly claims that this segment is preorally derived in 







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all groups of arthropods. But Bullock and Horridge clearly point 
out the chelicerate chelae are tritocerebral in affinity, i.e., postoral 
in embryonic derivation. In all arthropod embryos the region form- 
ing the mouth and labrum moves posteriorly, while the lateral seg- 
mental tissue migrates forward. In addition, Cisne points out that, 
while he does not have any nervous system preservation in his 
fossils, the gut in his specimen travels some distance anteriad from 
the mouth and loops around in front of all the cephalic musculature, 
including that of the antennal segment (fig. 6). This structural ar- 
rangement would really seem to preserve the most primitive of ar- 
thropod states with little or no movement of any of the cephalic 
segments anteriorly during development, with only the mouth 
migrating posteriorly, the antenna and legs all being postoral in 
orgin. Linking this adult trilobite cephalic anatomy to the protaspid 
evidence presented above would indicate that the four segments of 
the late protaspid are postoral in position. It is very important to 
remember that the adult position of somite derivatives in relation to 
the arthropod mouth does not correspond to the relative position in 
the embryos. The condition of trilobites presented by me in Table 2 
is thus based on Cisne 's observations of trilobite internal anatomy, 
but is not his interpretation. 

The pycnogonids have no deutocerebrum. The chelifores are sup- 
plied by nerves from the tritocerebrum which typically migrates 
from an embryonic postoral position to some forward location in the 
adult. This is a condition identical to that of the chelicerates. Wiren 
(1918) and Henry (1953) examined the Nymphon brain (fig. 7). In 
Nymphon a short circumesophageal commissure connects the large 
subesophageal ganglion with pedipalp and oviger nerves to the 
supraesophageal "brain," with the dorsal optic nerves and the 
anterior chelifore nerves. 


Fig. 6. A, dorsal and B, lateral views of the head and first three thoracic segments 
of the trilobite Triarthrus eatoni showing the skeletomusculature and digestive sys- 
tem. A = antennal segment; b=post frontal endoskeletal bar; C=post antennal head 
segments; dlm=dorsal longitudinal muscle; e=esophagus; fb=frontal endoskeletal 
bar; fdm=frontal dilator muscle of esophagus (not seen but postulated); h=hypo- 
stome; hom= horizontal muscles; i=intestine; m=metastome; mca=mouth cavity; 
s=stomach; T=thoracic segment; tpa=tergal posterior apodeme; vdm=ventral 
dilator muscle of esophagus, vim = ventral longitudinal muscle (modified from Cisne, 
1974). Note that the esophagus is anterior to the frontal bar and the segmental 
structures related to the antenna. 


h e vdm 


A Cj C 2 C3 T] T2 T3 



Dissections by me of other pycnogonids reveal this arrangement 
can vary somewhat (fig. 7) 1 . Dodecolopoda mawsoni has a large sub- 
esophageal ganglion from which the pedipalp and oviger nerves 
arise (this ganglion is a fusion of the second and third postoral gan- 
glia of the embryo), as well as paired nerves into the ventral part of 
the proboscis. The circumesophageal connectives are quite long and 
enshrine not only the esophagus, but a pair of proboscis levator 
muscles as well. This is a condition similar to that mentioned by Fry 
(1965). The chelifore nerves arise from the area where the commis- 
sures meet the brain. 

Colossendeis australis is similar in most respects to Dodecolopo- 
da, except that in its final molting stage to the adult the chelifores 
are lost. The commissures in this species again encircle the esopha- 
gus and proboscis levators, but there is no chelifore nerve. The only 
indication of the former presence of this appendage is the chelifore 
diverticula from the gut which arise behind the commissures, come 
around lateral to them, and proceed anteriorly to the point where 
the base of the chelifore would have been. These gut diverticula 
dead end at the base of the proboscis. 

This compression and movement of the interior ganglia forward is 
related to the distinctive location of the pycnogonid mouth. The 
mouth, unique among arthropods, is usually directed anteriorly as 
opposed to a primitive ventral or ventro-posterior orientation (Hess- 
ler and Newman, 1975). Both Dodecolopoda and Colessendeis dis- 
play a compression of the ventral nerve ganglia toward the anterior 
end of the body. This is opposed to the condition that Henry (1953) 
observed in Nymphon where each ganglion was located at the level 
of the segment it served. 

'Material supplied by Professor Joel W. Hedgpeth from his personal collection. 


Fig. 7. Central nervous systems of pycnogonids. A, Nymphon sp. (modified from 
Wiren, 1918), brain and anterior nerves in relation to the esophagus; B, trunk 
ganglia of Nymphon pixillae (modified from Henry, 1953); C, trunk ganglia of 
Dodecolopoda mawsoni; D, brain and anterior nerves in relation to the head anat- 
omy of D. mawsoni; E, brain and anterior nerves in relation to the head anatomy of 
Colossendeis australis; F, trunk ganglia of C. australis. spg=supraesophageal gan- 
glion, sbg=subesophageal ganglion, cec= circumesophageal commissure, opn= 
optic nerve, chn= chelifore nerve, rn= rostral nerve, ppn= pedipalp nerve, on = ovi- 
ger nerve, e= esophagus, chd=chelifore diverticulum, ppd=pedipalp diverticulum, 
ch= chelifore, pp=pedipalp, o=oviger, 1st wl=first walking leg, pr=proboscis, 
plm= proboscis levator muscle. 







Fig. 8. Phyletic relationships of the Cheliceriformes (modified from Firstman, 
1971), with cross-sections through the anterior trunk regions to display the relation- 
ships of Dohrn's membrane and the endosternites to the general trunk anatomy. A, 
hypothetical pre-chelicerate stage suggested by certain pycnogonids; B, Colossende- 
omorpha (Colossendeis); C, Nymphonomorpha {Endeis); D, Pycnogonomorpha 
(Pycnogonum); E, hypothetical ancestral merostome-arachnid; F, Limulus; G, hypo- 
thetical ancestral arachnid; H, tick; I, typical tracheate arachnid; J, Scorpionida; K, 
typical pulmonate arachnid; L, lungless spider. a=aorta, g=gut, n=central nervous 
system, DM = Dohrn's membrane, PVM = perineural vascular membrane, NIO= 
neural intestinal omentum, H=horizontal membrane, E— endosternite, VS=ventral 
suspensor muscle, DS=dorsal suspensor muscle, TM=transverse muscle, PIM 
= peri-intestinal membrane, TS=thoracic sinus. 




The work of Manton has come to make the structure and function 
of arthropod appendages very important to the modern phyletic 
theory of the group. Not as much detailed analysis has been done on 
appendage morphology in trilobites, chelicerates, and pycnogonids 
as in the uniramians and crustaceans. 

As mentioned above, Manton (1973a, b) considered the chelicer- 
ates. A divergent origin of this group from other arthropods was 
suggested by several factors. There was probably a small number of 
legs in the ancestral chelicerate form and thus no concomitant need 
for rigid co-ordination of gaits. There is a great importance to rock- 
ing movements in arachnids, with most limb joints being equipped 
with flexors and few extensors and only a limited promotor-remotor 
movement at one joint. Extension is achieved by regulating body 
fluid pressure (Ellis, 1944). The arachnid coxa is typically an im- 
mobile joint, while in the limulids the coxa is capable of a strong 
transverse action to engage the gnathobases during feeding as well 
as promotion-remotion. The origin of the chelicerates was thus dif- 
ferent from that of the uniramians or crustaceans, both of which in- 
volved multi-legged, well co-ordinated types. 

Firstman (1971) has demonstrated on the basis of gross dissec- 
tions the importance of the endosternite derived from vascular 
membranes in chelicerates. This structure is unique to chelicerates 
and to some extent pycnogonids. It functions as a point of origin for 
locomotor muscles, serving as a free-floating skeleton anchored by 
body wall muscles which insert on it. It is derived from the mesoder- 
mal vascular membranes surrounding the gut and nerve chord. 
These membranes have become impregnated with connective tissue 
as they became associated with body muscles. The body wall mus- 
cles serve to tense the endosternite as the animals move. This 
system serves as a substitute for the apodemal (ectodermal) endo- 
skeleton of uniramous and crustaceous arthropods which functions 
similarly. The endosternite is developed in inverse proportion to any 
apodemes that may be present, so that in the Order Solpugida an en- 
dosternite has been completely replaced by an extensive apodemal 
system. Firstman observes that the pycnogonids have a perivisceral 
sinus similar to the endosternal-associated sinus of chelicerates that 
lack booklungs. On this basis he associates the pycnogonids with 
the chelicerates (fig. 8). «. 


There is a question as to whether the chelicerate limbs are primi- 
tively uniramous or biramous. The appendages of all chelicerates, 
living and extinct, are uniramous, except for some exceptions in 
Limulus. The sixth prosomal appendage of Limulus has a small 
lobe, the flabellum, attached to the dorsalmost projection of the 
coxa, and the respiratory appendages of the mesosoma have gills 
developed on the posterior surfaces of a rather broad, flat, laterally 
directed lobe. The significance of these structures is not clear. 
Stdrmer (1944) considered these as indications that the chelicerate 
limb was primitively biramous. He homologizes the flabellum and 
respiratory lobes of Limulus with the filamentous branch of the tri- 
lobite limb. Kaestner (1968) seems to have some reservations about 
such an interpretation for the respiratory appendages of the meso- 
soma, though he is not specific. The appendages of the Cambrian 
Aglaspis prosoma (the most primitive known chelicerates) are uni- 
ramous (Raasch, 1939). Only the first appendage is chelate and none 
of the aglaspid postoral appendages seem to have gnathobases (fig. 
9). This might seem to indicate that the appendage morphology of 
the limulids may be quite specialized. 

It is perhaps important to note that the flabellum occurs only on 
the one prosomal appendage, which, besides biting in a transverse 
plane, serves to push food forward into the anterior part of the Lim- 
ulus food groove. The flabellum itself is directed back over the gill 
appendage chamber and may serve a special function in setting up 
respiratory currents. The respiratory appendages arise embryoni- 
cally as a flap from which the medial, more distal elements of the 
limb develop. One might thus legitimately interpret the respiratory 
portion of the mesosomatic limbs as specializations of the coxa. 

Little has been published on pycnogonid locomotory morphology 
except for some observations by Cole (1901), Prell (1911), Arita 
(1937), and Morgan (1971). Examination of preserved pycnogonids 
by me indicate that the movement in pycnogonid leg joints is very 
restrictive. The body-first coxa joint is almost immobile, capable of 
only slight dorso-ventral movement, if at all. The first coxa joint is 
the only one capable of promotion-remotion. All the other joints in 
the leg move dorso-ventrally. Viewing of movie film of various pyc- 
nogonids in motion reveals a great variety of gaits in pycnogonids 
(Schram and Hedgpeth, 1978). An extensive analysis is not pos- 
sible here, but some generalizations can be made. 1) Pycnogonids 
typically execute rather inaccurate and unco-ordinated stepping 
movements, although there are some exceptions, such as Deco- 



1 I th segment 


antenor prosomal appendage 

last prosomal appendage 

B \ 

Fig. 9. Anatomical features of Cambrian aglaspids noting the uniramous character 
of the limbs. A, Urarthrus instabilis, posterior most extremity from the ventral side; 
B, Aglaspis spinifer, whole body; C, chelate first prosomal appendage with epistoma 
of ,4. spinifer; D, last prosomal and first abdominal appendages of A spinifer. From 
Stfrmer, 1955.) 

lopoda which moves in a well co-ordinated manner. Others, e.g., 
Colossendeis, can be characterized as "stumbling about" when they 
move. 2) The promotor-remotor action is generally limited. Because 
of the arrangement of legs on a pycnogonid body in a radial configu- 
ration, the animals can sometimes move without any promotion- 
remotion depending on the direction of travel, and Dodecalopoda 
doesn't appear to use it at all. 3) The importance of the "rocking" 
pattern in individual leg motion (similar to that described above for 
chelicerates) is very marked, though in some forms it seems to be 
rather labored and deliberate. In general, pycnogonids seem to 
share many features in common with the chelicefate pattern of loco- 
motion, such as "rocking," inaccurate stepping movements, and de- 


emphasis of promotor-remotor action at a specific joint. This agrees 
with what is found in Limulus and arachnids. Terrestrial chelicer- 
ates do not have extensor muscles in some of the leg joints, but use 
fluid pressure to extend the leg, as may also be the case in pycno- 
gonids. The segments of legs of at least some pycnogonids are not 
all equipped with extensor and flexor muscles. 

Relatively little is known concerning trilobite limbs. Of the hun- 
dreds of trilobite genera, only six are known at all well (Stdrmer, 
1939; Harrington, 1959). On this limited basis trilobites have often 
been characterized as having similar appendages. Though trilobite 
appendages are alike in general form, having a strong telopod and a 
"filamentous branch" arising from the coxa, an examination of 
these few species reveals a potential for a wide range of functional 
variations (fig. 10). 

Olenoides (Whittington, 1975b), Triarthrus (Cisne, 1975), Naraoia 
(Whittington, 1977), and possibly Cryptolithus (Bergstrom, 1972; 
Campbell, 1975) possess medially directed spines or setae on the 
coxa which may have functioned like Limulus coxae, or possibly 
utilized a pushing action more like crustaceans. Phacops (Sturmer 
and Bergstrom, 1973), Naraoia, and Cryptolithus have spines and 
"gnathic" structures on more distal segments of the telopod. Cer- 
aurus has a completely unadorned telopod, no "gnathic" structures 
at all. The morphology of the filamentous branch exhibits even more 
diversity (the use of terms like "preepipodite" or "exite" leads to 
phyletic and anatomical conclusions which are not necessarily justi- 
fied). Bergstrom (1969) gives some convincing arguments against 
the filamentous branch being able to function in respiration. Berg- 
strom points out that the cuticle of the filaments is relatively thick. 
Though the filaments are flexible they do not collapse or fold as a 
structure with a thin cuticle would do, and the filaments are pre- 
served equally as well as the trilobite telopod. This would seem to 
imply a cuticle on these filaments too thick to sustain a respiratory 
exchange of gases, though Whittington (1975b) seems to disagree. 
Bergstrom also raises a question about the mechanics of getting 
body fluid out into the filaments and back, and the ability of the tri- 
lobite body plan to sustain the necessary high body fluid pressure to 
achieve it. Bergstrom feels that the apparently thin cuticle on the 
underside of the trilobite pleura is a better candidate for the respira- 
tory surface. 

Cannon and Manton (1927) illustrate how the filamentous branch 

Fig. 10. Trilobite appendages. A, Phacops, anterior view (modified from Sturmer 
and Bergstrom (1973); B, Triarthrus eatoni, anterior view (from Cisne, 1975); C, 
Cryptolithus tesselatus, ventral view (from Campbell, 1975) D, C. tesselatus, ventral 
view (from Bergstrom, 1972); E, Ceraurus, anterior view (modified from Stdrmer, 
1939); F, Olenoides serratus, anterior view (modified from Whittington, 1975b); G, 
Naroia compacta (from Whittington, 1977). Filamentous branches of A, B, E, F, and 
G actually extending into the plane of the figure. 



can act as a device to set up a filter feeding current (see below). But 
Bergstrom (1972) discusses how the filamentous branch of Crypto- 
lithus could have been used in a more "aggressive," non-filtering, 
feeding action. He suggests that in Cryptolithus the filaments are 
restricted to only the distal part of the filamentous branch and are 
directed downward. He believes these filaments were probably used 
to stir up sediments over which the animal walked. Campbell (1975) 
disagrees with this arrangement. Whittington (1975b) analyzed 
Olenoides tracks and trails and concluded their legs moved meta- 
chronally, and that the "filamentous branches" may have assisted 
in this. 


Cisne (1973, 1975) stated that trilobites feed in the manner of 
cephalocarids and other primitive crustaceans. Marshall and Orr 
(1960) give four criteria for a filter feeder: 1) a filter; 2) a means of 
creating a flow of water through the filter; 3) some way of scraping 
the filter and getting the food to the mouth; and 4) an exit for the fil- 
tered water. Cannon and Manton (1927) and Barrington (1967) give 
some lucid accounts of various filter feeding devices. We need be 
concerned with only three types in this account (fig. 11). 

The primitive crustacean filter feeding apparatus is composed of a 
series of superficially imposed, ventrally directed paddles (fig. 
11G-I). This is the type found in the cephalocarids. On the forward 
stroke, the space between the paddles is increased and water is 
sucked up between the limbs. Food particles in the water are 
trapped on the setae of the limbs. On the backstroke the bulk of the 
water is pushed out from between the limbs in a propulsive stream, 
but a backwash current is created on the anterior surface of the pad- 
dles which carries a stream of water dorsally, helping to clean off the 
setae, and sweeping the particles into a midventral food groove. The 
action of the opposed coxal setae and the anterior current in the 
food groove created by the action of the hypostomal flap moves the 
food to the mouth. There it is trapped with the assistance of mucous 
secretions of the epistome and the maxillary setae. 

It is difficult to compare any possible trilobite mode of filter feed- 
ing with the above crustacean type. Tiegs and Manton (1958) felt 
there might be some difficulty in attempting to oppose trilobite 
gnathobases, as in the chelicerates, or coxal setae, as in the Crusta- 
cea. Cisne (1975) and Whittington (1975b, 1977) reconstruct trilo- 



Fig. 11. A-C, Cannon-Manton hypothesis of trilobite limb function with A, cross- 
sectional view, and B and C, ventral views at successive stages (arrows indicate flow 
of water). D-F, Stdrmer hypothesis of trilobite limb function with D and E cross-sec- 
tional views at successive stages and F a ventral view of stage E. G-I, current flow 
around the limbs of a primitive crustacean type (lateral views). J, current flow 
around the functioning expodites of a mysid crustacean. 



bite legs with opposed coxae. The ventro-laterally directed telopod 
of the trilobite is generally conceded to be a "walking" type of limb. 
The telopod may be supplementary to feeding if some of the seg- 
ments of the leg can function as subchelae, e.g., Cryptolithus or Ole- 
noides. The only structure on the leg that apparently can function in 
filter feeding is the laterally directed filamentous branch. Cannon 
and Manton (1927) thus proposed a different mode of filter feeding 
in the trilobites from that of crustaceans (fig. 11A-C). They noted 
the filamentous branch is placed parallel to the pleura. The only way 
this might function to set up a feeding current is if the branch vi- 
brated back and forth, the filaments directed ventrally. This vibra- 
tion under the rigid pleura would serve to set up a backwash current 
along the dorsal edge of the filamentous branch, moving water 
medially. Tiegs and Manton suggest a large labrum or hypostome, 
e.g., like that seen in the remopleurids, combined with the action of 
the pharynx must have served to create a suction to pull the food 
current forward. But the form of the trilobite hypostome varies 
from the very small, e.g., Dimeropyge, to quite large, implying that 
the hypostome might not have functioned similarly in all trilobites. 

Stormer (1939) postulated a different use of the filamentous 
branch to set up a feeding current. In his analysis of the filamentous 
branches or Cryptolithus he noted that they seemed to be arranged 
so that the filaments were directed posteriorly, i.e., were parallel to 
the pleura (fig. 11D-F). He assumed all trilobites were so oriented. 
When the filamentous branches were depressed ventrally, they 
would suck in a volume of water into the subpleural space. Stormer 
then postulated that when the filamentous branches were raised so 
that the distal tips touched the pleura, closing off the subpleural 
space, this would force water forward toward the mouth. However, 
it appears to me that this elevation would not only force water for- 
ward, but also ventrally through the filaments, medially between 
the leg bases, and posteriorly. Furthermore, when the filamentous 
branches would be depressed to allow more water to flow into the 
subpleural space, all the currents set up by the elevation, including 
the one moving forward toward the head, would be broken by tur- 
bulent flow induced by the depression. The main defect of the 
Stormer hypothesis is that it does not allow for separate currents, 
one for getting food and the other for moving it to the mouth, as 
does the Cannon-Manton scheme. The Cannon-Manton hypothesis 
seems to be the most functionally feasible. 


Thus the trilobite mode of feeding would be different than that of 
the primitive crustaceans, KkeHutchinsoniella, and is only remotely 
similar to that found in the "schizopodous" malacostracans, those 
with "biramous" thoracic limbs (fig. 11). In this latter group the 
exopod of the leg is directed laterally and somewhat posteriorly and 
rotated in a circular pattern to describe a cone. A vortex, created up 
the center of the cone, directed medially, directs a feeding current 
into a midventral food groove. The arrangement of the leg com- 
ponents help direct the current forward and the action of the maxil- 
lary setae assist in capturing the food particles as the food stream 
passes through them. 

Cannon and Manton suggest that if one wishes to relate them, all 
of these types of filter-feeding mechanisms could be only derived 
from some strictly hypothetical form. 


A kind of convergence exists in the appendage structure of trilo- 
bites and malacostracous crustaceans; both exhibit a biramous con- 
dition. The oft-quoted similarity here, though, is superficial and the 
phyletic import attached to this is minimal. The exite-like filamen- 
tous branch of trilobitomorphs arises from a basal segment (a coxa 
or precoxa) of a seven- or eight-segment telopod, while malacostra- 
cans have a two-segment leg base off which can arise exite gill flaps, 
a filamentous exopodite, endites, and a telopodic-like endopod of 
five segments. Though the two main branches of the eumalacostra- 
can limb lend it a biramous character, the wide array of possible en- 
dite and exite structures betray its polyramous ancestry. 

Similarity between crustacean and trilobite limbs disappears 
when the primitive crustacean limb type is examined (fig. 12). Bor- 
radaile (1926) was the first to point this out and the recent work of 
Sanders (1963) and Hessler (1964) on cephalocarids reinforces these 
considerations. (Indeed, I would recognize three basic limb types in 
arthropods: the uniramous telopods of uniramians and chelicerate- 
pycnogonids; the biramous type of the trilobitomorphs; and the 
polyramous, foliaceous type of the crustaceans.) The basic crusta- 
cean limb, as illustrated by the trunk appendages of cephalocarids, 
is thin and leaf-like, composed of a basal protopod off which arises a 
flap-like epipodite; a foliaceous exopod; a weakly telopoditic endo- 
pod; and several endites (fig. 12). This foliaceous limb can best be 




Fig. 12. Thoracic limb of the cephalocarid crustacean, Hutchinsoniella macrocan- 
tha. pr=protopod, ep=epipodite, ex=exopodite, e=endopodite, en=endites. 

termed polyramous. The number of exites and endites that can be 
developed on this basic form in different crustacean groups is vari- 
ous. This plan is elaborated upon in the course of the crustacean 
radiation in the branchiopods, ostracods, many of the maxillo- 
podans, and the primitive malacostracans. It is only in the higher 
malacostracans and the cirripeds that there is a deviation from this 
plan (fig. 13). 

Any similarity, as stated above, between the trilobite biramous 
limb and that of the higher crustaceans is purely convergent. The 
convergence arises from the functional similarities the parts of the 







thoracic & maxillary 




radiation on the generalized 
body plan 


generalized ancestral typo 


bent hie -.pelagic 
body togmosls 


Fig. 13. Phyletic relationships of the classes of Crustacea with the ancestral envi- 
ronment and main anatomical exploitation of each class given. Representative 
thoracic appendages illustrate modifications and variations which can take place on 
the ancestral type. 

limbs share. The trilobite telopod and the malacostracan endopod 
are walking or pushing structures which require their long, cylindri- 
cal, well-muscled arrangement to develop motive power. The trilo- 
bite filamentous branch and the schizopod exopod are swimming 
and sometimes filtering elements. 


Cisne (1974) points out the marked similarity of trilobite body 
muscles and those of cephalocarids (Hessler, 1964). But it is also 
interesting to note the similarity of these to other arthropods, e.g., 
the musculature of chilopods (Manton, 1965). Though these similari- 
ties are interesting, the possibility of convergence must again be 
raised. Schram (1969) has already indicated a convergent develop- 
ment in the complex "caridoid" musculature of hoplocaridans and 

Cisne also discovered remnants of the liver and its diverticula in 
Triarthrus cephalon. The trilobite thorax apparently carried only 


the hindmost portions of the gut toward the anus. In this regard the 
trilobite cephalon is more akin to the chelicerate prosoma than the 
typical crustacean cephalon, and the trilobite thorax-pygidium is 
thus more like a chelicerate opisthosoma than a crustacean thorax 
and abdomen. The cephalocarids, however, have a pair of simple 
diverticula from the anterior midgut extending forward into the 
cephalon (Hessler, 1969). 


The separate and distinct nature of Uniramia, Crustacea, and 
Chelicerata seems to be well established on the basis of embryology 
and leg morphology. What then are the possible phyletic affinities 
(table 3) of the Trilobitomorpha and the Pycnogonida? 

In this regard it should be noted that much information is coming 
to light in the restudy of the Middle Cambrian Burgess Shale trilo- 
bitomorphs under the direction of Prof. Whittington and his col- 
leagues at Cambridge (Whittington, 1971, 1974, 1975a, 1977; 
Hughes, 1975). These trilobitomorphs were generally thought to 
possess trilobite-like limbs but exhibit a variety of body forms. The 
actual trilobite radiation is basically an elaboration of one particular 
body type. Analysis seems to be indicating a great trilobitomorph 
radiation in the earliest Paleozoic, whose taxonomic affinities may 
be with many arthropod groups, i.e., not all trilobitoids may be 
related to trilobites. 

Trilobites have characters that relate them to different groups 
depending on the selection. The only really sound crustacean simi- 
larities with trilobites lies in the nature of the body musculature and 
in both possessing anamorphic development. But there is a possibil- 
ity of convergence in the muscle structure. Generally similar modes 
of habit might be expected to produce a similarity in musculature. 
The distinct nature of the basic appendage types and differences in 
the larval postoral segmentation in the two groups seems to 
preclude any relationship. 

There are a few trilobite similarities with chelicerates: the restric- 
tion of the hepatic organs to the cephalon, the possession of four 
postoral segments in the protaspid larval stages, and possibly bi- 
ramous appendages. These characters would seem to be stronger 
than those which relate trilobites to crustaceans. It is quite possible 
the hepatic location could be convergent. The embryonic evidence is 
perhaps significant, though there must always be an element of 

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doubt since we can never be sure of what we are actually dealing 
with in the trilobite larva (unless someone comes up with a fossilized 
trilobite germinal disc). Trilobites, however, exhibit anamorphic 
development, while chelicerates are epimorphic. The supposed bi- 
ramous nature of the chelicerate limb can only be doubtfully as- 
sessed, especially in view of what little is known of the most primi- 
tive merostomes, the Aglaspida. (Raw (1957) allies chelicerates and 
trilobites based on a detailed analysis of gross external morphology 
of the dorsal exoskeleton of olenellids. His analysis, however, is an 
excellent example of the opening quotation of this paper from Man- 
ton— unsound comments based on imprecise knowledge of the 
meaning of animal shapes.) 

Trilobitomorph affinities to uniramians seem to be expressed only 
in that both are primarily multi-legged, homonomous forms. But 
their legs are not at all similar nor are their modes of feeding, and 
these among other factors seem to preclude any relationship. 

There are some totally unique trilobitomorph characters. These 
are: the body form with great lateral development of the pleura, the 
biramous structure of the leg, the possible mode of feeding either as 
a filter feeder or any other suggested modification thereof, and the 
first postoral limb as an antenna. The body form is distinctive in the 
trilobites proper, but it should be noted that some of the Cambrian 
trilobitomorphs exhibit deviations from this form. The leg structure 
and function is most important both in its being quite distinct from 
any other arthropod types and in the importance that Manton has 
placed on limb functional morphology in separating arthropodous 

Important potential lines of evidence relevant to trilobitomorph 
affinities, such as brain structure, nerve innervation patterns, and 
early embryonic details will probably never be known. The evidence 
available at present indicates to me that establishing trilobitomorph 
relationships with uniramians and crustaceans is difficult, but, on 
the other hand, possible with chelicerates. Trilobitomorphs really 
appear to be rather unique, especially when judged on criteria 
similar to those used by Manton to set off the other arthropodous 
phyla. Thus a separate phylum, Trilobitomorpha, is suggested for 
these forms for the time being. 

Attempting to determine pycnogonid phyletic affinities has 
always posed a problem (table 4), partly due to a real neglect in at- 



Table 4. Character states of Pycnogonida compared with the other groups of living 
arthropods. Items indicating similarity (PROS) are commented upon: +=good 
point, c=possibly convergent, (c)=slight chance of convergence. Items arguing 
against affinity stated under CONS. 





Proboscis (protrusive 
lips of onychophorans) 

Ventral organs 

Few legs on body 

1st appendage chelate 
Four postoral initial 

embryonic segments 





Uniramous leg 
Leg function in 

+ (c) 


+ (c) 

No deutocerebrum 


Liquid-suctorial feeders 


Uniramous legs 


"Caprellid" form 


No deutocerebrum 

Anterior mouth 




Body form 


tempting to gather information on their basic biology. They have 
been allied to every major taxon of arthropods at one time or anoth- 
er. Tiegs and Manton (1958), albeit only half seriously, suggested 
that they might be related to onychophorans. They compare the 
pycnogonid proboscis to the lips of Peripatus which can be extruded 
over the jaws. They also note that the ganglia of both pycnogonids 
and onychophorans develop from ventral organs in the embryo. 

Nothing seems to relate the pycnogonids to the crustaceans. The 
body form of some pycnogonids is generally similar to that of 
caprellid amphipods or certain asellote isopods, but this is almost 
certainly due to a similarity of epizoic habit. This reflects part of the 


problem with pycnogonids in general. They are undoubtedly a very 
ancient group which has become highly specialized to a particular 
mode of life. 

There are several chelicerate-like characters possessed by the pyc- 
nogonids: first limb being chelate, four postoral segments in the ear- 
liest embryonic phases, endosternite similarities, similarities in the 
structure and function of the uniramous legs, liquid suctorial feed- 
ing habits, and the lack of a deutocerebrum. All these items seem to 
be very telling. The nature of the embryonic segmentation is quite 
interesting, but its full significance can be better evaluated when 
the earliest embryonic conditions of the pycnogonids are more fully 
understood. The limited promotor-remotor movement of the legs 
and the importance of the rocking movement of the appendage are 
most reminiscent of conditions seen in various arachnids. The lique- 
faction-suctorial mode of feeding in both pycnogonids and chelicer- 
ates is, given other anatomical features, startling, though possibly 

Unique pycnogonid characters are the mouth orientation and, to 
some extent, the body format. On this last point, Lehman (1959) has 
shown that the Devonian pycnogonid, Palaeoisopus problematicus, 
had a multi-segmented abdomen. The pycnogonid abdomen or meta- 
soma has thus been lost in the course of evolution of this group. 

Looking at all the available evidence, it would appear that the 
pycnogonids have a strong connection with chelicerates proper 
Some of the characters joining these groups could be convergent 
but I feel this is an instance where the numbers of similarities are so 
high and of such a nature as to be more easily explained by postulat 
ing actual relationship rather than invoking multiple convergences 
especially with other unquestioned characters uniting the groups 
(Schram and Hedgpeth, 1978). The pycnogonids are, however, prob 
ably a very early offshoot of the line leading to Chelicerata: a sep 
arate subphylum status for Pycnogonida should be maintained 
within a phylum Cheliceriformes, separate from the subphylum 


A summary taxonomy for the arthropodous phyla is presented 
here. It is understood now that the arthropodous condition is a 
grade of organization similar in character to a pseudocoelomate, or a 
lophophorate, or any other grade. If one does not wish to abandon 


monophyly, then each of the phyla in this outline might be consid- 
ered a subphylum, and the subphyla as superclasses. 

Phylum: Uniramia, Manton, 1973 
Subphylum: Onychophora Grube, 1853 
Subphylum: Myriapoda Latreille, 1796 

Class: Chilopoda Latreille, 1817 

Class: Diplopoda Gervais, 1844 

Class: Symphyla Ryder, 1880 

Class: Pauropoda Lubbock, 1866 

Class: Arthropleurida Waterlot, 1934 
Subphylum: Hexapoda Latreille, 1825 

Class: Protura Silvestri, 1907 

Class: Collembola Lubbock, 1862 

Class: Diplura Borner, 1904 

Class: Thysanura Handlirsch, 1908 

Class: Pterygota (Insecta) Brauer, 1885 

Phylum: Crustacea, Pennant 1777 

Class: Cephalocarida Sanders, 1955 
Class: Branchiopoda Latreille, 1817 
Class: Ostracoda Latreille, 1806 
Class: Maxillopoda Dahl, 1956 

Subclass: Copepoda Milne-Edwards, 1840 

Subclass: Mystacocarida Pennak and Zinn, 1943 

Subclass: Branchiura Thorell, 1864 

Subclass: Cirripedia Burmeister, 1834 
Class: Malacostraca Latreille, 1806 

Subclass: Phyllocarida Packard, 1879 

Subclass: Hoplocarida Caiman, 1904 

Subclass: Eumalacostraca Grobben, 1892 
(Several superorders) 

Phylum: Cheliceriformes, nov. 

Subphylum: Pycnogonida Latreille, 1810 
Subphylum: Chelicerata Heymons, 1901 
Class: Merostomata Dana, 1852 
Subclass: Xiphosura Latreille, 1802 
Subclass: Eurypterida Burmeister, 1843 
Class: Arachnida Lamarck, 1801 


Phylum: Trilobitomorpha Stdrmer, 1944 
Class: Trilobitoidea Stdrmer, 1959 
Class: Trilobita Walch, 1771 


To some extent the question of whether a taxon is polyphyletic or 
monophyletic is semantic. It depends on where one wishes to "draw 
the line." In the context of the present discussion, it is sufficient to 
state that if the "phyla" in question cannot be derived from an im- 
mediate common ancestral type, they are polyphyletic. Naturally, 
one could continue to postulate ancestral, hypothetical types and 
eventually draw all the divergent arthropodous strains together to 
some "segmented worm." Each of those hypothetical intermediates, 
however, must be considered as an "arthropod." One could do this, 
but the end product would be a rather extensive "paper phylogeny" 
for which there would not be any concrete evidence. Manton (1973b) 
has laid down an important tenet or law of phylogeny which is im- 
plicit in all her work. Every stage in a postulated series connecting 
one morphological form with another must be completely functional 
in all its aspects. To postulate going from a stage A to stage B via 
any stages which are functionally impossible is not allowable. Pro- 
found differences of embryology and locomotory morphology in the 
end points of the clade must be reconciled. 

Therefore, when I delineate ancestral types for the various arthro- 
podous phyla, I am merely elucidating forms which could have 
given rise to the "phylum" in question and not making comment on 
what might have preceded such stages. Speculations along these 
lines (e.g., Melnikov, 1971) are interesting, but do not easily lend 
themselves to functional proof. 

We might now delineate a possible ancestral condition for each ar- 
thropodous group adding to the initial efforts of Manton (1973a, b) 
in this regard. 

The Uniramia are evolved from a form characterized as multi- 
legged, with a soft body, with lobopodial limbs, manipulating food 
(incipient biting) with the tips of the future jaws, omnivorous in 
diet, with a basically annelidan embryonic pattern, and at least ini- 
tially epimorphic development. 

The Crustacea have evolved from a form that was multi-legged, 
having polyramous and foliaceous appendages, manipulating food 


with the base of the future jaw, a filter feeder, with a "crustaceoid" 
embryonic pattern, and at least initially anamorphic development. 

The Cheliceriformes are evolved from a form with relatively few 
legs with possibly uniramous lobopodial limbs, possibly manipulat- 
ing food with several leg bases with the assistance of some limb tips, 
carnivorous in diet, with "cheliceriform(?)" yolk-modified embry- 
onic pattern, and epimorphic development. 

The Trilobitomorpha are derived from a form that was multi- 
legged, with biramous appendages, a detritus feeder, an unknown 
embryonic pattern, and anamorphic development. 


Certain monophyletic sensibilities may be offended by recogniz- 
ing four arthropodous phyla. The establishment of the concept of 
arthropods as a convergent phenomenon is based on more than just 
gross anatomy with little or no reference to function or any other 
aspects of the biology of the animals. To quote Manton (1973b, p. 
317), "The evidence of comparative functional anatomy and habits 
of life has thrown more light on the course of evolution and their 
directing forces than has any other type of evidence, and an im- 
mense mass of hitherto meaningless structure has become intellig- 
ible." This points out the fact that when we describe a taxon, at no 
matter what level, we cannot deal exclusively with gross anatomy 
as if in a vacuum. We must of necessity look at all the aspects of the 
living system insofar as is possible. This must of necessity include 
anatomical, functional morphologic, genetic, ecologic, geographic, 
biochemical aspects of an entity, rather than what we have come to 
be satisfied with when we have heretofore described a taxon. 


I thank the following persons who have read the manuscript: Dr. 
Niles Eldredge, Dr. Sidnie Manton, Prof. Joel W. Hedgpeth, Dr. 
Robert Hessler, and Prof. Harry Whittington. This does not neces- 
sarily imply agreement with the author's ideas or his presentation. I 
would also like to thank Prof. Hedgpeth at whose invitation this 
paper was written and whose library, collection, and laboratory 
facilities at Oregon State University Marine Science Center were an 
invaluable resource for the production of this paper. Work was sup- 
ported in part by NSF grant GB 35484. The specimens of pycno- 


gonids provided for dissection and analysis and the motion pictures 
of leg movements (taken by John C. McCain and Stephen V. 
Shabica) were obtained during the tenure of NSF grant GA 18348 to 
Prof. Hedgpeth. This paper was written in part while a Research 
Affiliate of the Illinois State Geological Survey whose facilities and 
personnel assisted in producing the final manuscript. 


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