Invertebrate Anatomy OnLine
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the link. A
glossary and chapters on supplies and laboratory techniques are also available
through this link. Terminology
and phylogeny used in these exercises correspond to usage in the textbook by
Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
by far the largest and most diverse animal taxon, includes chelicerates,
insects, myriapods, and crustaceans as well as many extinct taxa such as
segmented body primitively bears a pair of jointed appendages on each segment. The
epidermis secretes a complex cuticular exoskeleton which must be molted to
permit increase in size. Extant
arthropods exhibit regional specialization in the structure and function of
segments and appendages but the ancestor probably had similar appendages on all
segments. The body is typically divided into a head and trunk, of which the
trunk is often itself divided into thorax and abdomen.
gut consists of foregut, midgut, and hindgut and extends the length of the body
from anterior mouth to posterior anus. Foregut
and hindgut are epidermal invaginations, being derived from the embryonic
stomodeum and proctodeum respectively, and are lined by cuticle, as are all
epidermal surfaces. The
midgut is endodermal and is responsible for most enzyme secretion, hydrolysis,
coelom is reduced to small spaces associated with the gonads and kidney. The
functional body cavity is a spacious hemocoel divided by a horizontal diaphragm
into a dorsal pericardial sinus and a much larger perivisceral sinus. Sometimes
there is a small ventral perineural sinus surrounding the ventral nerve cord.
hemal system includes a dorsal, contractile, tubular, ostiate heart that pumps
blood to the hemocoel. Excretory
organs vary with taxon and include Malpighian tubules, saccate nephridia, and
organs also vary with taxon and include many types of gills, book lungs, and
nervous system consists of a dorsal, anterior brain of two or three pairs of
ganglia, circumenteric connectives, and a paired ventral nerve cord with
segmental ganglia and segmental peripheral nerves. Various
degrees of condensation and cephalization are found in different taxa.
is derived with centrolecithal eggs and superficial cleavage. There
is frequently a larva although development is direct in many. Juveniles pass
through a series of instars separated by molts until reaching the adult size and
reproductive condition. At
this time molting and growth may cease or continue, depending on taxon.
extinct trilobites are among the earliest known arthropods, appearing first as
Cambrian fossils and departing forever in the Permian. Far
from being primitive, however, they are already highly derived, specialized
arthropods when they make their first appearance and undergo relatively little
change over their 300 million year history. These are the best known, to science
and the public alike, of fossil invertebrates and are common worldwide.
Although there are eight orders (Fig. 17-7*) and
over 15,000 species in 150 families, trilobites exhibit a relatively uniform
body plan that includes a heavily calcified dorsal exoskeleton, segmented body
with paired, biramous, segmental appendages, and tagmosis consisting of anterior
cephalon, middle thorax, and posterior pygidium. The
ventral surface is uncalcified and its details, including appendages, rarely
body is dorsoventrally flattened, or depressed.
The eponymous three lobes lie side by side and
consist of a median axial lobe flanked by a pleural lobe on either side. With
the exception of the antennae, the apendages are biramous and similar, except
for size, over the length of the body. Most are relatively small, 3-10 cm
although the full range is 0.5 mm -70 cm.
The appendages have heavy gnathobases whose teeth
face, and form, a median food groove. Presumably,
movements of the appendages move food particles anteriorly in the food groove to
the posterior-facing mouth where ingestion occurs. This
primitive arthropod feeding mechanism appears in many other taxa including
The lenses of trilobite compound eyes are
mineral, being composed of calcite, rather than being organic as are those of
all other arthropods.
A recent interpretation of arthropod phylogeny
abandons the name Trilobitomorpha and places trilobites in the taxon Trilobita C. In
this scheme Arthropoda P is
divided into the sister taxa Schizoramia sP (with
biramous appendages and including Crustacea, Trilobita, Chelicerata) and
Atelocerata sP (with
uniramous appendages and including Hexapoda and Myriapoda). In
this revision Trilobita and Chelicerata are sister taxa in Arachnomorpha SC,
which is itself the
sister taxon of Crustaceomorpha SC within
fossils and casts of fossils are available from biological supply companies. This
exercise is based on actual fossils but either fossils or casts can be used. The
teaching specimens in the laboratory may belong to several species and in
recognition of this the descriptions below are sufficiently general that they
can be used with most species. The
ventral surface of most specimens will be embedded in rock or, if exposed, will
show little detail and consequently most of your study will be of the dorsal
excellent and exhaustive treatment of trilobite morphology and identification is
provided by Gon (2001).
Study the dorsal surface of a trilobite using a
dissecting microscope whenever the size of the specimen dictates. The
dorsal surface is covered by the calcitic, readily fossilized exoskeleton known
as the dorsal shield (Fig.
delicate, uncalcified, and rarely fossilized ventral membrane extends from the
doublure across the ventral surface.
The dorsal shield is conspicuously divided into
three side-by-side longitudinal lobes.
The central, or median, lobe is the axial
lobe and it is the thickest
of the three. In
the living trilobite it contained most of the viscera, including, presumably,
the gut, heart, and segmentally ganglionated ventral nerve cord (Fig. 17-2A). On
either side of the axial lobe lies a smaller pleural
is separated from the axial lobe by a longitudinal axial
three lobes are responsible for the name “ trilobite”.
addition to the three longitudinal lobes, the dorsal shield is also divided into
three transverse tagma consisting
of an anterior cephalon, or head, a middle thorax, and a posterior pygidium.
The cephalon bears
the sense organs and mouth and is usually relatively large. Internally
it housed the brain and anterior gut, including the stomach and digestive ceca
(Fig 17-2B). Dorsally
the segmental construction of the cephalon is not obvious (Fig. 1, 17-1). The
trilobed nature of the body, however, is readily apparent on the cephalon. The
axial lobe extends anteriorly onto the cephalon where it is known as theglabella and
the two pleural lobes are present at the genae,
or cheeks. The
glabella may bear transverse or oblique glabellar
furrows that mark the
divisions between the segments of the head. The heavy margin of the cephalon is
its anterior and lateral border.
Each gena usually bears a compound
eye although eyes are absent
in some taxa. Each
eye is composed of numerous ommatidia whose lenses, or facets, are visible on
the surface. In
most trilobites the lenses are numerous (up to 15,000 in each eye) and
contiguous so that each is hexagonal in shape. A
common cornea covers al the lenses of the eye. Such
eyes are holochroal. The schizocroal eyes
characteristic of one trilobite order have larger ommatidia whose circular
lenses are widely spaced and do not touch (Fig. 17-2B). Each
lens is covered by its own cornea. What
kind of eyes does your specimen have? ____________ Unlike all other arthropods,
trilobites lenses are mineral, made of calcite, rather than organic cuticle. Each
lens is a single calcite crystal.
Figure 1. The dorsal surface of the
ptychopariid trilobite, Elrathia. This
26 mm specimen, of unknown provenance, was purchased from a biological supply
In many taxa each of the two posterior corners of
the cephalon bears a genal
spine, which may be quite large (Fig. 17-7E,G). Of
considerable importance in trilobite taxonomy and in trilobite biology are thefacial
sutures on the genae. These
grooves arise at the anterior margin of the gena and pass posteriorly through
the middle of the gena, often separating the eye from the glabella or sometimes
bisecting the eye, and then ending on the lateral or posterior margin or the
cornerangle of the gena. The
location of the posterior end of the furrow is taxonomically important. In
Fig 1 the suture terminates on the posterior margin of the gena and is
suture that ends on the lateral margin is proparian and one that ends at the
genal angle is gonatoparian.
The facial sutures are ecdysial lines along which
the old exoskeleton splits during ecdysis. The
facial suture divides its gena into a medial fixigena and
a lateral librigena (Fig
ecdysis the fixigena remains fused with the glabella, whereas the librigena is
liberated from it. The
combined fixigena and glabella are known as the cranidium.
Look at the ventral surface of the cephalon. If
you could study the ventral membrane of a perfectly preserved cephalon you would
see more obvious segmentation that is apparent dorsally (Fig. 17-1B). The
trilobite cephalon is thought to be composed of the acron and four fused
appendages are a pair of uniramous antennae and three similar pairs of biramous
pair of appendages is associated with a cephalic segment.
On the ventral surface, corresponding in position
to the glabella is a longitudinal ridge covered by a sclerite, the hypostome (Fig
your specimen has been carefully cleaned, the hypostome may be visible. The
hypostome may homologous to the crustacean labrum. Unlike
the ventral membrane, the hypostome is sclerotized and is often present in
The mouth opens at the posterior edge of the
hypostome and faces posteriorly toward the anterior end of the food groove. The
gut is J-shaped with the esophagus and stomach located in the central lobe
(glabella) of the cephalon (Fig 17-2B). Large
digestive ceca extend from the stomach into the genae. The stomach is the
anterior-most region of the gut and the esophagus extends posteriorly to connect
with the backward-facing mouth (Fig. 17-5B). The gut was a long tube housed in
the axial lobe and ending at a posterior ventral anus on the pygidium (Fig
17-2A, 17-3, 17-5B).
remainder of the body is the trunk,
divided into thorax and pygidium, both of which are conspicuously segmented (Fig
1). The thorax,
which is the middle region of the body, is usually, but not always, much larger
than the pygidium. The
thorax may consist of 2-60 independent segments articulated, fore and aft with
each other by flexible articular membranes. Each
segment has a central axial
ring from which extend, one
right and one left, two pleurites. The
combined axial rings of all the segments make up the axial lobe, just as the
combined pleurites on each side make up the two pleural lobes. In
some taxa the pleurites bear lateral spines (Fig 17-7A).
The flexibly articulated segments of the thorax
allow for enrollment. In this maneuver the animal arches the body dorsally and
brings the ventral surfaces of the pygidium and cephalon in contact with each
other (Fig 17-5). This
protects the vulnerable ventral membrane and appendages from attack. Most
trilobites were able to enroll but some were better adapted for it than others.
the thorax, the pygidium is
composed of appendage-bearing segments but in this case the segments are fused
into a rigid platform and are not connected by flexible articular membranes (Fig
1). The pygidium is the posteriormost tagma. Although
fused, the pygidial segments are distinct. Pygidial
segments and their appendages diminish rapidly in size posteriorly. The
axial lobe extends posteriorly onto the pygidium but tapers to a blunt point
right and left pleural lobes also extend onto the pgidium where they join each
other posterior to the end of the axial lobe. The
thickened margin of the pygidium is the pygidial
border. The relative size of the pygidium and cephalon varies with
taxon and is an important character in trilobite classification and
identification (Fig 17-7).
is known of the internal anatomy of these extinct arthropods but some
information is available from X-ray studies and the remainder is conjecture
based on our knowledge of extant arthropods.
With the exception of the doublure and hypostome,
the ventral surface consists of the delicate, weakly calcified ventral membrane
which almost never fossilizes. The
segmental appendages are ventral but, like the ventral membrane, are rarely
present in fossils.
Along its margins the dorsal shield turns
ventrally to form a thick, marginal, ventral
shelf of calcified exoskeleton known as the doublure (Fig 2, 17-1, 17-5C, 17-3). The
doublure typically fossilizes along with the dorsal shield. If
your specimen has been carefully cleaned you may be able to see the doublure and
the hypostome but
little else on the ventral surface. The doublure consists of cephalic, pleural,
and pygidial regions.
Figure 2 Ventral surface of the Elrathia specimen
in Fig 1. Trilob57L.gif
*Hyphenated call-outs, such as this one, refer to
figures in Ruppert, Fox, and Barnes (2004). Call-outs
without hyphenation refer to figures embedded in this exercise.
Fortey RA. 1997.
Classification, in Kaesler RL (ed.) Treatise on Invertebrate Paleontology, Part
O Arthropoda 1, Trilobita, revised. Vol. 1: Introduction, Order Agnostida, Order
Redlichiida. Geological Society America & Univ. Kansas, Boulder, CO. 530pp.
Fortey RA. 2000.
Trilobite, Eyewitness to evolution. Vintage
Books, New York. 284 pp.
Fortey RA. 2004.
The lifestyles of the trilobites. Am. Sci. 92(5):446-453.
Gon S. 2001. A
pictorial guide to the orders of trilobites. Privately published and available
for $15 from S. Gon, 1604 Olalahina Pl, Honolulu, HI, 96807. 90 pp.
Gon S. A guide to the orders of trilobites. www.aloha.net/~smgon/ordersoftrilobites.htm
Ruppert EE, Fox RS, Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
1 trilobite, real or cast
1 dissecting microscope
are available from Carolina Biological Supply Company