last revised 28 February 2009 by SMGIII
Trilobite Internal Anatomy
This drawing ©2004 by S. M. Gon III
In the figure above, a longitudinal sagittal section taken through the midline of the axis of a phacopid trilobite reveals some of the major systems. The thick dorsal exoskeleton (composed of a complex structure of chitin and protein, thickened and hardened by calcite (calcium carbonate [CaCO3]) and much thinner ventral shell (uncalcified), are shown in dark brown. The dorsal circulatory vessel (heart) is shown in red. Modern arthropods have an open circulatory system in which internal organs are bathed in hemolymph (blood) which is circulated through contractions of the dorsal circulatory vessel.
The digestive system (yellow) starts at the mouth (adjacent to the hypostome), which feeds forward into a stomach which occupies much of the glabellar space. From the stomach, a long alimentary canal sends food backward to exit at the anus. Accessory digestive organs, sometimes referred to as gastric diverticulae occur both above and to each side of the stomach (see another example below).
We presume that trilobites had a ventral nervous system, either a pair of nerve cords connected at segmental ganglia (swellings), or perhaps a single cord with segmented nodes (shown above in dark green). The brain would be an enlarged frontal ganglion receiving sensory input from eyes and antennae, etc. Its precise placement is conjectural, but most modern arthropod brains are anterior to the mouth, with the paired central nerves passing to each side of the esophagus, rejoining at a subesophageal ganglion.
In the figure above, the muscular system is not shown (but see below). The reproductive system of trilobites is also very poorly understood. There is some evidence that eggs were brooded at the front of the cephalon (as they are in horseshoe crabs today), but the nature and location of copulatory organs has never been documented.
This dorsal view features the digestive system (dark blue), showing the
frontal placement of the expanded stomach, and the lateral and central
caecae (yellow), which were thought to have acted much as a liver. The
large cephalic lobe of a phacopid wasn't occupied by philosophical
considerations, but by its most recent meal! The long, median alimentary
canal (intestine) ended at a posterior anus.
Figure after Bergstrom 1976
A complex network of subcuticular genal caecae (above) is
Trilobite cross section
This drawing ©2004 by S. M. Gon III
|In the cross-section (left), through the thoracic
region of a phacopid trilobite, we see how the organ systems were placed
centrally amidst the muscular system that
worked the limbs, allowed for enrollment, and changed body posture. The
gills, borne on the exopodite, are shown in pink;
part of the circulatory and respiratory system. Oxygen-rich hemolymph (arthropod
blood) would enter the hemocoel (an open circulatory system bathing the internal
organs and muscles) via the legs, and would be moved along the length of
the animal via the long dorsal vessel (heart) shown here as a red oval. Below
the heart is the alimentary canal (yellow) and the ventral nerve cords (olive).
The muscle masses are shown in purple. The majority of organ systems occupies
the axial lobe of the animal. The pleural exoskeleton provided
protection for the limbs and gills, but the "guts" of a trilobite occupied
the axial lobe. Some workers speculate that the excretory system of
trilobites might have occupied the pleurae, exiting as ducts (associated with
the Panderian organs) where the pleural doublure meets the thin ventral cuticle.
Finally, the spines on the limbs, especially heavy on the coxae (base segments),
were acted as teeth and jaws, breaking up food before being passed forward
to the hypostome and mouth. This "jaw at the base" of the leg is called a
gnathobase (translating literally as "jaw