The cephalon is the head

Head

In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....

 section of an arthropod

Arthropod

An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

. It is a tagma, i.e., a specialized grouping of arthropod segments

The word cephalon derives from the Greek κεφαλή (cephale), meaning "head". It is rather used in Trilobite terminology. In insects, head is a preferred term. In arachnid

Arachnid

Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...

s and crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, the cephalothorax

Cephalothorax

The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...

 is derived from the fusion of the cephalon and the thorax, and is usually covered by a single unsegmented carapace. In relation with the arthropod head problem

Arthropod head problem

The arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other...

, phylogeny

Phylogeny of Malacostraca

The phylogeny of Malacostraca is the arrangement of the Malacostraca classes in the Crustacea subphylum and the relationship of the malacostracan orders.-Introduction:...

 studies show that members of the Malacostraca

Malacostraca

Malacostraca is the largest of the six classes of crustaceans, containing over 25,000 extant species, divided among 16 orders. Its members display a greater diversity of body forms than any other class of animals, and include crabs, lobsters, shrimp, krill, woodlice, scuds , mantis shrimp and many...

 class of crustaceans have five segments in the cephalon, when not fused with the thorax to form a cephalothorax.

In the Late Precambrian or Lower Cambrian proarticulatan species Praecambridium sigillum

Praecambridium sigillum

Praecambridium sigillum resembles a segmented trilobite-like arthropod, though many experts place it within the proarticulatan family Dickinsoniidae. It is from either the Late Precambrian or Lower Cambrian of Ediacara Hills, Australia, about 590 to 530 million years ago. This beast is known from...

, that resembles a segmented trilobite-like arthropod, the term is also used to describe the anterior part of the animal. The Agnostida

Agnostida

Agnostida is an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian. They are present in the lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders...

, an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian, also have a cephalon.

The Thylacocephala

Thylacocephala

The Thylacocephala are a unique group of extinct arthropods, with possible crustacean affinities. As a class they have a short research history, having been erected in the early 1980s. They typically possess a large, laterally flattened carapace that encompasses the entire body...

, a unique group of extinct arthropods, with possible crustacean affinities, thought to date from the lower Cambrian, while the class has a definite presence in Lower Silurian marine communities and survived to the Upper Cretaceous, has also a cephalon.

Trilobite cephalon

The cephalon of trilobites is highly variable with a lot of morphological complexity. The glabella, the expression of the axial lobe in the cephalon, forms a dome underneath which sat the "crop" or "stomach". Generally the exoskeleton has few distinguishing ventral features, but the cephalon often preserves muscle attachment scars and occasionally the hypostome

Hypostome

A hypostome , is a calcified harpoon-like structure near the mouth area of certain parasitic arthropods including ticks and mites , that allows them to anchor themselves firmly in place on a host mammal while sucking blood...

, a small rigid plate comparable to the ventral plate in other arthropods. A toothless mouth and stomach sat upon the hypostome with the mouth facing backwards at the rear edge of the hypostome.

Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described. The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific ecological niche

Ecological niche

In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

s.

The lateral fringe of the cephalon is greatly exaggerated in the Harpetida

Harpetida

Harpetida is one of the nine orders of the extinct arthropod class Trilobita. They lived from the Upper Cambrian to the Late Devonian period....

, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch. Highly complex compound eyes are another obvious feature of the cephalon.

Facial sutures

When trilobites moulted

Ecdysis

Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...

, the librigenae ("free cheeks") separated along the facial suture to assist moulting, leaving the cranidium (glabella + fixigenae) exposed. Trilobite facial sutures can be roughly divided into three main types (proparian, gonatoparian, and opisthoparian) according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge). Early Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 trilobites belonging to the suborder Olenellina

Olenellina

Olenellina is a suborder of the order Redlichiida of Trilobites. The suborder contains two superfamilies: Olenelloidea , and Fallotaspidoidea . The Olenillina are some of the most primitive trilobites...

 (like Fallotaspis

Fallotaspis

Fallotaspis is a Cambrian trilobite genus found in the United States and Morocco.-Sources:***...

) lacked facial sutures. Other later trilobites also lost facial sutures secondarily.