Cephalon (arthropod head)

For the biotechnology company, seeCephalon.

Thecephalonis theheadsection of anarthropod.It is atagma,i.e., a specialized grouping of arthropodsegments.The word cephalon derives from the Greek κεφαλή (kephalē), meaning "head".

Insects

Head of aEuropean hornet(Vespa crabro)

Ininsects,headis a preferred term. The insect head consists of five segments, including three (thelabial,maxillaryandmandibular) necessary for food uptake, which are altogether known as the gnathocephalon and house thesuboesophageal ganglionof the brain, as well as the antennal segment, and an ocular segment, as well as a non segmented fused section of the head where the archicerebrum is housed known as the acron. See alsoarthropod head problem.

Chelicerates and crustaceans

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  The crustaceanCherax warsamsonicus
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  The amplypygid cheliceratePhrynus asperatipes

Incheliceratesandcrustaceans,thecephalothoraxis derived from the fusion of the cephalon and the thorax, and is usually covered by a single unsegmentedcarapace.In relation with thearthropod head problem,phylogenystudies show that members of theMalacostracaclass of crustaceans have five segments in the cephalon, when not fused with the thorax to form a cephalothorax.

Proarticulata

Praecambridium sigillum

In the Late Precambrian or Lower CambrianProarticulataspeciesPraecambridium sigillum,that superficially resembles a trilobite, the term is also used to describe the anterior part of the animal.

Thylacocephala

The head of theThylacocephalais also referred to as a cephalon: the cephalon is usually obscured by the carapace. Thylacocephala are a unique group of extinct arthropods, with possible crustacean affinities, thought to occur from the lower Cambrian, but with certainty between the LowerSilurianand the Upper Cretaceous.

Trilobites

Thetagmatain a trilobite

Morphology of the Trilobite cephalon

1 – fixigena; 2 – librigena; 3 – glabella

1 – preocular area; 2 – palpebral area; 3 – postocular area; 4 – posterolateral projection; 5 – occipital ring; 6 – glabella; 7 – posterior area; 8 – lateral border; 9 – librigenal area; 10 – preglabellar area

Cephalon of the trilobitePhacops ranafrom theDevonianof northwestern Ohio.

The cephalon oftrilobitesis 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 thehypostome,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 specificecological niches.

The lateral fringe of the cephalon is greatly exaggerated in theHarpetida,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

Main article:Facial suture (trilobite)

When trilobitesmoulted,the librigenae ( "free cheeks" ) separated along the facial suture to assist moulting, leaving the cranidium (glabella + fixigenae) exposed.Trilobitefacial sutures can be roughly divided into three main types (proparian,gonatoparian,andopisthoparian) according to where the sutures end relative to the genal angle (the edges where the side and rear margins of the cephalon converge). EarlyCambriantrilobites belonging to thesuborderOlenellina(likeFallotaspis) lacked facial sutures. Other later trilobites also lost facial sutures secondarily.

See also

• Carapace
• Facial suture (trilobite)
• Pygidium
• Tagma (biology)
• Thorax