Longitudinal cross section (LS) of A. Tenebrosa. Sections stained with Mayer’s Haematoxylin and Eosin.

The mouth leads into the pharynx which opens into the coelenteron. Lining the inside of the pharynx is a pair of ciliated grooves called siphonoglyphs. These pharyngeal siphonoglyphs are responsible for polyp inflation by pumping water in via inward ciliary action. Within the coelenteron are vertical partitions (septa) which divide the internal space of the coelenteron into radial compartments. Septa can be complete or incomplete and the number may differ with body size. The number and pattern of septa can be used as a diagnostic feature in the classification of anthozoans. Extending from the inner margin of each septum is a trilobed septal filament. The middle lobe is referred to as the cnidoglandular band containing nematocytes and enzymatic gland cells. The two lateral lobes known as the flagellar bands and are densely flagellated.    

Longitudinal cross section (LS) of A. Tenebrosa
showing the trilobed septal filament.

Tissues and body compartments - Body wall has three tissue layers, epidermis (outer epithelium), gastrodermis (inner epithelium) which lines the gastrovascular cavity and joins the epidermis at the mouth and mesoglea (gelatinous, extracellular matrix) which lies between the two epithelia.

The gastrovascular cavity, also known as the coelenteron is a blind cavity and acts as a hydrostatic skeleton (main supportive structure) in addition to various other functions such as digestive, excretory, circulatory, absorptive and reproductive. 

Nervous system consists of epidermal and gastrodermal nerve nets that are connected by neurons via the mesoglea. The synapses of neurons conduct in both directions, allowing multidirectional conduction of nerve impulses into sensory cells, epitheliomuscular cells and nematocysts, forming a neuromuscular system. Being radially symmetrical, multidirectional conduction seems the most logical form of impulse conduction.

Musculature - Both the tentacles and oral disc contain sheets of longitudinal and radial epidermal muscular fibers respectively. The remaining muscles are gastrodermal with a sheet of circular muscle encircling the column wall, longitudinal and radial septal muscles and a circular musculature circling the pharynx. All muscules are epitheliomuscles, derived from the ectoderm. The longitudinal septal muscles are referred to as retractors. The retractors play a role in the retraction of the oral disc and tentacles when the animal is exposed to air. They are attached from the pedal disc to the oral disc, such that contraction of the muscles shorten and pull both the oral disc and tentacles into the column thereafter, the sphincter muscles constrict and close over the opening.