Internal Anatomy

Body Cavities
All chitons, including A. gemmata have a large body cavity or visceral sac which consists of the digestive tract and accessory organs. ^(3,10) The body cavity is not coelomic as it is not covered with peritoneal tissue.⁽³⁾ The body cavity is divided into two compartments: a small anterior and a large posterior compartment.^(3,10) These two compartments are divided by an incomplete partition of connective tissue (the diaphragm).^(3,10) The oesophagus and radular sheath pass through the diaphragm where there is a large opening for the passage of the radular retractor muscles. ^(3,10)

Digestion
Like all chitons, A. gemmata uses its well-developed radula to scrape it food off the substratum (Figure 1).^(3,10) The rounded mouth is located ventrally, in the centre of the head, and expands when the radula protrudes while feeding (Figure 1).⁽³⁾ A short buccal tube leads from the mouth to a complex buccal cavity.⁽³⁾ The roof of the buccal cavity bears a pair of buccal glands that secrete mucus, but these glands are not homologous to salivary glands of other molluscs.^(3,10) Sensory cells on sub-radula organs protrude through the buccal tube and are thought to be chemosensory and used for testing the substratum for food.^(3,10)

Figure 1. Internal anatomy of a chiton

Posterior to the radula, the pharynx is expanded into a diverticulum which is lined with glandular tissue.⁽³⁾ The role of this tissue is unknown, however salivary glands are located near this tissue within the pharynx.⁽³⁾ The secretion of these glands consists of alkaline phosphatase and other powerful enzymes which break down a range of carbohydrates.⁽³⁾ The stomach is a wide sac (tube-shaped in some other species) and lays between the lobes of the digestive gland.⁽¹⁰⁾ The role of this digestive gland is unclear but is thought to aid in digestion.^(3,10) Extracellular digestion occurs in all chitons including A. gemmata.^(3,10)

Excretion
The exretory organs in chitons are two large nephridia.^(3,10) Each nephridium opens via a nephridiophore (Figure 1) into the exhalent chamber (for more informatoin go to external morphorlogy for description on gills) to release the final urine. ^(3,10)

Muscular System
The musculature of chitons in complex.⁽¹⁷⁾ There are 4 major groups of muscles. The role of the four groups of muscles is to keep the valves firmly together and closely attached to the body wall.⁽¹⁷⁾  The muscles also provide flexibility to the animal.⁽¹⁷⁾

There are two rectus muscles that run along the median line of the animal to keep it extended.^(3,17) The pair of rectus muscles connects the anterior of the jugal sinus of each valve with the anterior part of the body wall of the proceeding valve.^{(3, 17)} There are also paired oblique muscles, which diverge antero-laterally towards their attachment on the body wall beneath the preceding valve.^(3,17) The pair oblique muscles, like the rectus muscles also attach to the antero-ventral surface of the jugal sinus of each valve, however both of these muscles are absent under the tail valve.^{(3, 17)}

Transverse muscles are unpaired and consist of short dorso-ventral fibres and form a flat elastic cushion between individual valves.⁽¹⁷⁾  There is also a lateral longitudinal muscle that encircles the animal just within the outside margins of the valves. If these muscles are contracted it causes the chiton to curl up (like an armadillo roll) and serves as a defense mechanism when the chiton is uplifted from the substarte.^{(3, 17)} The girdle also contains longitudinal and lateral oblique fibres that allow the animal to flex and together with foot muscles provides a firm attachment to the substratum.⁽¹⁷⁾ They also serve to minimize desiccation when exposed to air and to facilitate ventilation of the gills and release gametes when the girdle is raised.⁽¹⁷⁾ The latero-pedal muscles consist of an anterior and posterior pair which strongly connect the each valve to the muscular foot.⁽¹⁷⁾