Digestive System

Most simplistically the generalised arthropod digestive tract can be divided into the foregut, midgut and hindgut. The midgut is derived from endoderm, the inner most tissue layer, during embryo development. However, both the foregut and hindgut are envaginations derived from the outermost tissue layer, the ectoderm, and so are cuticularised (Ruppert et al. 2004), that is, are lined by the secreted cuticle or exoskeleton.

Immediately after entering the through the mouth, ingested material moves via the short, vertical oesophagus into the cardiac stomach (Saxena 2005). The opening to the cardiac stomach is guarded by valves with inward pointing setae (Saxena 2005), preventing backflow (Kunze and Anderson 1979). Collectively, the cardiac and pyloric stomach chambers are termed the proventriculus (Ruppert et al. 2004). Central to the many cuticular ossicles (teeth) and plates (Kunze and Anderson 1979) of the proventriculus are three teeth of the cardiac stomach, two lateral and one dorsal, forming the gastric mill (Ruppert et al. 2004, Saxena 2005, Kunze and Anderson 1979). The gastric mill is powered by a complex arrangement of muscles (Saxena 2005) and grinds particles into a fine powder, performing a much greater role in mechanical digestion than the mandibles or external mouthparts (Kunze and Anderson 1979). This process is supplemented by enzymes of the hepatopancreas, a large digestive gland adjoined to the foregut but extending into the abdomen (Saxena 2005).

Only after being reduced to a very fine particle size is ingested material then permitted to enter the pyloric stomach, requiring passage through a complex collection of channels formed through folds and ridges of the foregut cuticle lined with fine setal filters (Ruppert et al. 2004, Saxena 2005). The cardiopyloric valve then directs particles upwards into the dorsal region of the pyloric stomach. This region houses a channel allowing larger particles a bypass straight into the intestine of the midgut (Ruppert et al. 2004). Smaller digestible particles move into the muscular filter press of mid region of the pyloric stomach and then into the ventral gland filter, which is finer again than the cardiac setal screen (Ruppert et al. 2004). Multiple ossicles and cuticular plates force particles through the gland filter and into the large digestive ceca of the midgut (Ruppert et al. 2004, Saxena 2005).

The midgut, composed of the intestines, digestive ceca and the hepatopancreas, is the site of absorption and most chemical digestion, which is extracellular (Saxena 2005). The hepatopancreas is an important absorptive organ, also secreting enzymes aiding in the mechanical digestion of the proventriculus gastric mill (Saxena 2005). In hermit crabs it is very large and coiled into the abdomen (Matthews 1953). Only the finest particles are permitted to enter the hepatopancreas, a complex chitinous filter of the foregut protecting its entrance (Saxena 2005). Both the hepatopancreas and digestive ceca are blind ending (Saxena 2005).

The intestine continues to the hindgut, which is cuticularised like the foregut, but short and simple in structure (Ruppert et al. 2004). Water is pumped into the hindgut via the anus to aid in excretion of faeces (Ruppert et al. 2004), which is periodically scooped out of the gastropod shell, the anus being located to the posterior of the abdomen.

The cuticularised foregut of a hermit crab with stylised internal structures and select external ossicles. Illustration: Storm Martin 2012, adapted with modification from Kunze and Anderson 1979 and with reference to Ruppert et al. 2004 and Saxena 2005