Anatomy & Physiology

The anatomy and physiology of hermit crabs is similar to that of a generalised decapod crustacean. Hermit crabs have an open circulatory system known as a haemocoel, where the body cavity is filled with the circulatory medium or 'blood'. This medium is pumped around the body by a heart and though an open system, complex networks of vessels still supply the gills and organs. Dardanus megistos has multiple spindle shaped gills associated with each of the pereopods, though housed internally, beneath the carapace of the thoracic region. Water is drawn in through inhalant apertures at the base of the pereopods and out through exhalant apertures near the mouth by the beating of the scaphognathite, an epipod (extension from appendage base) of the second maxillae. Nitrogenous waste is excreted by paired antennal glands comparable in structure and function to the vertebrate nephridia. The digestive system consists of a complex cuticularised foregut which is the major site of digestion, a endoderm derived midgut which includes the intestines and hepatopancreas and is responsible for absorption and finally a simple cuticularised hindgut. The anus is located towards the posterior of the abdomen and so the hermit crab periodically cleans out it's shell.

Hermit crab anatomy. Sensory, nervous, respiratory, circulatory, digestive, excretory and reproductive systems illustrated. Male shown, ovaries of females located in similar position to testes, gonopores on coxae of third pereopod. Illustration by Storm Martin 2012, with reference to Ruppert et al. 2004.