Digestion
The foregut (stomodeum) develops from the ectoderm. It is called pharynx before passing through the central nervous system, and esophagus inside the CNS. Shortly afterwards it empties into the midgut.
The midgut (mesenteron) is the largest organ in harvestmen and fills most of the opisthosoma. Like in other arthropods it is derived from the endoderm. Unlike the fore- and hindgut, which are derived from ectoderm, it has no cuticular lining. The midgut is surrounded by muscle cells, trachaeae and intermediate tissue, which does not form a fat body like in scorpions and solifuges. The epithelial cells of the midgut are often infected by rickettsia-like parasites, like in some other arachnids. Resorptive cells, which may be unique to harvestmen, contain lipid droplets, glycogen and mineral spherites. These spherites are also common in many arachnids. They seem to function in heavy metal excretion and mineral storage. Overwintering juveniles store calcium and phosphates in it needed for molting. Silicon is present throughout the life, but decreases with age, as does the number of spherites.
The hindgut is a short invagination of the ectoderm, linking the midgut to the anus. It can be dilated and shortened by muscles.
Read more about this topic: Harvestman Anatomy
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