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I) is in many ways closely allied to the Trematoda, from which, however, it is distinguished by the '- want of a digestive system.
The digestive system consists of a simple or bifurcated sac, opening through the mouth by means of a "pharynx bulbosus," adapted to act primarily as a sucker, and secondarily, when drawing blood, as an aspirator.
Some of the central cells remain in clumps as "germ-balls," others form a mesenchyma in which "flame-cells" arise; others again give rise to muscles; and at the thicker end of the body, rudiments of the brain and digestive system are observable.
The digestive system has a slender gullet, a large crop and no gizzard; in some Hemiptera the hinder region of the mid-gut forms a twisted loop with the gullet.
The males are unlike the females in most species; only in Eosphora digitata, Rhinops vitrea, Proales werneckii, and the Seisonaceae a complete digestive system is present.