Article: Morphology and function of dichoporite pore-structures in cystoids

The cystoids, a diverse and artificial group of extinct Palaeozoic echinoderms, are characterized by the possession of pore-structures developed in the thecal wall and composed of thecal canals which open in thecal pores. Thecal canals may occur singly to form dipores, or in sets perpendicular to plate sutures to form rhombs. Five basic types of pore-structure are recognised: pectinirhombs, cryptorhombs, humatirhombs, humatipores, and diplopores. The first two are composed of dichopores, thecal canals which connect external pores and through which sea-water flowed in life. The dichopores of pectinirhombs open in slits; those of cryptorhombs open in pores, one of which is sieve-like. Pectinirhombs and cryptorhombs are characteristic of, and confined to, the super-families Glyptocystitida and Hemicosmitida respectively; humatirhombs characterize the superfamily Caryocystitida. Five types of pectinirhombs and one type of cryptorhomb are recognized.Pectinirhombs and cryptorhombs agree closely with the paradigm of an exchange system and were respiratory structures. Evolution of pectinirhombs proceeded from less to more efficient types independently in all families of Glyptocystitida. The three rhombiferan superfamilies probably acquired their rhombs independently. Rhombifera and Diploporita are regarded as separate classes. The former contains two Orders: Dichoporita and Fistuliporita.