Comparative morphology and phylogenetic significance of Gregory’s diverticulum in sand dollars (Echinoidea: Clypeasteroida) Author Ziegler, Alexander Author Lenihan, Jennifer Author Zachos, Louis G. Author Faber, Cornelius Author Mooi, Rich text Organisms Diversity & Evolution 2016 New York, N. Y. 2015-08-28 16 1 141 166 http://dx.doi.org/10.1007/s13127-015-0231-9 journal article 10.1007/s13127-015-0231-9 1618-1077 13173149 Mellitidae Data were obtained for the following four mellitid genera: Encope ( Fig. 6a–e ), Mellitella ( Fig. 6f–j ), Leodia ( Fig. 6k– o ), and Mellita ( Fig. 7 ). Gregory’ s diverticulum was present in all species analyzed, except for the extant species Encope emarginata and the two fossil species Encope secoensis and Mellitella falconensis . Relatively comprehensive X-ray imaging series are available for Mellita isometra ( 6–81 mm ), Mellita quinquiesperforata (1.5– 75 mm ), and Mellitella stokesii ( 4– 60 mm ). Gregory’ s diverticulum was found to be fully distended with sand up to a test length of 20 mm in Leodia sexiesperforata , 21 mm in M. isometra , 23 mm in M. quinquiesperforata , 30 mm in Mellita grantii , 35 mm in M. stokesii , 36 mm in Encope micropora , and 48 mm in Mellita longifissa . Remnants of the organ could still be discerned up to the following test lengths: 36 mm ( L. sexiesperforata ), 40 mm ( M. grantii , M. quinquiesperforata ), 42 mm ( E. micropora ), 54 mm ( M. isometra ), and 60 mm ( M. stokesii ). In 8 out of 18 specimens of Encope (44 %), 16 out of 37 specimens of Leodia (43 %), 68 out of 77 specimens of Mellita (88 %), and 43 out of 47 specimens of Mellitella (91 %), sediment was found inside the diverticulum. The extent of Gregory’ s diverticulum is particularly impressive in the Eastern Pacific species M. longifissa ( Fig. 7k–n ). Observations on the formation of ambulacral and interambulacral lobes In 2 mm large specimens of Echinarachnius parma and Mellita quinquiesperforata as well as 3 mm large specimens of Dendraster excentricus , Gregory’ s diverticulum was observed to be present in the form of a tubular cecum that did not yet bear any ambulacral or interambulacral lobes. However, small lateral protrusions of the ring duct could be discerned in the interambulacra ( Fig. 7p ). As the animal matures, each of these protrusions becomes subdivided into the ambulacral and interambulacral lobes ( Fig. 7q–s ). In M. quinquiesperforata , Fig. 6 Presence and shape of Gregory’ s diverticulum in the Mellitidae ( Encope , Mellitella , Leodia ). The figure shows selected X-ray images. a , b Encope michelini (9, 16 mm test length); c , d Encope micropora (36, 42 mm); e Encope wetmorei (14 mm); f–j Mellitella stokesii (15, 19, 26, 35, 46 mm ); and k–o Leodia sexiesperforata (5, 8, 20, 24, 26 mm ). All images show an aboral view with ambulacrum III facing upwards. gd Gregory’ s diverticulum, in intestine, re rectum, st stomach. Scale bar = 1 mm Fig. 7 Presence and shape of Gregory’ s diverticulum in the Mellitidae ( Mellita ). The figure shows selected X-ray images. a–e Mellita grantii (23, 26, 29, 30, 37 mm test length); f–j Mellita isometra (6, 10, 12, 21, 26 mm); k–n Mellita longifissa (16, 20, 29, 48 mm); o Mellita tenuis secondary protrusions on ambulacral and interambulacral lobes started to form at a test length of about 5 mm ( Fig. 7q ), while tertiary protrusions started to form at approximately 10 mm test length ( Fig. 7r ). The onset of lobe formation varies among species, as fully formed ambulacral and interambulacral lobes were already observable in specimens of 1.5 mm ( Sinaechinocyamus mai , Fig. 3f ) or 2 mm ( Scaphechinus griseus , Fig. 4f ) test length.