Mellitidae (Mooi and Chen, 1996)

Ziegler, Alexander, Lenihan, Jennifer, Zachos, Louis G., Faber, Cornelius & Mooi, Rich, 2016, Comparative morphology and phylogenetic significance of Gregory’s diverticulum in sand dollars (Echinoidea: Clypeasteroida), Organisms Diversity & Evolution (New York, N. Y.) 16 (1), pp. 141-166 : 159-160

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 ,

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

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.

Figures

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,

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

Fig. 3 Absence of Gregory’s diverticulum in the Rolutidae (a–e) as well as presence and shape of the organ in the Taiwanasteridae (f–j) and Echinarachniidae (k–t). The figure shows selected X-ray images. a, b Heliophora orbicularis (15, 46 mm test length); c–e Rotula deciesdigitatus (17, 22, 50 mm); f–j Sinaechinocyamus mai (1.5, 3.5, 6, 6.5, 7.5 mm); k–o

Fig. 4 Absence of Gregory’s diverticulum in Marginoproctus (a–e) as well as presence and shape of the organ in the Scutellidae (f–o) and Dendrasteridae (p–t). The figure shows selected X-ray images and a photograph of a dissected specimen. a–e Marginoproctus djakonooi (3, 3, 5, 6, 9 mm test length); f–j Scaphechinus griseus (2, 6, 9.5, 35, 36 mm);