Scutellidae, Gray, 1825

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 : 157-158

Scutellidae

The scutellid taxa analyzed comprise the extant genus Scaphechinus and the fossil genus Parascutella . Gregory’ s diverticulum is present in Scaphechinus griseus ( Fig. 4f–j ) and Scaphechinus mirabilis ( Figs. 1a and 4k– o ), but was not identified in Parascutella jacquemeti , Parascutella leognanensis , and Parascutella paulensis . However, these fossil specimens were either quite large or missing Aristotle’ s lantern. A relatively comprehensive X-ray imaging series is available for S. griseus (2– 41 mm). Gregory’ s diverticulum was found to be fully distended with sand up to a test length of 41 mm in S. griseus , while remnants of the organ could still be discerned up to a test length of 64 mm in S. mirabilis .

k–o Scaphechinus mirabilis (13, 15, 18, 21, 64 mm); and p–t Dendraster excentricus (5, 5.5, 6, 9, 37 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

In 26 out of 26 specimens of Scaphechinus (100 %), sediment was found inside the diverticulum.

Figures

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);

Fig. 1 Semischematic illustration of the two principle shapes of Gregory’s diverticulum in sand dollars. The figure is based on volume renderings of μCT data obtained from specimens of a Scaphechinus mirabilis (21 mm test length) and b Mellitella stokesii (17 mm). The shape of Gregory’s diverticulum can be discerned due to the presence of X-ray-opaque sediment contained within the organ. The course of the predominantly empty main digestive tract is drawn in white (primary siphon not shown). White numbers denote ambulacra (I–V) and interambulacra (1–5), while black letters (A–J) and numbers (1–5) indicate ambulacral or interambulacral lobes, respectively. Both images