Figure 1

Historic and contemporary representations of the general anatomy of the irregular and "regular" sea urchin digestive tract. (A) and (B) constitute the first graphic representations of the gastric caecum (black arrow in A) as well as the dilation of the anterior stomach (white arrow in B). (A) Spatangus purpureus - aboral view, modified from Hoffmann [8]. (B) Paracentrotus lividus - aboral view, modified from Tiedemann [20]. Numbers indicate homologous body parts in "regular" and irregular sea urchins according to Lovén's system [29]: Roman numerals (I-V) indicate ambulacra, whereas Arabic numerals (1-5) indicate interambulacra. (C) Spatangus purpureus - aboral view of a dissected specimen. (D) Paracentrotus lividus - aboral view of a dissected specimen. di = dilation, es = esophagus, gc = gastric caecum, in = intestine, re = rectum, st = stomach. Not to scale.