Developmental stages of Spirorbis borealis. 1: Blastula stage. 2: Gastrula stage. 3: Further development of collar and setae in larvae (still within egg capsule). 4: Larvae more vermiform, mid-body region is largest (ready to leave capsule). 5: Early shell secretion stage. 6: Later shell secretion stage. 7: Adult displaying arrangement of operculum, branchiae, shell gland, generative glands and alimentary tract. 8: Cephalic and thoratic regions of adult, displaying arrangement of operculum and branchiae, setae and the foreigut of the alimentary tract. Adapted from Schively, 1897.