Platynereis Atoke T-Shirt
Worms seem taciturn and pedestrian, and yet these creatures may hold the secrets to the differences between the eyes of vertebrates and invertebrates. At least one rather well studied polychaete (many bristles or legs) annelid may provide the background to understanding the origins for the profound differences in ocular morphology found between chordates and non-chordates and can shed light on the question of the monophyletic versus polyphyletic appearance of eyes. Platynereis dumerilii (closely related to our cover species—Platynereis bicaniculata) spends its adult life in a self spun silky tube in the sublittoral zone, between the seashore and the edge of the continental shelf, and represents a frequent tasty morsel for many of the wading birds. The immature Platynereis, called atokes, are free swimming and must attach to the bottom as an adult, or epitoke, to spin a tube. The animal has lifelong development of segments and may have up to 75 segments through a process known as homonymous segmentation, each segment having a pair of appendages called parapodia. The head, or prostomia, has peristomal cirri (specialised appendages seen on the cover image), two pairs of sensory appendages, and two pairs of eyes These are old organisms—very old! These animals have probably changed little since the Cambrian explosion (BJO, February 2004), and may well have existed in a similar form since well before that, in the Precambrian fauna as represented in Ediacaran fossils.