Figure 2

The ommatidium of Jaekelopterus rhenaniae (Jaekel, 1914). (a–d) The specimen, where the first ommatidium was discovered (red arrow), d: brown: rhabdom with eccentric cell dendrite in the centre, yellow: receptor cells, black spots: possibly screening pigments. (b) shows the rosette of the ommatidium in crossection under different contrasts. Note the bright spot of the presumed eccentric cell in the centre of the rhabdom. (c,d,f) Rosette of (a) magnified, and its schematic drawings. (e) Ommatidium of the trilobite Schmidtiellus reetae Bergström 1973 and its homogenous rhabdom. (g–j) Cross sections of ommatidia of Limulus (quoted from⁶⁹ Figs. 2 and 3)), [white circle indicates the rosette, yellow circle the pigmental periphery, blue arrow the situation of the rhabdom, (j). (k–o) SEM of the compound eye of Jaekelopterus rhenaniae (Jaekel, 1914), total aspect (k) to one rosette (m–o) and individual rhabdom (o), [white circle indicates the rosette of receptor cells, yellow circle the pigmental periphery, yellow arrow the situation of the rhabdom, (n)]. (p–u) Different sensory rosettes of J. rhenaniae (Jaekel, 1914) and their interpretative drawings. Note the bright patch in the centre of the rhabdom, comparable to (a–d), and (g–j). (q–r) In black and white to show contrasts different from (s–u) in colour, [white circle indicates the rosette of receptor cells, yellow circle the pigmental periphery, blue arrow the situation of the rhabdom]. p, q GIK 188; (r–t), w, u GIK 190; ec eccentric cell; exc exocone, erc, element of receptor cell forming the outer part of the rhabdom; p, pigment cells; rc, receptor cells; rh, rhabdom; su sensory unit. Blue arrows indicate the dark rhabdomeric ring with the relics of the presumed dendrite of the eccentric cell inside.