Development of the Squid’s Visual System

Like other cephalopods, squid have magnificent visual systems, the development of which has been too little studied in recent times. The structure of the eye (Fig. 1) is described in another chapter (Saibil, 1990), while the adult organization of the optic lobe is well described in Loligo (Kopsch, 1899; Young, 1974). This chapter will review the development of both, and the methods appropriate to that task, in an account drawn entirely from the literature; works on coleoids other than teuthoids are cited only when evidence is lacking in squid and to identify areas in which interesting findings would be forthcoming. The visual system will be interpreted conservatively so as not to include the extraocular photoreceptors in the paraolfactory vesicles, found in squid (Mauro, 1977; Young, 1978); the development of another system of cephalopod extraocular photoreceptors found in octopods, the epistellar body, has already been described (Sacarras:), 1956). The prospects for developmental studies on squid have recently improved with the prolonged culture of healthy animals in captivity (Hanlon, 1990). Squid mating behavior is described for Loligo pealei by Arnold (1962, 1990); embryos are obtained by methods described by Arnold (1974, 1990) and Hanlon (1990). Developmental stages are provided for Loligo vulgaris by Naef (1928), as amended by Meister (1972), and by Arnold (1965a, 1974, 1990) who provides a comprehensive illustrated timetable for Loligo pealei; a table of the events of eye morphogenesis at closely spaced intervals (Marthy, 1973) cross refers to these.