<P> By utilizing the iris sphincter muscle, some species move the lens back and forth, some stretch the lens flatter . Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length . In addition, the pupil shape can be used to predict the focal system being utilized . A slit pupil can indicate the common multifocal system, while a circular pupil usually specifies a monofocal system . When using a circular form, the pupil will constrict under bright light, increasing the focal length, and will dilate when dark in order to decrease the depth of focus . Note that a focusing method is not a requirement . As photographers know, focal errors increase as aperture increases . Thus, countless organisms with small eyes are active in direct sunlight and survive with no focus mechanism at all . As a species grows larger, or transitions to dimmer environments, a means of focusing need only appear gradually . </P> <P> Prey generally have eyes on the sides of their head so to have a larger field of view, from which to avoid predators . Predators, however, have eyes in front of their head in order to have better depth perception . Flatfish are predators which lie on their side on the bottom, and have eyes placed asymmetrically on the same side of the head . A transitional fossil from the common symmetric position is Amphistium . </P> <P> The eyes of many animals record their evolutionary history in their contemporary anatomy . The vertebrate eye, for instance, is built "backwards and upside down", requiring "photons of light to travel through the cornea, lens, aqueous fluid, blood vessels, ganglion cells, amacrine cells, horizontal cells, and bipolar cells before they reach the light - sensitive rods and cones that transduce the light signal into neural impulses, which are then sent to the visual cortex at the back of the brain for processing into meaningful patterns ." While such a construct has some drawbacks, it also allows the outer retina of the vertebrates to sustain higher metabolic activities as compared to the non-inverted design . It also allowed for the evolution of the choroid layer, including the retinal pigment epithelial (RPE) cells, which play an important role in protecting the photoreceptive cells from photo - oxidative damage . </P> <P> The camera eyes of cephalopods, in contrast, are constructed the "right way out", with the nerves attached to the rear of the retina . This means that they do not have a blind spot . This difference may be accounted for by the origins of eyes; in cephalopods they develop as an invagination of the head surface whereas in vertebrates they originate as an extension of the brain . </P>

Which of the following best describes the likely evolutionary steps that led to a complex eye