Frontal Eye

Visual perception (Wiki) 
This article on human vision is provided for reference. 

Frontal Eye Circuitry, Rostral Sensory Pathways and Brain Organization in Amphioxus Larvae (BIOSIS Previews) 

Frontal Eye Circuitry, Rostral Sensory Pathways and Brain Organization in Amphioxus Larvae: Evidence from 3D Reconstructions (Goog)

Full length PDF online.  I got PDF through library.   

    "The frontal eye consists of a pigment cup, two transverse rows of receptor cells, and clusters of neurons whose close association with the medial receptor cells suggests they may function as an integral part of the eye complex.

Neurites from both the receptor cells and neurons supply the ventrolateral nerve tracts, which consist mainly of axons arising from sensory cells located at the rostral tip of the larva. A core group of 3-4 rostral fibres on each side innervate two ventral giant cells located just behind the cerebral vesicle in the primary motor centre (PMC).  The circuitry suggests these cells may be responsible for triggering the larval startle response. The ventrolateral tracts also include two types of axial dendrite-like fibres:

    (i) a single unpaired fibre, a forward continuation of the principal dendrite of the left giant cell, which is the main target for synapses from neurons in the frontal eye; and

    (ii) sets of paired fibres from cells in the tectum, a dorsal cortex-like structure located at the back of the cerebral vesicle through which the dorsal sensory nerves pass in transit to the PMC. 

...The visual `system' described here for amphioxus larvae is more like that of vertebrates than has previously been recognized. Specifically:

    (i) the medial nerve cells of the frontal eye appear to form local circuits with relay and integrative functions similar to those of the retina, involving cell types that resemble specific retinal interneurons; and

    (ii) output is directed to a region at the back of the posterior c.v. that resembles the vertebrate midbrain, and which may be its homologue. This region has a dorsal tectum and, like the midbrain, includes the anterior part of a ventral zone of motoneurons and reticulospinal interneurons."

Molecular evidence for dim-light vision in the last common ancestor of the vertebrates (Goog) 
"Animal vision is mediated through pigments belonging exclusively to the opsin family. These are members of the G-protein-coupled receptor family that bind retinal [1]. Based on function and phylogenetic relationship, vertebrate visual opsins can be clustered in five groups... Rh1 is used for seeing under dim light conditions (scotopic vision), while the others permit full colour (photopic) vision in bright light [2], [3], [4] and [5].

"... both photopic and scotopic vision evolved in the stem vertebrate lineage and must have been in place in the Cambrian by about 522–518 Ma [10] and [11].

Early vertebrate evolution probably took place in a brightly lit environment and thus the earliest stem vertebrates were probably diurnal and inhabited shallow waters. However, at some stage after photopic vision was already in place, scotopic vision also evolved in the stem vertebrate lineage, which implies that a behavioural or ecological shift — perhaps a move into deeper water or to nocturnality — occurred in an ancestral vertebrate. What drove this shift can only be conjectured, such as the emergence of large macrophagous predators..."  

Apical Organs, Epithelial Domains, and the Origin of the Chordate Central Nervous System (Goog) - 1994 
Only abstract available online.  I got the PDF through the library.  
from the abstract   
The apical organ is a key structural landmark in a wide range of invertebrate larvae, but its homolog in chordates has not been identified.
Structural and biochemical evidence both support the idea that the apical organ, in amphioxus, has been incorporated into the frontal eye complex. Structural and positional similarities between this eye-like structure and paired eyes in vertebrates suggest the two may be homologous. The implication is that the cells of the neural retina in vertebrates may be derivatives of the primitive apical organ.

Rhabdomeric and ciliary eyes – Pharyngula - ScienceBlogs

Ruiz, M.S., and Anadón, R. 1991c.
Some considerations on the fine structure of rhabdomeric photoreceptors in the amphioxus, Branchiostoma lanceolatum (Cephalochordata).
J. Hirnforsch. 32: 159–164.