JOHN BUCK 759 



ceptihly more apt, in many instances, than tlie appellation "search- 

 light" which has also been used. Thus neither the mantle photo- 

 phore of the squid AbraUopsis- (Fig. 1) nor the skin organ ot cer- 

 tain other squids (Fig. 2) has much resemblance to the camera-type 

 scpiid eye, or indeed to any known eye, though each has at least an 

 analog of the proper parts in proper sequence. Furthermore, even 

 with an ostensibly eye-like structure, the eye-photophore comparison 

 loses force if the photophore resembles the eye of some other or- 

 ganism rather than the animal's own, as, for example, the thoracic 

 photophores of schizopod shrimps (Fig. 3) seem to mimic the camera 

 eye of squid or vertebrate more closely than a shrimp eye. (Actually, 

 as we shall see, there are indications of arthropod affinities in this 

 photophore.) 



Nevertheless, there do seem to be eye-photophore similarities, par- 

 ticularly among deepwater (bathypelagic) Crustacea, that are so de- 

 tailed that it seems captious to doubt that they have a more than 

 accidental significance. In considering these it will be helpful first 

 to recall that present-day arthropods have at least three types of eye, 

 the ocellus, or simple eye, which has usually a single corneal lens and 

 may form a camera-type image; the faceted or compound eye, which 

 forms a mosaic (apposition) or superposition image as the combined 

 product of its many individual optical units (ommatidia) ; and the 

 stemma, found in larval insects and corresponding roughly to a 

 single ommatidium. There is still disagreement as to the function and 

 evolutionary status of the various eye types, but the variations in 

 known structure are so great as perhaps to justify, for present pur- 

 poses, the statement that the presence of an anterior lens, and the 

 question of whether, if present, it is a corneal thickening or has 

 sei:)arated completely from the surface cuticle, are developmental de- 

 tails of secondary significance. So also the question of whether, in 

 development, the hypodermis folds in to form a single or double- 

 layered ocellar retinal cup or bulges out to form a hemispherical array 

 of visual units (compound eye — Fig. 10) may be secondary to the 

 fact that each resulting structure is an optical organ. This is par- 

 ticularly pertinent in regard to evolutionary questions, since two 

 structures may have undergone much secondary transformation since 

 separating from an ancestral stem-type. If eyes and photophores are 



*As in other fields of Zoology, there have been many taxonomic changes among 

 the bathypelagic squids and shrimps during the past half-century. Since, however, 

 nomenclature is a field for specialists, I have retained the species names used in 

 the original reports. 



