400 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



of the light-bathed, small-eyed rays of the continental shelf. The retinal 

 photomechanical changes have teen eliminated in adult bathypelagic 

 teleosts, and the pigment-epithelial cells are often devoid of pigment and 

 processes. Summation of visual cells in optic nerve fibers is greatly in- 

 creased (compare Fig. 135 with Fig. 72, p. 177). 



The eyeball maintains a substantially normal external form in a ma- 

 jority of deep-sea species. Such normally-shaped eyes may attain a diam- 

 eter equal to more than half the length of the whole head, as in Zenion 

 hololepis. Beyond this point, the relative volume of the eye could scarcely 

 be increased without serious encroachment upon other cephalic struc- 



Fig. 136 — ^Tubular eyes of deep-sea fishes. 



a, Odontostomus hyalinus. x 13. After Brauer. ar- accessory retina; cr- chief retina, b, eye 

 of Argyropelecus sp. superimposed upon outline (dotted) of normally-shaped teleost eye of 

 the same lens-size. After Hesse. 



tures. So, in. many species the 'telescopic' (better, tubular) form of eye- 

 ball has been evolved: 



The relationship of the tubular ocular shape to the normal can be 

 easily expressed (see Fig. 136, also p. 212 and Fig. 84) : the tubular eye 

 is like the axial core of a normal eye, the rest of which has been thrown 

 away to make more room in the animal's head for a very large core. But 

 in the teleosts the tubular form is not attained, phylogenetically or de- 

 velopmentally, in any such simple manner. Commonly, both tubular and 

 normal eyes occur in the same family. Both kinds even occur in different 

 species of the same genus, as in the bathypelagic genus Evermanella. 

 In at least some cases (e.g., in Argyropelecus, Ichtbyococcus, Dissom- 



