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ADAPTATIONS TO MEDIA AND SUBSTRATES 



Beebe, that the adult form of this fish is the deep-sea Idiacanthus, known 

 since the work of Peters in 1876 (Fig. 139). The stylophthalmus larva 

 has the eyeball at the end of an enormously long stalk, which is sup- 

 ported by a unique rod of cartilage, rooted on the skull and containing 

 a muscular insert near its base, which enables the eye to be waved about. 

 The rod, together with the optic nerve and the filamentous eye muscles, 

 is ensheathed by skin which (over the front of the eyeball) contributes 

 to the cornea in the usual way. 



Fig. 139 — Idiacanthus fasciola. After Beebe. 



a, head of stylophthalmus larva; eye-stalk cartilage shown in black, b, c, 16mm. larva 

 and adult female; the straight lines under the drawings express the relative body lengths, 

 d, head of 45mm. postlarva, showing eyes retraaed into head and skein-like, unshortened, 

 eye-stalk cartilage, e, head of 35mm, transitional adolescent with skin flap raised to show 

 coiled cartilage in anterior portion of orbit. 



In post-larval stages, the eye-stalk shortens to pull the eyeball into 

 a normal position in the head. The cartilaginous rod cannot shorten, 

 however, so it bursts out of the stalk sheath and becomes a tangled 

 skein (Fig. 139d), which is later tucked into the anterior part of the orbit 

 and covered by the skin of the head (Fig. 139e). The cartilage is even- 

 tually resorbed during adolescence. The adult Idiacanthus eye is rela- 

 tively large, but is of normal shape. The male is degenerate, never 

 getting beyond an essentially post-larval condition except as regards the 

 reproductive system. Unlike the female, it has a huge photophore on 



