CHAPTER VIII 

 THE EYES OF PROTO-CHORDATES 



The Chordates constitute a phylum characterized by a dorsal 

 tubular nerve-cord, a dorsal supporting axis (a notochord) and pharjTi- 

 geal gill-slits ; the last two, however, may be temporary in duration. 

 The Vertebrates constitute a sub-phylum within the Chordates which 

 possesses as distinctive characters a head and skull, a brain with eyes, a 

 vertebral column, and (generally) paired limbs. Stumbling on the 



FiG. 232. — B.iLAyoGLOssvr. 



The long tongue-like proboscis (Pr) resembles an acorn (/3aAo(i'Of, an 

 acorn ; yXioaax, a tongue). 



border-line between Invertebrates and Vertebrates are three classes of 

 animals (Proto-chordates) possessed of a rudimentary nerve-cord, a 

 notochord and gill-clefts — the Hemichordates, the Tunicates, and the 

 Lancelets. Apart from the pelagic Tunicates, these lowly creatures 

 are either sessile or burro\\ing in habit. 



The HEMiCHOEDATA are typified in Balanoglossus, a worm-like 

 marine creature burrowing in the sand and mud of most seas (Fig. 232). 



Fig. 233. — The Eyes in the Tornaria Larva of Bala.\oolossvs. 

 Antero-posterior section through the apical plate showing the anterior, 

 EA, and posterior, EP, eyes (after Morgan). 



The nervous system arises as a longitudinal groove of ectoderm which 

 becomes tubular but gives no evidence of visual out-pouchings. In the 

 larv£e (tornaria) of some species situated on the apical plate there are 

 two eye-spots consisting of cup-shaped depressions of clear cells 

 surrounded by pigment (Fig. 233), but in the adult there are no special 

 sense organs (Spengel, 1893 ; Stiasny, 1914). 



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