96 FROM FISH TO PHILOSOPHER 



importantly concerned in the regulation of the composi- 

 tion of the internal environment in all the higher ani- 

 mals, appears to have first assumed this role in the 

 Amphibia. 



The pituitary is not only the most important gland of 

 internal secretion in the body, but it is also the most 

 complex, because it is composed of several different 

 types of tissue, each having a different physiological 

 fxmction. All parts of the pituitary are of ancient origin, 

 but there is Httle information on their specific fimction 

 below the mammals, and almost no information on their 

 evolution. The most we can say is that a 'pituitary gland' 

 is present in all vertebrates. In the cyclostomes, the low- 

 est Hving vertebrates, the pituitary is connected by nerve 

 fibers with the ^pineal eye' in the top of the head. This 

 pineal eye remains something of a mystery: it was pres- 

 ent in certain ostracoderms (cephalaspids and pter- 

 aspids) and in a few Mesozoic Amphibia, while it was 

 particularly prominent in some of the Mesozoic reptiles; 

 it persists, in a degenerate form, in the living archaic 

 lizard, Sphenodon, though here it has lost its visual func- 

 tion as it had probably lost that function in the Mesozoic 

 animals. In the elasmobranchs and teleosts what is called 

 the 'pineal organ' is no longer an eye and presents no 

 trace of photoreceptor structure, and in the mammals 

 it appears as the pineal gland,' the function of which 

 (if any) is unknown. 



The pineal eye had doubtless functioned as an 'eye* 

 in the ostracoderms, serving to guide these bottom-living 

 animals through the lights and shadows of relatively 

 deep water. Of greater iaterest is the fact that it was 

 connected by nerve fibers with the pituitary gland, which 

 secretes, among many other hormones, one that causes 

 expansion of the amoeba-hke cells (chromatophores) in 

 the skin that are charged with pigment (most frequently 

 the black pigment, melanin) ; it is largely by contraction 

 or expansion of these chromatophores that the lower ani- 

 mals change their color, and it is possible that the pineal 



