428 



VERTEBRATE LIFE AND ORGANIZATION 



Later a.1 eye 



nostril 



Sensory 

 f ie-lds 



Pineal" 

 eye 



Figure 22.3. HemJcyclaspis, a representative ostracodenn of the early Devonian pe- 

 riod. This fish was about eight inches long. (Modified after Stensio.) 



rear end. iMore advanced fishes have lungs or air-filled swim bladders 

 and hence are more buoyant. They do not have flattened heads or 

 large pectoral appendages, and they have evolved symmetrical tail fins 

 (Fig. 22.9). 



The ostracoderm head was rather unusual. A single median nostril 

 was present on the top ot the head in the best known species. There 

 was a pair of lateral eyes and a single, median pineal eye on the top 

 of the head posterior to the nostril. Professor Young of University 

 College, London, has removed the pineal eye from primitive living 

 fishes (lampreys) and finds that they no longer undergo the rhythmic 

 color changes (light at night and dark in the day) observed in the 

 usual diurnal cycle. These color changes are known to be controlled 

 by the hypothalamic portion of the brain and by the pituitary gland, 

 hence the pineal eye must affect these organs. The hypothalamus and 

 pituitary gland also control many other physiologic activities, so it is 

 possible, as Young has postulated, that the pineal is an organ that in 

 these animals adjusts the rate of activity to changing conditions of 

 illumination. Three regions of the ostracoderm head, one dorso-medial 

 and a pair of dorsolateral areas, contained small plates beneath which 

 were enlarged cranial nerves. These peculiar areas may have been sen- 

 sory fields, or areas beneath which lay muscles modified for the produc- 

 tion of electric shocks. Much of the ventral surface of the head was 

 covered with small plates forming a flexible floor to the gill region, or 

 pharynx. Movement of this floor presumably drew water and minute 

 particles of food into the jawless mouth. The water then left the 

 pharynx through as many as ten pairs of small gill slits, but the food 



