ii2 VERTEBRATES WITHOUT JAWS iv. 15- 



organs to light is not found in other fish-like vertebrates. Indeed the 

 receptors are not strictly lateral line organs but pigmented epidermal 

 cells. The sensitivity curve shows a sharp peak at 530 m/x, this being 

 the region of the spectrum at which light penetrates farthest into sea 

 water. The pigment is probably a porphyropsin (Steven, 1950). 



In hag-fishes (Myxine) the head and cloacal regions are more 

 sensitive to light than is the rest of the body. The impulses from the 

 skin are conducted through the spinal nerves in these animals, not 

 the lateral line nerves. 



16. Habits and life-history of lampreys 



We have very little information about the life of lampreys during 

 the time that they are in the sea. They are caught in considerable 

 number attached to other fishes. It is not known how many years a 

 lamprey spends in the sea, but it returns only once to the river for 

 spawning and dies after this act. The up-river migration of L. fluvi- 

 atilis occurs in the autumn, for instance large numbers come up the 

 River Severn and are caught in traps on the way, for use as food. The 

 spawning migrations of lampreys may take them for hundreds of 

 miles, for example, those of the eastern Pacific ascend to the head- 

 waters of the Columbia River. They are said to perform remarkable 

 feats of climbing, leaping from stone to stone and hanging on by their 

 suckers. During this period of migration some lampreys assume 

 brilliant orange and black colour patterns. On the other hand, lam- 

 preys land-locked in the lakes of New York (Petromyzon marinus 

 unicolor) feed in fresh water and ascend only a few miles up streams 

 to breed. 



Once in the river the lampreys do not feed again but live over the 

 winter on the reserves accumulated in the form of fat, especially under 

 the skin and in the muscles. During the winter the gonads ripen pro- 

 gressively and the secondary sexual characters begin to become appar- 

 ent only in February. The females then develop a large anal fin, while 

 in the male a penis-like organ appears (Fig. 47) and the base of the 

 dorsal fin becomes thickened. 



Spawning occurs in the spring and is preceded by a form of nest- 

 building. Numerous lampreys collect together, usually at a place 

 below a weir where the water is shallow and rather swift, and the 

 bottom both stony and sandy. Stones are then dragged by the mouth 

 in such a way as to make a small depression. Fertilization is secured 

 by a process of copulation in which the male fixes by the sucker on to 

 the fore-part of the female and the two then become intertwined and 



