RACES OF LAMPREYS 



119 



metamorphosis by which the ammocoete larva changes into an adult 

 lamprey. The mouth becomes rounded and its teeth, tongue, and 

 complex musculature develop. The paired eyes (previously buried) 

 appear; the olfactory organ becomes internally folded, and the olfac- 

 tory nerve and tracts much enlarged. The naso-hypophysial sac grows 

 backwards to the gills. In the pharynx the gills develop into sacs 

 opening to the branchial chamber. Changes also take place in the 



B 



Fig. 80. Diagram to show distribution of iodoproteins, at first in exoskelctal struc- 

 tures, as in many invertebrates and in tunicates (a). Some of this material is 

 concentrated in the pharynx. This tendency is exaggerated in amphioxus and the 

 ammocoete larva, and in the adult lamprey and later animals this pharyngeal 

 material forms the thyroid. A. Many invertebrates and tunicates ; n. Amphioxus ; 

 c. Ammocoetes; D. Metamorphosis of ammocoetes; e. General vertebrate type. 

 (After Gorbmann, A., in Comparative Endocrinology. Wiley, New York.) 



intestine. The yellow-brown colour of the larva gives place to the 

 black with silver underside of the adult. The animal more and more 

 frequently leaves the mud and finally migrates to the sea to begin its 

 parasitic life. 



18. Races of lampreys, a problem in systematics 



Besides the river lampreys, such as L. fliwiatilis (Linn.), which 

 show this characteristic migratory life-history, there are also in various 

 parts of the northern hemisphere small brook lampreys ('prides'), 

 such as L. planeri Bloch, which remain throughout their life in fresh 

 water. These prides are very abundant in many English rivers and 

 streams, but since the greater part of their life is passed in the am- 

 mocoete stage they are not often seen. The larvae remain in the mud 



