260 THE EYE IN EVOLUTION 



primitive type of vertebrate eye showing characteristics differing 

 markedly from those of Fishes. 



THE LAMPREYS (PETROMYZONIDiE) 



The lampreys are large eel -like creatures found mainly in the seas and 

 rivers of the northern hemisphere ; the sea lamprey {Petromyzon marinus), 

 about 3 feet in length, and the fresh-water river lampern {Lanipetra fluviatilis), 

 about 2 feet in length, eat worms and small crustaceans and are also ectoparasites 

 on living fishes to which they attach themselves and feed by rasping off the flesh. 

 From the latter species smaller brook lamperns (sand-prides) have presumably 

 been derived ; these do all their feeding as larvae and after metamorphosis to 

 the advilt form, breed and then die. Related genera are Mordacia and Oeotria 

 from the coasts of Chile and Australasia, and Ichthyomyzon from the western 

 coasts of North America (Fig. 270). 



The hfe-cycle of the lamjDrey is interesting and complex. The 

 larva, or Ammocoetes (sometimes known as the " pride " when it was 



Fig. 270. — The Sea Lamprey, Petromtzon maris us. 



There are two unpaired median fins and a relatively large eye ; behind 

 the eye are seven point-like gill-slits. For the head of the lamprey, see 

 Fig. 862. 



thought to be a different species), is a small creature without a sucking 

 mouth and with a solid spinal cord in which a medullary cavity 

 subsequently develops ^ ; the eyes are extremely rudimentary and lie 

 beneath the skin. Before metamorphosis the larva burrows in mud 

 and the non-functional eyes are covered with opaque integument. At 

 metamorphosis during the latter half of July, at the age of 2 to 4 years, 

 great changes occur as the ammocoetes leaves the mud or sand and 

 transforms into the eel-like adult, changes which include the develop- 

 ment and emergence of the eyes. The simple and relatively undiffe- 

 rentiated retina of the larva (retina A) rapidly becomes transformed 

 into the functional adult tissue (retina B) and as it does so the over- 

 lying skin atrophies and becomes transparent. The adult organ is 

 rapidly formed, neither regressive, atrophied nor degenerate in type, 

 but primitive in nature and embryonic in certain characteristics, 

 particularly in the structure of the optic nerve. 



It nteresting that the animal also possesses pineal and parietal " eyes," 

 a subjf ich will be fully discussed in a later chapter. ^ 



' Compare p. 239, footnote. ^ p. 711. 



