452 CYCLOSTOMATA 



because it is in many ways different from the adult, and does 

 not become like the adult until it undergoes a metamorphosis. 

 For a long time it was supposed to be a distinct animal both 

 by zoologists and country people, the former calling it Am- 

 mocoetes and the latter a " niner ". A Strasburg fisherman 

 called Baldner, seems to have convinced himself more than 

 200 years ago that " niners " grew into lampreys, but his correct 

 conclusion was not generally accepted till long afterwards. 

 Among the differences between larval and adult form, we may 

 note that the eyes of the larva are far below the surface, that 

 the mouth of the larva is horse-shoe-shaped rather than cir- 

 cular, and that the larva has no teeth. The upper lip of the 

 larva is somewhat hood-like and the much shorter lower lip is 

 included within it. Niners remain niners for three to five years, 

 and the change to the adult form takes seven or eight months 

 in the case of the brook-lamperns (September to April). 



When the larva is hatched it burrows into the mud, where 

 it lives on microscopic organisms wafted into the mouth by 

 ciliary action. It is for some time sightless, the eyes being far 

 below the surface. Both these points are of evolutionary inter- 

 est : the abundance of cilia in the front part of the alimentary 

 tract is a primitive feature, recalling the state of affairs in the 

 lancelets ; and the position of the eye reminds us that the eye 

 of Vertebrates is a " brain-eye," i.e. that it grows from within 

 outwards, whereas the eyes of Invertebrates are skin-eyes 

 arising on the surface. As the brain is developed from an in- 

 sinking of superficial ectoderm cells, the contrast between brain- 

 eye and skin-eye is not so sharp as at first sight appears. It 

 may also be noted that the eye of the hag remains hidden 

 throughout life ; it never reaches the surface at all. 



The larva lives a rather sluggish life wallowing in the mud, 

 especially in slow-flowing reaches or backwaters of the river. 

 Alcock x has discovered the interesting fact that the skin 

 secretes a digestive ferment which protects it from the injurious 

 action of Bacteria and Fungi. 



Different Kinds of Lampreys 



Petromyzon. — For the most part northern forms, such as 

 Pctromyson marinus, about a yard long, which spawns and 



l Jonrn. Anat. Physiol., xiii., pp. 612-637. 



