THE CYCLE OF LIFE 465 



Neunaugen], they are blind ; for their eyes, growing out from 

 the brain as vertebrate eyes always do, have not yet reached 

 the surface. There are seven gill-slits on each side, and 

 these have been popularly counted in as eyes. These 

 curious old-world creatures are often regarded as the young 

 of eels, but, as a matter of fact, they are far below the level 

 of fishes on the genealogical tree of animals. Whence have 

 they come and what future is before them ? 



The Sea Lamprey (Petromyzon marinus), whose larvae 

 the niners are, is a strong muscular animal, sometimes a 

 yard long, abundant in the Mediterranean and the North 

 Atlantic. It also occurs in the American lakes, having in 

 this case dispensed with its normal journey to the sea. 

 The colour is greyish green with darker spots. The struc- 

 tural peculiarities are numerous. For apart from the 

 absence of jaws, limbs, and scales, which we have already 

 mentioned, there is a circular adhesive disc around the 

 mouth and lined with rows of horny teeth, there is a very 

 muscular protrusible ' tongue ' bearing horny plates for 

 rasping with, there is an unpaired nostril far back on 

 the top of the head, like a porpoise's blow-hole, and there 

 are curious gill-purses. Their habits are not less remark- 

 able. They attach themselves to living fishes and rasp 

 the flesh and suck the blood. They take a very firm hold 

 of their victims and make deep holes, and they are some- 

 times carried for long distances by large fishes, such as 

 salmon. They migrate in spring or early summer from 

 the sea (or the lakes of the State of New York) to the rivers, 

 usually changing to a more yellowish colour ; they make 

 nests of stones, and they die after spawning. * 



It is a very general fact of Natural History that when 

 the habitats of adult and young are different, the cradle- 



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