150 THE STORY OF ANIMAL LIFE 



The Lancelet also differs from the true verte- 

 brates, in that it has no limbs. There is a fring- 

 ing fin along the body, but it is not comparable 

 with the fins of fishes. It differs also in possess- 

 ing no teeth. 



In one respect, however, the Lancelet reminds 

 us of a fish : and that is in the arrangement of 

 its muscles ; these form a successive series of 

 overlapping masses on each side of the body, as 

 in a fish. 



The development of the Lancelet presents us 

 with an instance of the two-layered larva, or Gas- 

 trula. This shows that Amphioxus is a compar- 

 atively primitive type. But it has been suspected 

 that it is less primitive than it looks, and that it 

 has degenerated from some higher form, owing to 

 its preferring a dull mode of existence, half-buried 

 in sand or mud. 



There is a huge gap between the Lancelet and 

 the true vertebrates. The lowest form of the 

 latter is Ammoccetes, the larva of the Lamprey 

 (Petromyzon). The latter, even in the adult form, 

 has no true limbs, though there are fringing fins. 

 The notochord sheath issupplemented, however, by 

 cartilage bars which are equivalent to the begin- 

 nings of the vertebrae of the back-bone. The gills 

 are very different from those of other true verte- 

 brates, and it has no jaws. Teeth it has, however, 

 on the tongue and the lining of the mouth. Prob- 

 ably this creature is greatly altered by adaptation 

 to its peculiar mode of life, so that no certain 

 conclusions can be drawn from it regarding the 

 structure of primitive fishes. It has a sucking 

 mouth, by means of which it hangs on to fishes, 

 while it rasps away their flesh with its rough 

 tongue. When not thus engaged, it hangs on to 



