DEVELOPMENT 



331 



a marked groove. The 

 upper Icp^ 



In the Lampreys {Petromyzonidae) , for example, the newly 

 hatched larva is so unlike the parents that it has received 

 distinct generic and specific names {Ammocoetes branchialis), and 

 is popularly known as a Pride or Niner. It is curiously worm- 

 like in form, and differs from the adult in having rudimentary 

 eyes buried beneath the skin, a horse-shoe shaped mouth, with 

 a small transverse lower lip and a hood-like upper lip, and no 

 teeth (Fig. 119): the entrance to the mouth is surrounded by a 

 number of fringed barbels forming a perfect strainer. The 

 small external gill-openings lie in 

 Prides are hatched 

 some ten to fifteen 

 days after fertilisation 

 of the eggs, and re- 

 main in the nest for 

 about thirty days. 

 They then wander 

 down the stream, 

 and having selected 

 a suitable spot, bur- 

 row in the sand or 

 mud. They live 



buried in tubes for 

 three or four years, 

 quite blind, and feed- 

 ing on minute organ- 

 isms or on organic 

 matter contained in 

 the mud. In their 

 mode of life, and 

 particularly in the 

 manner in which they obtain their food, these larvae bear 

 a marked resemblance to the Httle sand-dwelling Lancelet 

 (Amphioxus) and to the other very lowly vertebrates, the 

 Sea Squirts or Ascidians. The minute particles comprising 

 the food are carried through the mouth into the pharynx 

 by currents of water produced by the action of special 

 ciUated cells working in unison. The particles become entangled 

 in strings of mucus, secreted in a groove (endostyle) in the floor 

 of the pharynx (which, in the higher vertebrates, becomes the 

 thyroid gland), and are swept into the stomach by bands of 

 ciHa. At the end of three to five years a metamorphosis occurs, 

 and the larva quite suddenly assumes the characters of the 



Two 



Fig. 119. 



views of the head of the larval Lamprey 

 Pride (Ammocoetes branchialis), X 4. 



