ii 4 VERTEBRATES WITHOUT JAWS iv. 17 



17. The ammocoete larva 



The eggs contain a considerable quantity of yolk, but their cleavage 

 is total and proceeds in a manner not unlike that of the frog. After 

 about three weeks the young hatches as the ammocoete larva, about 

 7 mm long. At first this is a tiny transparent creature, but its larval 

 life lasts for a long time, during which it grows into an opaque eel-like 

 fish, up to 170 mm long (Fig. 76). 



Fig. 76. Ammocoete larva of Lampetra planeri, showing the effect of shining a narrow 



beam of light on to various parts of the side of the body. Illumination of 1, 2, or 8 is 



followed by movement after a few seconds, but no movement follows illumination at 



points 3-7. (From Young.) 



end. 



Fig. 77. Young ammocoete larva of lamprey fixed while feeding on green flagellates 

 and detritus and then stained and cleared. 



an. auditory sac; br. brain (covered by meninges); e. eye; end. endostyle \f.c. food cord in 

 pharynx; h. heart; /. liver; m. mid-gut; oes. oesophagus; v. velar fold. 



This portion of life is spent buried in the mud, the animals emerg- 

 ing only occasionally to change their feeding-ground, presumably if 

 the mud is not sufficiently nutritious. There is no sucker, the mouth 

 being surrounded by an oral hood rather like that of amphioxus (Fig. 

 77). The paired eyes are covered by muscles and skin. The head at 

 this stage is little sensitive to light, but the animal quickly begins to 

 swim if the tail is illuminated. We have seen already (p. 108) that in 

 lampreys there are photoreceptors in the tail, connected with the 

 lateral line nerves. In the larva these are the main photoreceptors, and 

 they ensure that the animal lies completely buried. 



If a number of larvae are left in a vessel with a layer of mud on the 



