AMPHIOXUS 



ii 



driven forwards and sideways up the gill-bars, and round the 

 peripharyngeal bands, where food particles become entangled 

 in it. The food particles have been swept into the mouth 

 with the current of water made by the cilia of the wheel-organ 

 and gill-bars. Mucus and food then get carried into the 

 hyperpharyngeal groove and back to the intestine. By this 

 means the food is carried safely back through the pharynx as 

 by a moving stairway, and is not lost with the water which 

 streams out through the gill-slits. 



The ciliary method of feeding is primitive. From the 

 nature of its mechanism it can only supply particles of food 



Fig. 9. — Amphioxus : view of the dorsal portion of the pharynx and gill- 

 slits (gs), showing the nephridia («). 



on, opening of the nephridium into the atrium ; pg, primary gill-bar ; s, 

 synapticulum ; sg, secondary gill-bar. 



of small size, and therefore it can only occur in smallish 

 animals. In higher forms in which other methods such as 

 biting or sucking have been adopted for procuring food, the 

 endostyle is no longer required to secrete a mucus " fly- 

 paper " ; it becomes modified in a most striking way and 

 gives rise to the thyroid gland (see p. 399). 



Atrium. — The gill- slits do not open directly to the outside 

 world but into a cavity known as the atrium, which in its 

 turn opens to the exterior near its posterior extremity by the 

 atriopore. On the right side of the body (but not on the left) 

 the atrium extends back behind the atriopore as a blind sac 



