CEPHALOCHOKDA 135 



anteriorly, and posteriorly it is central in position. It is made 

 up of cells which have become highly vacuolated, and the 

 spaces are filled with a gelatinous fluid. It is surrounded 

 by a tough connective-tissue sheath. The large vacuoles of 

 the notochord are maintained in life in a turgid state, and the 

 sheath provides an external resistance. It thus forms a central 

 elastic skeleton which gives rigidity to the body, and it is bent 

 by the action of the muscles on each side. 



The alimentary canal is modified for the capture, digestion, 

 and absorption of food, and for respiration. The oral cavity 

 has a wide margin produced into cirri. The margin is kept 

 distended by a skeleton of gelatinous bars which are produced 

 into each cirrus. The cavity gradually narrows to the mouth 

 opening, bounded by the velum. The oral cavity in front of 

 the mouth or velum is raised into ciliated folds which form 

 what has been called the wheel organ. The wheel organ is 

 interrupted on the roof of the oral cavity by a pit, called 

 Hatschek's pit, or the preoral pit. The velar margin is pro- 

 duced into a series of processes which are called velar tentacles. 

 These structures are best seen in prepared specimens of young 

 Amphioxus and in sections. 



The functions of these organs are not very apparent. 

 Hatschek's pit appears to be a mucin-secreting gland, and 

 may thus serve to begin the process of providing mucin for 

 entangling the food. It is possible also that, situated as it 

 is on the roof of the oral cavity, it acts as an environmental 

 organ an organ, that is to say, designed to give warnings by 

 internal secretions of changes in temperature and other features 

 of the water. The cilia of the wheel organ and of the velar 

 tentacles maintain and direct a current carrying food into the 

 pharynx. 



Beyond the velum the canal suddenly expands into the 

 large pharynx. The pharynx occupies the whole of the meta- 

 pleural part of the body ; it is compressed laterally, but verti- 

 cally it extends from near the notochord to the ventral region 

 of the atrial cavity. At the anterior end the walls of the 

 pharynx present a pair of ciliated bands, the peripharyngeal 

 bands, which encircle the wall in an oblique direction and 

 merge dorsally and ventrally in glandular grooves. The 



